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over all the humps at about 25 knots, where there is a in canals) that “the wave at the critical speed changes range of about 200 horse-power. The curve for go feet of from the repeating to something approaching the solitary water is, however, for 5000 horse-power at 26 feet, or about 500 horse-power above the next highest curve. It The author next considered the waves accompanying the would therefore pay better, according to these model ex vessel. Transverse waves should tend to become longer periments, to run a 26-knot trial of a destroyer in a depth and longer for the same speed as the depth diminishes of 20 feet to to feet rather than at a depth of 90 feet; the until, at the critical depth, these should be of the isolated saving in power for the given speed due to the use of | type. In shallower water, past the critical depth, there shallower water would be about 600 to 700 horse-power. would be no transverse wave corresponding to the critical

We have been obliged to depart from the text of Mr. depth. Actually as the waves became longer if they did Harold Yarrow's paper in order to give the facts con not lengthen as rapidly as investigation would lead one to tained in his diagram. Limits of space prevent detailed suppose they would be travelling faster than their natural reference to other particulars brought forward by these speed, and must be dragged by the boat. The increased suggestive trials, but enough has doubtless been said to resistance on approaching humps in the curve supports this show their interest and the wide field for further investi view. The isolated wave is non-repeating, and exists gation that is afforded by the numerical data now at only under certain relations of depth to speed. After passcommand. It will be remembered, as Mr. Harold Yarrow ing the critical point the transverse wave disappears, being pointed out, that the tank experiments were made in the replaced by confused water. The paper was accompanied usual way without propellers, and this would doubtless by diagrams illustrating these points, and it was considered have a considerable influence on the results ; but possibly a fair deduction that the waves formed by a ship closely a way may be found, as suggested by Sir William White, follow the laws of waves in open water. The effect of to add the propeller, and so bring the tank conditions the vertical sides of a canal in diminishing the loss of more nearly akin to actual practice. It will be remembered energy was pointed out by the author, and the manner in that the late Mr. Froude proposed to run a propeller, which the restricted width of a tank might have a similar wcrked by independent mechanism, at a speed of revolu effect was noted. The interference of the bow-wave system tions corresponding to that of actual practice, the model, on the stern-wave system was discussed, it being shown cf course, being towed by the carriage.

that the union of the bow and stern waves (the crests Mr. Marriner's investigation of the model results showed coinciding) resulted in a large resultant wave which that they should, as Mr. Harold Yarrow said, “be would carry away a great deal of energy. The velocity accepted with caution." In order to check these tank of diverging waves is much less than the speed of the data progressive trials were made with an actual destroyer vessel, speeds attained up to the present not being high on a carefully selected course off the mouth of the Thames. enough for waves to approach the critical speeds for the Fcur tugs were anchored to mark the course, which depths in which vessels usually run. Diverging waves extended from the East Girdler buoy across the Tongue apparently constitute the principal wave-making resistance Sand to a point east of the extreme north-east point of at speeds beyond the critical combinations of depths and the Tongue Spit. This gave water of depths varying speeds under consideration. The final conclusions of the between about 14 to 16 feet over the sand, and 100 to author were as follows :—" (1) The critical combinations 102 feet in the Queen's Channel. The revolutions of the of depth and speed do not depend on the size of the vessel. engines, the fore and aft inclination of the vessel, and ! (2) Of these critical combinations there is, for every vessel, the height of the stern wave were noted, indicator one more serious than the others, and where this worst diagrams also being taken. The data thus obtained at combination occurs depends largely on the length of the varying mean speeds were given by means of diagrams | vessel. (3) The depth to be avoided is given by the thrown on the screen. We have not space to repeat all equation d=1'?/10, and the resistance diminishes in both the records, but will take as an example the run made greater and lesser depths. The further away from this at a mean speed of about 22.2 knots, the steam pressure bad depth the better, especially on the deep side." hring 140 lb. In running from rather less than 50 feet In the course of the discussion which followed the readdepth into water of 20 feet, the revolutions increased from ing of these papers, Mr. A. F. Yarrow, in conformity with about 305 to more than 325; the inclination in a length of a suggestion made by Sir William White, proposed, and 26 feet decreased from approximately 51 inches to 43 or Mr. S. W. Barnaby seconded, a resolution that the 4! inches; and the approximate height of the stern wave Admiralty be urged to erect a measured mile, where ample fell from 20 inches above the deck level to a little more depth might be found, in proximity to Chatham and the than 10 inches below the deck level, or about 2 feet Thames. This resolution was put to the meeting by Sir 6 inches. Naturally any increase or reduction of resist John Durston, who occupied the chair, and was carried ance to the vessel would be accompanied by increase or unanimously. It was but the logical outcome of the facts riduction in the rate of revolutions of the engine, the brought forward. The measured mile in the Clyde at steam pressure being constant. Increased height of stern Skelmorlie has ample depth of water, and is, as Sir wave and greater inclination are also signs of increased William White said, the only satisfactory mile for highTraistance and a greater demand for horse-power.

speed trials ; a fact which, it is acknowledged, gives the It would be difficult to deal adequately with Mr. ship-builders of that district a manifest advantage over Marriner's paper within anything approaching the space those of other centres. Mr. Barnaby stated that when a we have at our disposal, and without the diagrams by destroyer built by his firm, John I. Thornycroft and Co., which it was accompanied. We can only hope, therefore, was tried on the Skelmorlie mile an increase of speed to give an idea of its scope, and refer our readers to the of i knot was obtained over that reached under the best original in the published Transactions of the institution. conditions on the Maplin mile, whilst an increase of The contribution consisted of a discussion, on a scientific 3 knots was reached as compared to running on the basin, of the results contained in the preceding paper, Maplin when the state of the tide was most unfavourable. the size of waves made by a vessel in her passage being Sir William White, in a letter to the secretary, read at a measure of the power absorbed in their formation. the meeting, heartily endorsed the suggestion of a deep

Is the height of the stern wave was seen to increase water measured mile off the Thames. He also pointed when the resistance of the ship increased abnormally, it out, with great regret, that it was necessary for Messrs. was to be assumed that anything which tended to retard Yarrow to go to Germany for their tank experiments, and the formation of waves would reduce the loss from wave trusted that the fact might furnish a fresh incentive making. The author cited the work of W. Froude and towards the establishment of a research tank at the nis son, R. E. Froude, of Lord Kelvin, D. W. Taylor, of National Physical Laboratory. the l'nited States Navy, and Prof. Horace Lamb. The In the discussion on the technical details of the two krmula for relation of length to speed of ordinary re papers, besides those mentioned, Mr. J. H. Varbeth, of prating waves in deep water was set forth, and also the the Admiralty, Mr. R. Saxton White, Captain Jackson, more complicated equation for shallow water. The equa R.V., Controller of the Navy, Mr. W. H. Whiting, chief tion showing critical depth for speed and critical speed for constructor, and Prof. Biles took part. Generally it may depth was given, and the conclusion was drawn (sup be said the views pxpressed by the authors were not dispurtrd by Scott Russell's equation for the solitary wave I puted, although Sir William White did not quite agrer

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with Mr. Marriner as to the importance the latter attached of upwards of 223,000 lives per annum. As an exceptionally to the comparative narrowness of the tank.

low rate of infant mortality had been maintained for On the Thursday's sitting Mr. Milton's paper on frac two successive years, it might be hoped that the warnings tures in large steel boiler plates was read and briefly dis uttered as regards infant hygiene, and more particularly cussed. It gave particulars of the failures, the reasons for infant feeding, were beginning to take effect. which could not be explained, of certain plates, and there It was, however, during the first half of life that the fore were, naturally, attributed to “heat treatment." It great fall in the death-rate had taken place. It was a is a term of exceeding comfort to the steel maker and the remarkable fact that in men, at all ages from forty-five engineer alike, for the former is able to put the blame to seventy-five, there had been a startling rise in the on the latter, and the latter to put the blame on the death-rate, and that in women, from fifty-five upwards, former, as no one can prove where the injudicious heat it had been practically stationary. At the ages when we treatment occurred. The controversy is an old one, dating should have welcomed a rise in the death-rate, and at back, at any rate, to the days of the Livadia's boilers. which only, in a hygienic Utopia, death ought to occurMr. Milton's paper is a suggestive contribution, and the eighty-five and upwards-it had fallen. Some of the facts he records may carry us some way towards a solu nerve centres went on evolving until middle life, 0.g. the tion of the problem in the more or less distant future. hand and arm centres. He had ascertained that among

Mr. William Gray in his paper gave particulars of the certain classes of operatives in Birmingham the hand and performances of certain steamers fitted with Parsons' arm centres did not reach their full maturity until about steam turbines. These were set forth in a table, which, the thirtieth year. Similarly with the weavers of Bradford as the author said, “ treated the matter from a purely and the potters of Staffordshire. At about forty-five the commercial standpoint." The discussion was largely of productiveness of the manufactory hand generally began to the same character.

diminish, and after that it contracted in an increasing The only remaining paper was not on the original ratio as time went on. The hand-failure of our operatives programme, but was read at the conversazione which after forty-five was premature, and due to excessive wear brought the proceedings to a conclusion on the Thurs and tear of the mechanism regulating manual movements. day evening. It was a contribution by Captain R. H. | But there were other centres in the brain which, reaching Bacon, R.N., entitled “Notes on the Causes of maturity later, retained their power longer. Orators Accidents to Submarine Boats, and their Salvage." secured their greatest triumphs between forty-five and fiftyThis paper, in harmony with the circumstances in which five, and it was with musical expression as with oratory. it was read, was of a popular nature, and was

The best antiseptic against senile decay was an active designed to show to the public at large that undue interest in human affairs, and those kept young longest anxiety as to the safety of submarine boats is not who loved most. The natural evolution of our nerve warranted by the conditions under which they are centres was largely interfered with and too often arrested employed. Dealing with the probability of water entering by unfavourable environment and deleterious habits of life the hull through the hatch (the cause of four serious or methods of work. It was a good working hypothesis accidents through which these boats have foundered), the that the natural life of man was one hundred, and that so author pointed out that the fitting of another water-tight far as it fell short of that it was “curtailed of fair prohatch at the base of the tower reduced the chance of portion." Every man, he thought, was entitled to his accident in the future. The danger from grounding, he century, and every woman to a century and a little more. said, “ was not very great," whilst the risk of the hull Dr. Francis Galton, F.R.S., in a paper on physical being crushed by the boat diving to too great depths records, suggested that on February 29 in each leap year argued the failure of the diving rudders, or too much there should be school reunions at which there might be water ballast. As to explosion through leakage of petrol, an opportunity for reviving early friendships, and at “ in a properly designed system leaks should be prac which, at the same time, the anthropometric and other tically non-existent." Another possible cause of ex

records of the pupils might be added to. plosion is due to hydrogen given off when batteries are

Each old boy would be represented by an envelope stored being charged, but as this operation is carried on only in the school library. This would contain his anthropowhen the boat is opened up for ventilation, “no danger

metric record to date, and he would be given printed exists." Altogether Captain Bacon's lecture was most re forms, containing a few well considered questions-health, assuring, and it is pleasant to learn that his optimistic profession, preferments, marriage, children, and general views are fully shared by his colleagues in the Service, both

remarks and would be asked to forward the filled-in forms officers and men. The risk of sinking-involuntary sink

to the school. ing—being so small, it is of less consequence that only

Many papers were read on infantile mortality and on over a limited area near shore is it possible to recover a

municipal milk depôts. submarine boat once she has gone to the bottom. For

In the unavoidable absence of Sir William Broadbent, a this reason Captain Bacon considers it inadvisable that

discussion on sanatoria for consumptives was opened by the Royal Navy should have a salvage plant of its own.

Dr. T. N. Kelynack, physician to the Mount Vernon Hospital for Consumption. To illustrate the enormous economic waste to the community caused by pulmonary

tuberculosis, Dr. Kelynack mentioned that in the metroTHE CONGRESS OF THE ROYAL INSTITUTE

politan district alone 40,000 people died of the disease every OF PUBLIC HEALTH.

year, and the monetary loss to London had been estimated

at 4,000,000l. THE congress of the Institute of Public Health, which

The provision of adequate assistance for the consumptive this year was held in London under the presidency poor demanded urgent attention. Sanatoria or hygienic of the Marquis of Londonderry, attracted a large number hospitals undoubtedly secured the best conditions for the of visitors, and much good work was done in the various arrest and alleviation of the disease. At present we were sections which met at the Polytechnic and at King's just muddling along, with no satisfactory organisation of College.

our resources and no rational cooperation. In a brief space it is impossible to deal adequately with A resolution was unanimously adopted urging the the valuable discussions and papers read.

Government to appoint a commission to deal with the Sir James Crichton Browne, F.R.S., in his presidential subject of the sanatorium treatment of the poor. address to the section of preventive medicine, chose as his The presidential address in the section of chernistry and subject the prevention of senility. It was, he declared, 1 bacteriology was given by Prof. R. T. Hewlett. It was on the reduction of the death-rate that the potency of a plea for the recognition of the place of the specialist in preventive medicine, as hitherto applied, stood forth con the various departments of public health. Proper adminis. spicuously declared, and that the promise of its future tration required a medical officer and his sanitary staff, a sovereignty might be discerned. Fifty years ago the death

discerned. Fifty years ago the death- bacteriologist, a chemist, and an engineer, all working rate of England and Wales stood at 22.5 per 1000 persons; , cordially together to a common end. For the smaller in 1903 it had dropped to 15.4-a fall of 7.1 per 1000, districts such a staff could be secured by grouping. Could representing, on the estimated population of 1903, a saving they expect effective action if the medical officer was a local practitioner who derived his livelihood by the good- observations are needed. The following suggestions are will of the local landlord ?

compiled from various sources. Prof. Hewlett also denounced the way in which chemists Spread a white cloth or piece of canvas upon the ground were taking upon themselves the bacteriological examin in any convenient open space. It is well to spread two ation of pathological material, and emphatically asserted cloths or pieces of canvas, one to be used before, the other that disease problems should be dealt with only by medical after, the total phase. Let each observer be furnished men. He also advocated that a course of biology should with several sticks, 4 feet to 6 feet long. be obligatory for candidates for the associateship of the About three minutes before the time of totality, let the Institute of Chemistry taking the subject of biological observer stand near the cloth with his back to the sun chemistry.

and watch the cloth intently. If bands or dark patches An interesting discussion, opened by Dr. Newman, of are seen, place one stick down in the direction in which Finsbury, was on the possibility of establishing a bacterio they lie; after this is done place a second stick in the logical standard of purity of milk. Dr. Newman suggested direction in which they are moving. Both of these operthe following standards :-(a) not more than 24-25 degrees ations should be done deliberately, not hurriedly, and the of total acidity at the time of sale, i degree being equi sticks left in position. valent to 1 c.c. of deci-normal NaOH solution ; (b) not During the total phase the observer is free to enjoy an excess of pus or blood ; (c) no B. coli, B. enteritidis, or the scene or make other observations, but it may be well B. enteritidis sporogenes; (d) non-virulent to guinea-pigs. to note it any

to note if any bands can be seen during totality, as some All the speakers, including Dr. Allan Macfadyen, Prof. i have asserted. Kenwood, Dr. Savage, Colonel Firth, Mr. Revis and ! At the close of totality the observer should be at the others, agreed that there was little possibility at present ) second cloth, or at another part of the single cloth (if he of fixing a standard, and Dr. Newman's suggestions did | uses but one), and should repeat the observations made not obtain general acceptance.

before totality, placing one stick down in the direction in Another discussion, on the relative efficiency of chemical | which the bands lie, and another in the direction in which and bacteriological methods for the examination of sewage they move. effluents, was opened by Mr. Dibden and by Dr. Savage. It will be seen that four sticks are needed for these There was a general agreement that chemical methods gave observations. If two persons make the records, one should a better indication of proper purification than bacterio confine his attention to the direction in which the bands logical ones, though, of course, bacteriological methods lie, the other to the direction in which they move. The alone were of service in detecting species of micro bands are likely to be somewhat faint and poorly defined, organisms.

so that extreme accuracy may not be possible. Lieut. Nesfield, I.M.S., gave an interesting demonstra The sticks should not be disturbed until after the eclipse, tion of a method devised by him for the sterilisation of when their direction should be determined with as much drinking water during a campaign. He had found that care as possible, either by a compass or, still better, by chlorine in the proportion of 2 grams per 100 gallons la surveyor's theodolite if one is available. If neither acting for five minutes effectually destroyed the organisms compass nor theodolite is at hand, an estimate of the of cholera, typhoid, and dysentery. His method consisted directions should be made. in carrying iron bottles of liquid chlorine, from which, by The velocity with which the bands travel is more means of a valve, the requisite amount was liberated into

difficult to determine. The estimates vary from the speed the water. After five minutes a powder of sodium sulphite of a man running to that of an express train. Several (2.2 grams) was added to the water, from which a double methods may be suggested :decomposition ensued, and the water was rendered abso (1) Let two persons work together, one having a watch lutely tasteless. For the soldier on the march another with the seconds marked on the face. Let him mark time method was devised, so that he could sterilise for himself

by calling out each second. The number of the second is a gallon of water. This consisted in adding to the vessel | not important, but a simple sound to mark the seconds of water a tablet containing iodide and jodate of sodium. is sufficient. Let the other observer watch the bands and This resulted in the liberation of free iodine in the water, see how many he can count per second. which acted in five minutes as an efficient germicide, and

(2) With one observer marking time as before, let the was then “ killed " so that the water was rendered pot

second observer note how many seconds elapse while a able, by the addition of another tablet of sodium sulphite.

band is passing between two objects the distance apart of In both processes the quantities of reagents employed are

which is known. so small as to have no effect on the human economy; the (3) Let a person run a short distance with the bands methods are rapid, and the reagents, &c., portable.

and see if he can keep up with them. If not, let him estimate how much faster the bands are moving than he can run.

(4) A mere guess at the speed is of some value.

The width of the bands and their distance apart can ECLIPSE SHADON BANDS.

best be determined upon the cloths mentioned above, and

it will add to the accuracy of the estimates if the cloths ONE of the most peculiar appearances attending a total are divided by seams or in some other way into strips of

eclipse of the sun is that generally known as the known width. A carpenter's rule will aid the observer in "shadow bands." They are long dark bands, separated making the estimates. The bands will probably be several by white spaces, which are seen on the ground or sides inches wide and separated by spaces the width of which of buildings just before and just after the total phase of is the same or greater. an eclipse, moving rapidly. It is probable that they are If the observer notices any other point connected with not real bands, but are composed of dark patches which the bands, such as their colour, whether they are straight seem to the eye to make long bands. Their cause is not or wavy, whether they are continuous bands or made up yet clearly known, as the observations in former eclipses of dark patches, whether they flicker or not, the informare quite discordant. The undersigned is very desirous of ation will be valuable. Still more valuable would be obtaining observations of them at various stations along photographs of the bands as they pass over the ground the line of totality, especially at places near the edge of or the side of a building or wall. the shadow, in order to compare with similar observations It is earnestly requested that anyone who will kindly made by himself and others. The observations require no | attempt the above, either in whole or in part, will send special instruments, and can be made by any careful his records to the undersigned. If for any reason the person. Information is desired upon the following observation seems unsatisfactory, either because the bands points :-(1) the direction in which the bands lie ; (2) the were not as distinct as expected, or for any other reason, direction in which they move ; (3) the velocity with which or if the record is only fragmentary, it will still be of thry move ; (4) the width of the bands; and (5) their value. The report should consist of a statement of the distance apart. All of these are likely to be different methods employed by the observer or observers in making before and after the total phase, so that two sets of the observations, and the results obtained with any Place .....

remarks upon the subject or upon other phenomena noted upon subsequent chemical actions or at least we cannot at the time of the eclipse.

A. LAWRENCE Rotch. correlate it with what is known of the physics of chemical Blue Hill Observatory, Hyde Park, Mass., U.S.A. activity. Finally, as will be seen later, it is hardly adequate

to account for the varying degrees of stability which may The observations may be summarised as follows :

apparently characterise the latent image. Still, there is

much in Mr. Bose's work deserving of careful consideraOBSERVATIONS OF SHADOW BANDS, AUGUST 30, 1905.

tion. He has by no means exhausted the line of investi

gation he has originated. (Situation and altitude)

Another theory has doubtless been in the minds of many.

I have said we must seek guidance in some photo-physical Observers .........

phenomenon. There is one such which preeminently con

nects light and chemical phenomena through the interBefore totality . After tolality

mediary of the effects of the former upon a component part 1. Direction of bands,

of the atom. I refer to the phenomena of photo-electricity.

It was ascertained by Hertz and his immediate successors 2. Direction of motion,

that light has a remarkable power of discharging negative 3. Velocity,

electrification from the surface of bodies—especially from 4. Width of bands, .........

certain substances. For long no explanation of the cause

of this appeared. But the electron-the ubiquitous electron 5. Distance apart,

-is now known with considerable certainty to be reRemarks :

sponsible. The effect of the electric force in the light wave Direction of the wind before totality........

is to direct or assist the electrons contained in the sub........, after

stance to escape from the surface of the body. Each totality.......... ........., and direction from which upper

electron carries away a very small charge of negative clouds (if any) came......

electrification. If, then, a body is originally charged negatively, it will be gradually discharged by this convective

process. If it is not charged to start with, the electrons THE LATENT IMAGE.'

will still be liberated at the surface of the body, and this M Y inclination has led me, in spite of a lively dread

will acquire a positive charge. If the body is positively of incurring a charge of presumption, to address

charged at first, we cannot discharge it by illumination. you principally on that profound and most subtle question,

It would be superfluous for me to speak here of the the nature and mode of formation of the photographic

nature of electrons or of the various modes in which their image. I am impelled to do so, not only because the

presence may be detected. Suffice it to say, in further

connection with the Hertz effect, that when projected subject is full of fascination and hopefulness, but because the wide topics of photographic methods or photographic

among gaseous molecules the electron soon attaches itself applications would be quite unfittingly handled by the

to one of these. In other words, it ionises a molecule of

the gas or confers its electric charge upon it. The gaseous president you have chosen. I would first direct your attention to Sir James Dewar's

molecule may even be itself disrupted by impact of the

electron if this is moving fast enough and left bereft of an remarkable result that the photographic plate retains con

electron. siderable power of forming the latent image at tempera

We must note that such ionisation may be regarded tures approaching the absolute zero-a result which, as

as conferring potential chemical properties upon the moleI submit, compels us to regard the fundamental effects

cules of the gas and upon the substance whence the progressing in the film under the stimulus of light un

electrons are derived. Similar ionisation under electric dulations as other than those of a purely chemical nature.

forces enters, as we now believe, into all the chemical But few, if any, instances of chemical combination or

effects progressing in the galvanic cell, and, indeed, genedecomposition are known at so low a temperature. Purely

rally in ionised solutants. chemical actions cease, indeed, at far higher temperatures,

An experiment will best illustrate the principles I wish fluorine being among the few bodies which still show

to remind you of. A clean aluminium plate, carefully chemical activity at the comparatively elevated tempera

insulated by a sulphur support, is faced by a sheet of ture of – 180° C. In short, this result of Sir James Dewar's

copper-wire-gauze placed a couple of centimetres away from suggests that we must seek for the foundations of photo

it. The gauze is maintained at a high positive potential graphic action in some physical or intra-atomic effect

by this dry pile. A sensitive gold-leaf electroscope is which, as in the case of radio-activity or fluorescence,

attached to the aluminium plate, and its image thrown is not restricted to intervals of temperature over which

upon the screen. I now turn the light from this arc lamp active molecular vis viva prevails. It compels us to regard

upon the wire gauze, through which it in part passes and with doubt the role of oxidation or other chemical action

shines upon the aluminium plate. The electroscope at as essential, but rather points to the view that such

once charges up rapidly. There is a liberation of negative effects must be secondary or subsidiary. We feel, in a

electrons at the surface of the aluminium; these, under word, that we must turn for guidance to some purely the attraction of the positive body, are rapidly removed photo-physical effect.

as ions, and the electroscope charges up positively. Here, in the first place, we naturally recall the views of Again, if I simply electrify negatively this aluminium Mr. Bose. This physicist would refer the formation of plate so that the leaves of the attached electroscope diverge the image to a strain of the bromide of silver molecule widely, and now expose it to the rays from the arc lamp, under the electric force in the light wave, converting it

the charge, as you see, is very rapidly dissipated. With into what might be regarded as an allotropic modification

positive electrification of the aluminium there is no effect of the normal bromide which subsequently responds speci

attendant on the illumination ally to the attack of the developer. The function of the

Thus from the work of Hertz and his successors we sensitiser, according to this view, is to retard the recovery

know that light, and more generally what we call actinic from strain. Bose obtained many suggestive parallels

light, is an effective means of freeing the electron from between the strain phenomena he was able to observe

certain substances. In short, our photographic agent, in silver and other substances under electromagnetic radia light, has the power of evoking from certain substances tion and the behaviour of the photographic plate when the electron which is so potent a factor in most, if not subjected to long-continued exposure to light.

in all, chemical effects. I have not time here to refer to This theory, whatever it may have to recommend it,

the work of Elster and Geitel whereby they have shown can hardly be regarded as offering a fundamental explana- |

that this action is to be traced to the electric force in the tion. In the first place, we are left in the dark as to what light wave, but must turn to the probable bearing of this the strain may be. It may mean many and various things. phenomenon on the familiar facts of photography. I We know nothing as to the inner mechanism of its effects assume that the experiment I have shown you is the meat

1 Address to the Photographic Convention of the United Kingdom, 1905. fundamental photographic experiment which it is now ir By J. Joly, F.R.S.

our power to make.

We inust first ask from what substances can light will be said that such an action must be in part of the Liberate the electron. There are many-metals as well as nature of a chemical effect. This must be admitted, and, in non-metals and liquids. It is a very general phenomenon, so far as the re-arrangement of molecular fabrics is inand must operate widely throughout nature. But what volved, the result will doubtless be controlled by temperachiefly concerns the present consideration is the fact that ture conditions. The facts observed by Sir James Dewar the haloid salts of silver are vigorously photo-electric, and, support this. But there is involved a fundamental processit is suggested, possess, according to Schmidt, an activity the liberation of the electron by the electric force in the in the descending order bromide, chloride, iodide. This light wave, which is a physical effect, and which, upon is, in other words, their order of activity as ionisers (under the hypothesis of its reality as a factor in forming the the proper conditions) when exposed to ultra-violet light. latent image, appears to explain completely the outstanding Photographers will recognise that this is also the order of | photographic sensitiveness of the film at temperatures far their photographic sensitiveness.

below those at which chemical actions in general cease. Another class of bodies also concerns our subject :-the Again, we may assume that the electron-producing power special sensitisers used by the photographer to modify the of the special sensitiser or dye for the particular ray it spectral distribution of sensibility of the haloid salts, e.g. absorbs is responsible, or responsible in part, for the eosine, fuchsine, cyanine. These again are electron-pro special sensitiveness it confers upon the film. Sir Wm. ducers under light stimulus. Now it has been shown by Abney has shown that these sensitisers are active even if Stoletow, Hallwachs, and Elster and Geitel that there is laid on as a varnish on the sensitive surface and removed an intimate connection between photo-electric activity and | before development. It must be remembered, however, the absorption of light by the substance, and, indeed, that that at temperatures of about – 50° these sensitisers lose the particular wave-lengths absorbed by the substance are much of their influence on the film. [See a paper by me those which are effective in liberating the electrons. Thus | read before the convention in 1894.) we have strong reason for believing that the

rous It appears to me that on these views the curious photo-electric activity displayed by the special sensitisers phenomenon of recurrent reversals does not present a must be dependent upon their colour absorption. You will problem hopeless of explanation. The process of photorecognise that this is just the connection between their | ionisation constituting the latent image, where the ion is photographic effects and their behaviour towards light. probably not immediately neutralised by chemical com

There is yet another suggestive parallel. I referred to bination, presents features akin to the charging of a the observation of Sir James Dewar as to the continued capacity--say a leyden jar. There may be a rising potential sensitiveness of the photographic film at the lowest attained between the groups of ions until ultimately a point is extremes of temperature, and drew the inference that the

attained when there is a spontaneous neutralisation. I fundamental photographic action must be of intra-atomic may observe that the phenomena of reversal appear to nature, and not dependent upon the vis viva of the molecule indicate that the change upon the silver bromide or atom. In then seeking the origin of photographic molecule, whatever be its nature, is one of gradually inaction in photo-electric phenomena we naturally ask, Are creasing intensity, and finally attains a maximum when a these latter phenomena also traceable down to low tempera- return to the original condition occurs. The maximum is tures? If they are, we are entitled to look upon this fact | the point of most intense developable image. It is probable as a qualifying characteristic or as another link in the that the sensitiser—in this case the gelatin in which the chain of evidence connecting photographic with photo bromide of silver is immersed-plays a part in the conelectric activity.

ditions of stability which are involved. I have quite recently, with the aid of liquid air kindly Of great interest in all our considerations and theories supplied to me by Mr. Moss, and made in the laboratory is the recent work of Prof. Wood on photographic reversal. of this society, tested the photo-sensibility of aluminium The result of this work is-as I take it—to show that the and also of silver bromide down to temperatures approach | stability of the latent image may be very various according ing that of the liquid air. The mode of observation is to the mode of its formation. Thus it appears that the essentially that of Schmidt-what he terms his static sort of latent effect which is produced by pressure or method. The substance undergoing observation is, how friction is the least stable of any. This may be reversed ever. contained at the bottom of a thin copper tube, 5 cm. or wiped out by the application of any other known form in diameter, which is immersed to a depth of about 10 cm. of photographic stimulus. Thus an exposure to X-rays in liquid air. The tube is closed above by a paraffin will obliterate it, or a very brief exposure to light. The stopper which carries a thin quartz window as well as latent image arising from X-rays is next in order of inthe sulphur tubes through which the connections pass. creasing stability. Light action will remove this. Third The air within is very carefully dried by phosphorus in order is a very brief light-shock or sudden flash. This pentoxide before the experiment. The arc light was used cannot be reversed by any of the foregoing modes of stimuas source of illumination. It was found that a vigorous lation, but a long-continued undulatory stimulus, as from photo-electric effect continued in the case of the clean lamp-light, will reverse it. Last and most stable of all is aluminium. In the case of the silver bromide a distinct the gradually built-up configuration due to long-continued photo-electric effect was still observed. I have not had light exposure. This can only be reversed by overdoing leisure to make, as yet, any trustworthy estimate of the it according to the known facts of recurrent reversal. percentage effect at this temperature in the case of either Prof. Wood takes occasion to remark that these phenosubstance. Vor have I determined the temperature mena are in bad agreement with the strain theory of accurately. The latter may be taken as roughly about Mr. Bose. We have, in fact, but the one resource-the -150° C.

allotropic modification of the haloid-whereby to explain Sir James Dewar's actual measurements afforded twenty all these orders of stability. It appears to me that the per cent, of the normal photographic effect at - 180° C. and elasticity of the electronic theory is greater. The state ten per cent, at the temperature of - 252°.5 C.

of the ionised system may be very various according as it With this much to go upon, and the important additional | arises from continued rhythmic effects or from unorganised fact that the electronic discharge-as from the X-ray tube shocks. The ionisation due to X-rays or to friction will or from radium--generates the latent image, I think we probably be quite unorganised, that due to light more or are fully entitled to suggest as a legitimate lead to experi- less stable according to the gradual and gentle nature of ment the hypothesis that the beginnings of photographic the forces at work. I think we are entitled to conclude action involve an electronic discharge from the light that on the whole there is nothing in Prof. Wood's beausensitive molecule; in other words that the latent image is tiful experiments opposed to the photo-electric origin of built up of ionised atoms or molecules the result of the photographic effects, but that they rather fall in with photo-electric effect on the illuminated silver haloid, upon what might be anticipated. which ionised atoms the chemical effects of the developer When we look for further support to the views I have are subsequently directed. It may be that the liberated laid before you we are confronted with many difficulties. electrons ionise molecules not directly affected, or it may be I have not as vet detected any electronic discharge from that in their liberation they disrupt complex molecules built | the film under light stimulus. This may be due to my up in the ripening of the emulsion. With the amount we defective experiments, or to a fact noted by Elster and have to go upon we cannot venture to particularise. It! Geitel concerning the photo-electric properties of gelatin.

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