ing on wood are concisely described; but the principal part of the work, and by far the most important, deals with the colour sensitising of emulsions, and the applications of such sensitised emulsions to the production of negatives in the many methods of dealing with and reproducing colour that are now in vogue.

The applications of the newer sensitisers are described in many scattered communications, and often with very little discrimination between the practically useful and the merely theoretically interesting. Mr. Klein states that he has included only those that have passed the test of time and been found to be thoroughly practical. It is in this that the value of the work lies, and we think that it would have been better to have restricted the volume to this aspect of the subject. The occasional references to the underlying scientific facts will not help the practical man, nor would they if they were free from the errors that now disfigure them. A volume of practical instructions is not the place for a page or two of chemical equations or the expression of theoretical views that have often been called in question. However, these occupy but little space, and scarcely interfere with the use of the book as a strictly practical manual.

Der Gegensatz zwischen geographischer und nichtgeographischer Variation. By Karl Jordan. Pp. 59; with 73 figures in the Text. (Leipzig: W. Engelmann, 1905.)

THE present treatise affords an excellent example of the light that may be thrown on questions of biological interest by the scientific use of entomological data. Dr. Jordan here presents a valuable résumé of some of the most important results of the elaborate investigation of the chitinous sex-organs of insects, more particularly the Papilios and Sphingidæ, carried on by him for many years past at the zoological museum at Tring. These researches, the detailed results of which have already appeared in the pages of "Novitates Zoologicæ, are of high interest, not only to entomologists, but also to all students of the methods of evolution.

or

It must, however, be confessed that the author's interpretations are less acceptable than his facts. Starting from the position that "species" have a real objective existence, he endeavours to show that new species could only have arisen from geographically isolated variations, not from variations occurring side by side with the parent form. The main fact on which he relies is that while "individual " "seasonal" variation of forms inhabiting the same locality is never accompanied by a variation in the sex-organs (with the single known exception of Papilio xuthus), the diverse geographical forms of a species are in very many cases found to be distinct from one another in sex-organs as well as in aspect. There is thus a correlation in the latter case which does not exist in the former, and which seems to the author to warrant the conclusion that these geographical forms only can occupy the position of incipient species. Some of the obvious objections to this view are dealt with by Dr. Jordan, others are left unnoticed.

A slight inaccuracy occurs on p. 177, where a figure of Byblia goetzius is said to represent B. ilithyia, while the true B. ilithyia bears the legend B. anvatara; both mistakes being repeated in the text. A more serious matter is the absence of any detailed reference to Mr. G. A. K. Marshall's work on this genus and his remarkable discoveries in the genus Precis. Some special recognition of these should have found a place, even in a treatise of general nature like the present. It will be gathered from what has been said that Dr. Jordan's conclusions are open to criticism. There can, however, be no

doubt as to the value of the researches so ably carried on by himself and others in connection with the ample material of Mr. Rothschild's museum at Tring. F. A. D.

Butter-making on the Farm and at the Creamery. By C. W. Walker-Tisdale and T. R. Robinson. Sixth edition, revised and enlarged. Pp. 162. (London: Office of the Dairy World, 1906.) Price 2s. 6d. net.

WE import into the United Kingdom perhaps twice as much butter as we make, and pay twenty millions yearly for it. Some, at least, of these millions would have been saved to the agricultural industry if our farmers and dairymen had given as much intelligent study to the principles of butter-making as, for instance, the Danes have done. Unfortunately, however, in such matters as the use of centrifugal creamseparators, the employment of pure bacterial cultures for " starters,' " and the general organisation of the industry, we did not lead the way; we were content to follow, and that, too, with somewhat halting footsteps. Even now the small butter-maker is often a sad empiricist. If cleanliness, for example, is an article of faith with him-and frequently it is nothe holds it as a dogma, not as reasoned knowledge. The little book under notice may help in the recovery of some of those lost millions. It gives an outline of approved present-day practice in buttermaking, though it does not purport to offer much in the way of theoretical explanation and discussion. Mainly it is an account of how best to conduct the operations of a small modern dairy. It is practical and simple; well suited for the elementary dairystudent, for the farmer's son who wishes to know something more than mere rule-of-thumb work, and for the private maker who supplies his own household from his own cows. The first few pages deal with the design, construction, and equipment of the dairy. Then cream is considered, and its separation ripening" are described, after which we pass to the churning and subsequent operations. A number of simple arithmetical examples are worked out to illustrate various points that arise. The last thirty pages deal, briefly and in a more technical manner. with the operations of a fully-equipped creamery, including pasteurisation" and refrigerating.

and "

The book does not profess to be much more than a useful note-book and practical guide, but as far as it goes it is excellent. C. SIMMONDS.

The Deinhardt-Schlomann Series of Technical Dictionaries in Six Languages: English, German, French, Italian, Spanish, Russian. By Kurt Deinhardt and Alfred Schlomann. Vol. i. The MachineElements and Tools for Working in Metal and Wood. Together with an Appendix, edited by P. Stülpnagel. Pp. 403; 823 illustrations. (London: Archibald Constable and Co., Ltd., 1906.) Price 5s. net.

THIS volume is the first of a series intended to aid engineers and others in reading technical works in any of the principal modern languages. Terms of general importance only are included; they are classified into subjects and many are accompanied by an explanatory sketch. Formulæ and symbols, serving as they do the purpose of an international language, are introduced wherever possible. The translations have been tested in workshops and offices in the various countries represented; so the work ought to prove of service in reading technical literature. The convenient pocket size of the dictionary, the systematic arrangement of its matter, and the full alphabetical index of words in each of the six languages should gain for it a sphere of usefulness among technical students.

LETTERS TO THE EDITOR

(The Editor does not hold himself responsible for opinions expressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts intended for this or any other part of NATURE. No notice is taken of anonymous communications.]

Osmotic Pressure.

THE publication of a paper by Mr. Spens in vol. lxxvii. Proc. Roy. Soc., p. 234, in which he criticises a relation between the vapour and osmotic pressures of a solution which Mr. Hartley and I had deduced (see same volume), seems to be an opportune moment for directing the attention of physical chemists to the necessity for an agreement as to what is meant by the term osmotic pressure.

Mr. Spens, following Duhem, points out that the osmotic pressure, defined as the difference between the pressure on the solvent and the pressure applied to a solution to seep it in equilibrium with the solvent, when the two are separated by a semi-permeable membrane, varies according to the nressure on the solvent. He suggests using a dennite pressure on the solvent, say its vapour pressure, as the standard.

I would point out that, by accepting this definition, one is necessarily bound to compare two solutions when they are under different conditions-not only on account of the different vapour pressures of different solvents, but also on account of the different pressures on the solutions themselves.

The following consideration will, I think, make this lear, and at the same time will suggest a more scientific

standard.

There seem to be two methods of examining directly the osmotic phenomena of a solution.

One, which I may call the osmotic "force" method, depends essentially on the determination of the rate at which the solvent will flow through a semi-permeable membrane into an infinite mass of solution when there is n pressure on the latter.

It is evident that if one knew the frictional resistance to the flow, the heat developed, &c., one could calculate the smotic "force " in absolute units.

I would mention, in parenthesis, that Mr. Hartley and I have made some comparative experiments in this direction with results which were not entirely unsatisfactory.

(2) All other direct methods give what may be called quilibrium pressures; they depend on the measurement of the pressure necessary to bring about a balance between the solution and the solvent. These equilibrium pressures Anot, on account of the compression of the solution, be measured under the same conditions.

An example will. show this plainly. The equilibrium pressure between a solution of 540 grams of cane-sugar in the litre of solution and the solvent (water) under atmospheric pressure is, in round numbers, 70 atmospheres. The equilibrium pressure for 750 grams in the litre is 134 atmospheres. In the actual measurements each solution had been compressed, in one case by 71 atmospheres and in the other by 135 atmospheres. The conditions were therefore not comparable.

If we could measure the osmotic "force" of these two elutions as in (1) then comparable results would be obtained, for in both cases the solution and the solvent would be under the same pressure (gravitational). Up to the present, so far as I am aware, no serious tempts to measure the osmotic force " have been sade, but I would suggest that, pending these, the relation between the vapour and osmotic pressures of a lucion as deduced by Mr. Hartley and myself may be wful for the purpose of comparing the osmotic pressures

different solutions.

46

This relation gives the osmotic pressure of a solution when it is under no pressure but its own vapour pressure. 1 knowledge of the vapour pressure, together with the density of the solvent, is all that is required for calculating

that pressure; while to apply the standard that Mr. Spens proposes, it is necessary to determine the increment in volume of the solution when unit mass of solvent enters it, and in some cases it may be necessary to obtain the coefficient of compression of the solution.

The experimental work saved by the adoption of the standard here proposed is apparent when it is remembered that, owing to the want of suitable semi-permeable membranes, the measurement of equilibrium pressures is confined to but a few substances dissolved in water. Foxcombe, near Oxford.

The Eruption of Vesuvius,

BERKELEY.

YESTERDAY I ascended the cone of Vesuvius up to the crater, being, I suppose, one of the first climbers after the eruption. The ascent was made from Torre Annunziata without any difficulties, but care had to be taken to avoid the courses of the avalanches of stones and ashes rushing from the cone and spreading over the slopes more than half a mile from the foot of the cone.

I estimated the new crater to have a diameter of about 3000 feet; the bottom was not visible, but the walls could be seen to a depth of about 1000 feet. The inner walls are nearly perpendicular, partly overhanging, and I saw pieces of the very narrow crater edge breaking down, in this way still enlarging the crater. The very regular stratified construction of the crater walls was visible. The height of the crater edge is very different from what it was before the eruption, being greatest on the west side, and diminishing in irregular steps to the north and east. At the point to which I ascended the aneroid showed an elevation of 3760 feet. From this point, which was on the southern side, the Somma was clearly visible over the lower northern edge of the crater. This shape of the crater may account for the fact that the showers of lapilli and other fragmentary products which destroyed the villages of Ottajano and San Giuseppe were given a direction to the north and east over the Somma.

The crater now closely corresponds to the descriptions of the great crater formed in 1822, and described by Forbes and Scrope. From the throat of the crater I heard a constant roaring, and saw that white clouds of vapour filled the huge hollow, but I did not see any ejections of stones or dust.

On descending I visited the points where the lava streams started from the foot of the cone. The first lava reached the surface on the morning of April 4 a little west of the Casa Firenze, but it soon stopped. Another stream started from Casa Firenze, destroying the buildings, and flowed half the way toward Bosco-Trecase. The lava which damaged a part of Bosco-Trecase started on April 6 a little lower on the slope, and divided into two parallel branches. The quantity of lava during this eruption was on the whole comparatively small. No lava came from the crater. The general characteristics of the eruption are the immense amount of volcanic ash, lapilli, and other fragmentary material ejected, and this makes the eruption of April, 1906, very similar to that of the year 79 A.D.

Visiting the destroyed village Ottajano on April 19, I made the following curious observation. A great number of the window glasses are broken, but among the others there are many regularly penetrated or pierced by circular holes one or two inches in size. These holes are as common on the northern and eastern sides of the houses as on the other sides, and they can therefore not have been caused by the showers of lapilli, which only came from the south-west. Some people ascribed these holes to the very heavy lightning which accompanied the fall of the lapilli, but I am not aware that electrical discharges may produce such effects.

It may be of interest to note that when visiting the volcanic vents of the Phlegræan Plain to investigate if any kind of volcanic activity was shown in connection with the eruption of Vesuvius I heard that the emanation of steam from the Solfatara diminished greatly during the days of the strongest eruption of Vesuvius: normal conditions set in later. HJ. SJÖGREN.

Naples, April 23.

On the evening of March 26, at 6.30 p.m., before dusk had set in, there was a large thunder cumulo-nimbus cloud about eight miles north of Johannesburg. The summit of this cloud was very sharp against a clear dark blue sky. There was no false cirrus. Six flashes of lightning darted from near the summit of the cloud into the clear sky. The longest path was about ten degrees. One flash returned to the cloud, the others finished in the clear sky. Before dusk set in this phenomenon ceased to occur. All the flashes were directed to that part of the sky from which the cloud moved.

In a well-known book on meteorology we read "it is impossible to say whether a flash of lightning moves from a cloud to the earth or in an opposite direction," and further that the lightning is instantaneous. Hann does not confirm these statements, and it is time that they were modified in English text-books. Quite frequently I have observed lightning flashes leaving a cloud for the earth, but fading away before reaching it; the opposite pheno

menon has not been observed. The paths of lightning shown by photographs taken in the Transvaal all indicate discharges from cloud to cloud, and from cloud to earth. The enclosed photograph, taken by Mr. T. N. Leslie at Vereeniging, is typical. Some flashes of lightning are instantaneous, the majority are not, but I do not think any exceeds a duration of a third of a second. The revolving wheel has been used, and shows that the duration is often certainly much longer than 1/40th of a second. Johannesburg, April 2. R. T. A. I.

Diurnal Variation of Ionisation in Closed Vessels. UNTIL Messrs. Campbell and Wood give us some more definite information as to the magnitude of the daily variation which they have found in the natural ionisation of air in closed vessels (NATURE, April 19, vol. lxxiii., p. 583), it is somewhat premature to go into a detailed discussion as to how this discovery will affect theories of atmospheric electricity. Still, the letter in NATURE of April 26 (vol. lxxiii., p. 607) on this question from Dr. O. W. Richardson calls for some remarks.

The facts are shortly:-(1) Messrs. Campbell and Wood discover that the natural ionisation of air in a closed vessel has a double daily period, the maxima being between 8 a.m. and 10 a.m. and between 10 p.m. and I a.m., the corresponding minima being at 2 p.m. and 4 a.m.; (2) the potential gradient in the lower atmosphere has, at most places, also a double period, the maxima being at about 8 a.m. and 8 p.m., and the minima at about 4 a.m. and midday. Thus, allowing for a certain amount of uncertainty in the exact determination of the times of the maxima and minima, we may say that the daily variations

of the natural ionisation and the potential gradient are similar.

In order to discuss a possible dependency of these two factors, Dr. Richardson assumes that "the distribution of the earth's field reduces itself to a case very similar to that between two plane electrodes immersed in a gas and maintained at a constant difference of potential." It is more than questionable as to whether this assumption is justifiable or not, for in atmospheric electricity we are dealing with constant quantities of electricity, and not with constant potentials. But, rather than follow up this objection, I would prefer to look at the problem from a different point of view, and show that the exact contrary conclusions can be deduced.

In discussing this problem, it is usual to accept that there is a negative charge on the earth's surface, and that the corresponding positive charge is a volume charge distributed in the atmosphere. Now all the measurements which we have of the daily variation of potential gradient have been made within a few metres of the surface. Within these few metres there can be, relative to the charge on the earth, very little volume charge, so what our measurements actually refer to is the charge on the surface, the relation being dv/dh=-470. The point to notice in this is that, with a given charge on the surface and the corresponding charge in the atmosphere above, the vertical distribution of the charge and the conducting state of the upper atmosphere do not in the slightest affect the potential gradient within a few metres of the surface. If the potential gradient changes there it can only be by a change in the surface charge on the earth.

If there is a penetrating radiation which, besides ionising the air in closed vessels, also ionises the air in the atmosphere, we should expect from Messrs. Campbell and Wood's experiments the ionisation of the air in all parts of the atmosphere to have a daily variation. Thus the air quite near the surface would twice a day be exceptionally conducting; one would expect that at these times there would be a greater loss of the surface charge, and so the remaining charge to be diminished, and with it the potential gradient. The consequence would be a daily variation of the potential gradient corresponding to the variation of ionisation, but the maxima of one corresponding to the minima of the other.

That such a relation does exist between the ionisation of the lower atmosphere and potential gradient has been shown by many observers situated in most parts of the globe. Thus from Messrs. Campbell and Wood's results one would expect minima of the potential gradient to occur at about 8 a.m. and 10 p.m.; this is the exact reverse of what really occurs.

Thus it would appear as if Messrs. Campbell and Wood have added one more to the many puzzling factors connected with atmospheric electricity. Manchester University.

GEORGE C. SIMPSON.

August Rainfall.

ACCORDING to Greenwich experience, August has been a very dry month considerably oftener about sun-spot maxima than about minima. This fact may be of some practical interest.

Using Mr. Nash's table (from 1815), let us confine our attention to the three years about the eight maxima and the three about the eight minima, i.e. twenty-four years in each division.

The driest August in the minima division was in '55. with 1.40 inches. But in the maxima division there are ten cases of lower values, ranging from 1.25 inches down to 0.45 inch, viz. '38, '49. '59. '61, '69, '71. '82, '83, '84. '93. Since 1837 no three-year group of this division has been without at least one such very dry August, two have had two, and one three.

The total August rainfall in those twenty-four-year groups is. in the sun-spot maxima division, 50.25 inches, in the minima division 66.50 inches, the higher value thus showing an excess of 16-25 inches (nearly one-third of the lower).

The sun-spot maximum we are now near (1905?) has not been here considered, but I may remark that in 1904 we had one of those low August values (1.24 inches).

ALEX. B. MACDOWALL.

AT THE HEAD OF LOCH FYNE.'

A LARGER number of contributors even

than those mentioned on the title-page have conspired to make this memoir authoritative and complete. It is descriptive of Sheet 37 of the 1-inch geological map of Scotland, an attractive work published in 1903, in which the north-east and southwest lines of the Caledonian earth-folding predominate, and are followed out in the trend of the intrusive masses. The memoir is illustrated by excellent plates, one of which is here reproduced; and the fact that part of the ground is familiar to the tourist gives it an additional interest.

The region described is cut, from corner to corner, by the noble inlet of Loch Fyne. The parallel reach

landscape. The fundamental rocks of the district are metamorphic, and formed a part of the Caledonian continent, on which the Old Red Sandstone gathered; and Mr. Hill points out how denudation is removing the Devonian lavas and lake-deposits in the northwest, and is revealing, in the sculpture of the old continent, a highland much like that of modern days. The ice-flows of the Glacial epoch, however, have moulded the present surface in many of its details, have left erratic blocks in quaint positions on the hills, and have deposited moraines and banks of gravel across the edges of the ancient schists.

The metamorphosed series is mainly of sedimentary origin, with many bands of limestone. The albiteschists (p. 15), which are "highly micaceous or chloritic rocks with grains or crystals of clear

of Loch Awe lies in the north-west, and Loch Eck, banked out by gravel terraces from the sea, comes in near Loch Long in the south-east. The traveller by land usually enters the region by the steep and rugged fastnesses of Glencroe, and leaves it by Glen Aray, if he is willing to face the rain-swept moorland above which Cruachan towers in the north. The geological surveyors, however, have become familiar with a side area practically untrodden by any visitor. Mr. Hill's appreciative introduction should be read with the aid of the hill-shaded Ordnance map, Sheet 37, one of the most beautiful products of a draughtsman who surely possessed a sentiment for "The Geology of Mid-Argyll." By J. B. Hill, with the collaboration of E N. Peach, C. T. Clough, and H. Kynaston, with petrographical totes by J.J. H. Teall and J. S. Flett. Pp. vi+166. Memoirs of the Geological Survey, Scotland. (Glasgow, for H.M. Stationery Office : J Hedderwick and Sons, Ltd., 1905.) Price 8s.

secondary albite," are of special interest. Dr. Teall supplies an analysis, showing 3.2 per cent. of soda and an equal amount of potash. This allows 28 per cent. of the rock to be formed of albite. "Green beds," which are hornblendic, and yet are not the intrusive epidiorites so familiar in Dalradian areas, occur in a band south-east of Loch Fyne, and may have been derived clastically from me preexisting basic igneous series (p. 18). Trulls of epidiorite occur, however, plentifully among the metamorphic rocks between Loch Awe and Loch Fyne. In the same region there are numerous later intrusions of quartz-porphyry and other igneous rocks, probably post-Silurian in age. "Kentallenite," described in detail by Mr. Hill in 1900 (Quart. Journ. Geol. Soc., vol. lvi., p. 531), and first known from the Appin promontory, occurs here and there, as a link between