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whose parents can afford them an, in some places fancy, educa.
A Palæozoic Ice-Age. tion that can in the nature of things be only attainable by the
The account by Dr. Wallace in NATURE (p. 55) of glacial rich?
deposits recently discovered in Australia is a most important and In view of the discussion upon the proposed Teaching Uni
welcome addition to our knowledge. But to us the surprising versity for London it is to be hoped that these things will not
circumstance is that Dr. Wallace appears quite unaware of the be overlooked amid the local questions and rival institutions.
fact that this is only an addition to a great series of discoveries, It is to be hoped on the one hand that those who will have the
by no means confined to Australia, affording evidence of a Palprivilege of learning in the greatest city in the world will not be deprived of the personal influence of its greatest men by
æozoic ice-age. That the deposits near Sandhurst are Palæozoic
may, in the absence of any indication to the contrary, be asrelegating these to some haven of laboratories where no bracing breath of students shall interfere with the inmates. On the
sumed, since they are clearly similar in position and character
to the well-known boulder beds of Bacchus Marsh, and these other hand it is to be hoped that London will so far honour
have been correlated with the strata containing ice-burne fragitself as not to be content until it sees its University a centre of
ments, amongst the marine beds west of Sydney and also at thought and investigation from which shall radiate new ideas!
Wollongong to the southward, and in Queensland to the northand discoveries to enlighten and benefit the whole nation.
ward. All these beds have been shown to be upper carboniferBefore I close there is a matter of great importance to
ous. A good account of the facts known up to 1886 may be which I fear sufficient importance is not attached by those
found in Mr. R. D. Oldham's paper on the Indian and Auswho are directing this matter and that is the great
tralian coal-bearing beds (Rec. Geo. Surv. Ind. xix. p. 39). objections there are to mixing up Universities and Colleges with
It is scarcely necessary to refer to the fact that extensive examining boards. We here in Trinity College, Dublin, suffer
| Palæozoic glacial deposits, of the same age as those of Australia, very much from the fact that a considerable number of our
have been found in several parts of India, some as far within students never reside here, but only come over for periodical
the tropic as lat. 18° N., others in the Salt Range of the Punexaminations. We only suffer in one way, while is London
jab, that the famous Dwyka conglomerates of South Africa are adopted this abominable arrangement it would suffer in two
similar and in all probability contemporaneous, and that boulder ways. We suffer because our degree is much less valued than
beds of very possibly the same geological date have been obit would be if all our students were compelled to reside. All
served in Brazil. We should not have mentioned these but for our students have not that education got by friction with their
the fact that the idea of a Palæozoic ice-age is apparently novel fellows and by contact with trained intellects which no exami
to Dr. Wallace. We do not think, however, that the reason nation can test, and which is such a valuable training, and in
why so well-informed a naturalist is unacquainted with geological consequence our degrees are the less valuable. London would
data long known to many is any mystery. It has become an suffer in this way, and it is a very serious way too. In addition
accepted article of faith amongst most European geologists to this London would suffer from the inordinate importance that
(there are, of course, exceptions) that no ice-age occurred before would be attached to extern examiners if the University ex.
the last glacial epoch, just as it is part of the geological creed amined London and extern students. So far we have escaped this
that the carboniserous fora was of world-wide extension, and as danger, but it is inevitable in London because the extern element
it has become the prevailing belief that the deep oceans have there would be large, influential and organized, while with us it
been the same since the consolidation of the earth's crust. Now is of little strength. The result would be to perpetuate and
the discoverers of glacial evidence in the carboniserous beds of intensify that horrible teaching for examinations which is so
India and Australia also assert that the carboniferous flora of necessary an evil in the case of the majority of students, but
those countries differed in toto from that of Europe and refrom which the leaders of thought should be exempt. It matters
sembled the jurassic flora of European regions, and some of not that the syllabus nor even that the very questions are ap
them add that the great southern flora of South Africa, India, proved by the professor, if the examination is conducted to any
and Australia must have inhabited a vast continent, part serious extent by an independent mind. The student will seek
of the area of which is now beneath the depths of the Indian a coach, who will probably teach him very well indeed, but
Ocean. Partly from Indian and Australian geologists being rewhose whole view of learning will be of ihe passing-an-ex
garded as heretics geologically, partly from other causes, the amination type, and who will infect his pupil with this miserable
evidence of ice action in India and Australia has been generally disease. Gradually the professor himself will be involved in the vortex, and the whole University will gradually look upon
ignored. No better proof could be afforded of the fact that the passing of examinations as the end of life for students, and
European geologists in general have omitted to notice the
series of discoveries in the southern hemisphere and in India his is the acme of coaching and the bathos of education.
than the publication of Dr. Wallace's paper. GEO. FRAS. FITZGERALD.
The glacial evidence as it now stands is extremely interesting Trinity College, Dublin, November 25.
and perhaps transcends in importance that of the Pleistocene glacial epoch. For as the effects of the carboniferous ice-age
were felt within the present tropics, either the earth's axis of The Stars and the Nile.
rotation must have shifted considerably, or else the refrigera
tion of the surface must have been due to a cause distinct AFTER reading Mr. J. Norman Lockyer's papers on the onnection of the orientation of Egyptian temples with the
from that supplied by the late Mr. Croll's theory, even when eliacal rising of certain stars, I was interested to find that a
supplemented by Sir R. Ball's amendment. istom still exists in the neighbourhood of the Second
Our own interest in the whole subject is chiefly due to the
circumstance that we happened in 1856 to be the first who met ataract having a strong resemblance to the old Egyptian
with the ancient boulder-bed in India, and suggested that it stom.
he i might be explained by the action of ice. The discoveries in The Nuba people of this part foretell the first rise of the ile by the heliacal rising of the Pleiades, or as they call it, i
Australia and South Africa were of course quite independent Turaya.” For Sirius they have no special name, calling it 1 "
of those in India, but were, we believe, slightly later in date.
W. T. BLANFORD. rely
November 20. “the driver” or “ follower " of the three stars
HENRY F. BLANFORD. rion). It must be remembered that the first sign of the rise at
Geology of Scotland. ady Halsa occurs at the beginning of June, reaching MAY I supplement Prof. Green's history of geological mapsouan about a week later, but for some days the increase is
ping in Scotland (NATURE, vol. xlvii. p. 49) by pointing out y slow, and scarcely perceptible except in the readings of ihať Mr. Cruchley published, on March 23, 1840, * A GeologiVile gauges.
cal Map of Scotland by Dr. MacCulloch, F.R.S., &c., published lhese Nuba people still preserve in their language many
by order of the Lords of the Treasury by S. Arrowsmith, Hydroient Egyptian words, and possibly we may have here a trace
grapher to the King.” This fine map is on the scale of four he old custom, the Pleiades being taken instead of Sirius
miles to an inch. From the omission of “the late” before account of the earlier date of the rise in the district of the
MacCulloch's name, it seems possible that the plates were in ond Cataract than in Egypt itself.
course of engraving before his death in 1835. H. G. Lyons, Capt. R.E.
GRENVILLE A. J. COLE. airo, November 14.
Royal College of Science for Ireland, Dublin.
ending in a spatula like a finger-nail. It is fixed on the thumb I ENTIRELY concur with Dr. Hurst's view that the supposed
of the right hand. The opera ion is only made on fruits which shall
be picked up the following day. The day after the operation the new species, described by the Rev. Hilderic Friend as L. rubescens is in reality Savigny's L. festivus. I may add a further
fig is quite ripe. The male flowers in those fig. are all aborted, reason for discarding the term L. terrestris, Lin., and substituting
and the females have never perfect seeds. The figs of the third L. herculeus, Sav., for our common large worm. Savigny himself
generation are larger, of an agreeable taste, and sweet - cented : used “Enterion terrestre" to indicate a worm differing consid.
but they are not operated upon, only because in August and erably from L, terrestris, Lin., in the position and extent of the
September, though the trees are much fuller of fruit than in clitellum ; moreover it belongs to the genus Allolobophora and not
May and June, the people have so much to do at that time. to Lumbricus at all.
They are seldom sold, and only eaten by the owners of the With regard to the second “new” species, A. cambrica,
trees, or else they are abandoned to the field-mice, birds, and recently described by Mr. Friend, I believe that it is merely a
dogs, which latter are very fond of them. These nilg fruits are variety of A. chlorotica, Sav.
full of sycophaga." According to the description it appears to differ from the latter
It will be seen that the instrument he has sent me is of a species in three points :-(1) colour; (2) extent of clitellum ;
different shape to the one he describes ; and the chief interest (3) number of spermathecæ.
lies in the fact that Pliny also describes the process as closely (1) Now, amongst my collection of British worms I find one,
corresponding with this modern method. He even uses a similar of which a water-colour sketch taken from the living specimen
term "nail" (óvuxas): TÉTTELY Oů dúvatai av un TIKVIC OF & closely resembles Mr. Friend's description of the colour of A.
έχοντες όνυχας σιδηρούς επικνίζουσιν· & δ' αν επικνισθη, τετάρτες cambrica. My notes as to size, habits, &c., agree with his
TÉTTETAL (Nat. Hist. xiii. 14). Further, the Prophet Amos description. I have carefully re-examined my specimen, and
describes himself as bölās sigmim; and the authors of the LXX, find that it agrees perfectly with A. chlorotica ; or, in other words,
writing in Alexandria, appear to have understood the expression I find that A. chlorotica may vary-as Hoffmeister knew that it
and translated these word by kví(wv gukáuiva. This is the same did vary—so much as to resemble A. mucosa, and I may suggest
verb as that which Pliny uses ; so that it would seem to be pretty that it is a mimetic resemblance,
certain that Amos performed identically the same operation on the (2) Further, with regard to the clitellum of A. chlorotica : in
figs as is still done in Egypt at this day. It will be noticed that the table given by the Rev. Hilderic Friend, it is stated to cover
the idea was to ripen the figs. It does not really do this, somites 29-36. As a matter of fact the next somite, 37, is because there are no seeds; but it does make the fiz nearly always included. This brings A. cambrica, Friend, into
sweeter. It also liberates the insects, and without doing this harmony with A. chlorotica, Sav.
the figs would be uneatable. Jerome is the only author, as far (3) Thus the only differential character left is the number of
as I know, who alludes to“ grubs " being inside the fig. spermathecæ ; and I cannot agree to the validity of a new
GEORGE HENSLOW. species on this single character ; several specimens should be examined to settle the point, as variation in this feature is known
Iridescent Colours. to occur.
I take a certain amount of credit to myself for the useful The article “Iridescent Colours" on p. 92 puts me ia faunistic studies on the earthworms of Great Britain, now being mind of a notice which I published thirty years ag, while I pursued by the Rev. Hilderic Friend, for, if I mistake not, I lived in the United States. It was entitled “Harmonies of put him in the way of recognizing their specific characters, Form and Colour" (Stettiner Entom. Zeitung, 1862, PE when, some years ago, I named lor him, with remarks thereon, 412-414), and a portion of it refers to the subject of the sundry consignments of some scores of worms which he sent to above-mentioned article in Nature, and may be of interest 13. me for that purpose.
WM. BLAXLAND BENHAM. its readers :The Dept. of Comparative Anatomy, Oxford,
"A fundamental observation, which proves the influence or November 21.
the intensity of light upon colour, may be made on some insects of metallic coloration, inhabiting a large area from north 12
south. About six years ago, while in Southern Russia, I took Egyptian Figs.
a walk during sunset, and was struck by the brilliancy of some The accompanying sketch represents an instrument used in metallic red Chrysomela, abundant in that locality. I found Egypt for removing the “eye” or top of the sycomore fig. It that it was the common C. fastuosa, which I did not recognize is a piece of hoop iron, blunt on one edge and tolerably sharp at once, because in the environs of St. Petersbury, where on the other, and fixed into the end of a stick. The fruit of I lived at that time, it occurs in its metallic-green variety, wib Ficus sycomorus, or “ Egyptian fig," seems to be invariably an iridescent blue stripe on each of the elytra. Srill fartber infested with the insect Sycophaga crassipes, Westw. ; which I north it assumes a more violet metallic colour. The same is am informed by Rev. T. F. Marshall, who has kindly given me the case with Chrysomela cerealis and C. graminis. The firs the name, is the same insect supposed to effect caprification in of these species is represented in St. Petersburgh in the blee Malta, judging from specimens which I sent him. This fig never variety (C. ornata, Ahrens), while the typical variety, occurring produces ripe seed in Egypt, though it has been introduced farther south, has purpli-h-red metallic stripes. It is evideo from the earliest times. Not only are the ancient coffins made therefore that the metallic colouring of these wide-spread of the wood, but it was adopted as the sacred “Tree of Life. species is gradually intensified from north to south, in the order
of the colours of the spectrum. We may imagine the aren which these beetles occupy, like an immense rainbow, reflected from their backs, violet in the north, red in the south; he violet perhaps connected in some way with the magnetic phen mena prevailing in the polar regions. The longicorn beetle (Callidium violaceum) undergoes the same variati in : violetin the north, blue in central Europe." C. R. OSTEN SACKEN
Heidelberg, Germany, November 27, It probably came from Yemen, where Dr. Schweinfurth saw many seedling trees growing spontaneously. The tree bears three crops per annum, in May, June, and August-Septem
The Afterglow. ber. Boys cut off the top of the figs of the first two crops only. Dr. E. Sickenberger, one of the professors in the School of
There has been for three weeks past a very remarkalıle Medicine, Cairo, informs me that the figs have no pleasant
newal of the afterglow. There is a quite deep secondary flavour until the operation has been performed :-“They then
glow after the stars are fully out. I should say that no sa become very sweet, but remain smaller than when not cut open.
| afterglow has been seen since 1886, or three years after The object is to let the insects escape. Those that are left
Krakatão eruption. There is also a great extension of the wc become walery and tasteless, and are full of namoos or syco: hazy atmospheric corona around the sun, very marked phaga." In his first description Dr. Sickenberger described | around the moon. I am unable, however, to make out 2014 the instrument as “a kind of thimble made of iron plate the pink colour on the outer edge of the haze, which was so e
acteristic of “Bishop's Ring,” and distinguishable at Honolulu ! The substance which has been found to be most satisfor two years. Apparently there has recently been a great re factory as a membrane-former is copper ferrocyanide. inforcement added to the material in the upper atmosphere, which When aqueous solutions of potassium terrocyanide and produces the afterglows. Is this owing to the August eruption in Alaska, which is said
copper sulphate are carefully brought into contact a to have distributed ashes at a distance of 250 miles ?
pellicle of copper ferrocyanide is formed where the two Prof. C. . Lyons, in charge of tidal observations in Honolulu,
solutions meet. In this condition the pellicle is much reports the period of highest mean tide to have extended itself
too fragile to sustain even slight differences of pressure ; this year into November, or fourteen months later than the last but by the following simple device, employed first of all similar period. The mean sea level is now over ten inches higher | by W. Pfeffer, satisfactory results have been obtained. than it was last April. It is also somewhat higher than has been If a cell siinilar to the ordinary porous pot of a voltaic shown hy any previous lide registers in Honolulu. Mr. Lyons battery be lowered into a solution of copper sulphate regards this as of special importance, taken in connection with while at the same time a solution of potassium ferrocyathe oscillation of the earth's axis, now established by the com. nide be poured into its interior, the two solutions meet bined observations at Berlin and Honolulu.
somewhere within the walls of the cell and deposit a film Honolulu, November 8.
SERENO E. BISHOP.
of copper ferrocyanide. Little diaphragms of membrane are thus produced stretching across the pores of the
cell-wall, which furnishes the necessary support, and by OSMOTIC PRESSURE.
taking suitable precautions a membrane may thus be
obtained capable of withstanding a pressure of several of the various properties which have found a common atmospheres.
explanation in the new theory of solutions, there. The behaviour of a solution when separated from pure are none perhaps to which more interest attaches than to solvent by such a semi-permeable membrane differs osmotic pressure ; and although, on account of the experi markedly from what takes place when an animal memmental difficulties, the observations as yet accumulated brane is employed. In the latter case, at the outset on this subject are but scanty, they have so largely con water adds itself to the solution ; the level of liquid and tributed to the novel ideas involved in the new theory, the pressure on the solution-side of the membrane thus that they merit special attention.
rise until a maximum pressure-head is attained, which, Since accounts of osmotic pressure are finding their roughly speaking, is greater the stronger the solution way into few English text-books, it may be worth while used. Seeing, however, that dissolved substance is conglancing at the main features which have led up to the tinually escaping from the solution through the mempresent state of the question.
brane, as soon as the maximum is reached the pressureIt has long been known that if an aqueous solution - | head begins to fall until eventually it vanishes, the say, of sugar-be separated from pure water by a piece levels of liquid on either side of the membrane being the of animal membrane, that movements of the water and of same. the sugar take place through the membrane. If the If, on the other hand, a semi-permeable membrane be solution be contained in an open vessel, the base of which employed, as before, a maximum pressure is attained ; is composed of membrane, on partially immersing the but since dissolved substance cannot leave the solution, vessel in water it is easy to see that more water enters this maximum pressure as well as the concentration of the the vessel than solution leaves it. The level of liquid solution remain constant. within rises above that without the vessel, different pres When this constant state of things is established the sures being thus set up on opposite sides of the mem excess of pressure on the solution-side of the membrane brane.
over that on the solvent-side, whatever it may mean, is To this process wherein currents pass through a mem termed the “ osmotic pressure" of the solution. It is branous septum, the terms “osmosis," “osmose," and therefore customary to reserve the term osmose to pheno“ diosmose" have been applied. The last of these is per mena relating to semi-permeable membranes ; diosmose haps to be preferred, as it serves to indicate that two being used in cases where, as with animal membranes, currents are involved in the phenomena. Investigations dissolved substance as well as solvent can traverse the carried out as indicated above were concerned with the membrane. It is obvious that when the pressure is measurement of what was termed the “endosmotic established as indicated above, the original concentration equivalent.” That is the ratio of the amount of water of the solution has been altered by the entrance of solpassing into the solution to the amount of dissolved sub vent, and the observed osmotic pressure refers of course stance passing in the opposite direction. Consistent to the solution having the final concentration. If, howmeasurements of this quantity could not be obtained, ever, we imagine the vessel containing the solution to be however, for it was found that the nature of the mem- closed at the top, a quantity of air being imprisoned over brane exercised a marked influence upon its magnitude. the solution, pressure may be set up by compressing this The kind of membrane employed, or, with the same air, only a small quantity of solvent being allowed to membrane, its thickness or freshness, or even the direc-enter. If, further, the air enclosure be tapped by a mation in which water passed through it, was of importance. nometer, measurements of the pressure may be taken, Thus in illustration of the last point, water passes more and by making the air enclosure and the volume of the eadily outwards through eels-skin, more readily inwards manometer small enough the quantity of solvent enterhrough frog's-skin.
ing while pressure is being established may be negTo obtain quantitative relations in this field it thus lected, the original concentration of the solution remaining vecame essential to eliminate the influence of the mem practically unaltered. This is the principle of the method rane, and more recently this end seems to have been employed in measuring osmotic pressure in absolute ttained by the use of membranes artificially prepared. I units.
These artificial membranes differ from those of animal The question now arises, “ Are these measurements rigin in the remarkable particular that although they really independent of the nature of the membrane ? Has llow water to pass through, they present a barrier to the difficulty which beset the older experiments been le passage of certain dissolved substances. On this overcome?" To this question an immediate answer is ccount they have been termed semi-permeable mem- | for thcoming, for, as pointed out by Prof. Ostwald, it manes, and by their use measurements of osmotic | follows from theoretical considerations that if the memressure have been made possible.
brane employed is really semi-permeable, the observed To carry out such measurements the first point to be osmotic pressure of a given solution must be the same, Jved was to obtain a membrane of sufficient strength. I no matter of what material the membrane is composed. For suppose we have a quantity of solution sure are the same. Moreover, on thermodynamical enclosed in a tube, one end of the tube being closed by a grounds it was established that on the above hypothesis membrane A, the other by a membrane B, and suppose as to the nature of osmotic pressure its magnitude should it possible that a pressure P can be developed on the be quantitatively connected with measurements of other membrane A when it separates the solution from pure physical properties of solutions, more especially those on water, which is higher than the pressure p similarly de- the lowering of the vapour-pressure, and of the freezing veloped when B separates the solution from pure water. point of a solvent produced by the presence of dissolved On immersing the tube in water, the latter will begin to material. In this way a mass of evidence was collected, pass through both membranes into the solution. When a general survey of which led to the foundation of the the pressure p is attained passage through B will stop, new theory of solutions. On this theory the dissolved but that through A will continue ; but as soon as the substance, if the solution be dilute, is supposed to behave pressure on the solution rises above p, water will be forced as if it were gaseous, the pressure it exerts—the osmotic out through B. The pressure P will thus never be pressure-being equal to the pressure which it would exert attained, water will continuously enter through A, and if it were gasified, and occupying, at the same temperapass out at B. We will thus have a machine capable ture, a volume equal to the volume of the solution. of doing an infinite amount of work, which is impossible. Unfortunately measurements of osmotic pressure have Similar reasoning shows that p cannot be greater than P; only been made on few substances, and only for solutions it follows therefore that the pressure developed on each in water, but on turning to all the available observations membrane is the same, that the osmotic pressure must be to see how they support this novel conclusion, the most independent of the nature of a truly semi-permeable superficial examination serves to show that an agreement membrane.
does not exist. Unless in the case of sugar, for no sub Actual observations are on record in which the stance of known formula which has yet been investigated osmotic pressure did appear to vary with the membrane does the osmotic pressure agree with the corresponding employed. A sugar solution, for example, exhibited a | gaseous pressure. These substances consist of salt solu much lower osmotic pressure with a membrane of Prussian tions, and they invariably give higher osmotic pressure blue or calcium phosphate than with copper ferrocyanide, than theory demands. Similar disturbing influences From the preceding argument it is concluded, however, have been observed when other physical properties al that these membranes giving the lower values were not these solutions were measured, and to account for the quite firm or not quite impermeable to the dissolved | facts an additional hypothesis has been put forward by substance; the highest value is thus taken as the measure | Dr. Svante Arrhenius. of the osmotic pressure which is nearest the truth.
Salt solutions are electrolytes, they conduct the electric On glancing at the results which have been obtained, the current, and undergo simultaneous chemical decompos. first point which strikes one is the extraordinary magni tion into their constituent ions. Experiment shows that tude of the pressures thus set up. In the case of a i per such electrolytic solutions give high osmotic pressures cent. aqueous solution of nitre the pressure attains the more particles appear to bombard the semi-permeable value of 2} atmospheres. This value increases with the membrane than if the dissolved substance behaved as a strength of the solution till at 3'3 per cent. it is no less gas. The new hypothesis states that this is really the than 6 atmospheres, this pressure being the highest which case, the additional number of particles being produced any membrane yet prepared has been able to withstand from the breaking up of the dissolved substance. It With substances like sugar, other things being the same, states that in a solution which can be electrolyzed a po: the pressure is not so great, but in all cases, in order to tion at least of the dissolved substance exists already keep it within workable limits, the solutions employed decomposed or dissociated into its ions, and that although have to be dilute.
these ions cannot be separated by diffusion they are si Striking as the results are themselves, their explanation far independent that each can exercise an effect on the is not less remarkable. The original measurements of 1 semi-permeable membrane. osmotic pressure were made with the purpose of eluci- The extent of this electrolytic dissociation is sur dating the movement of liquids in plant cells, and posed to vary with the chemical nature of the dissolve: naturally the substances examined were such as occur substance, and to increase with the dilution. In ver in the vegetable organism-aqueous solutions of sugar, dilute solutions it may be complete, the whole of the dis gum, dextrin, and the nitrate, sulphate, and tartrate solved substance being supposed to exist in the stated of potassium. For some years after these observations ions. were made, they lay comparatively unnoticed, until Prof. The second hypothesis gives, therefore, some expla van't Hoff, of Amsterdam, turned them to a use undreamt | nation why the osmotic pressure of a salt solution of by their discoverer. From a study of the properties of greater than that of a non-electrolytic solution of suga: dilute solutions van't Hoff came to the conclusion that it further fixes the limits between which the osmotic pre the osmotic pressure was due to the bombardment of the sure ought to vary in the case of an electrolyte, for the molecules of the dissolved substance on the semi-ferme- lower limit should be that of undissociated gas, the higte able membrane. For when the osmotic pressure is should be that of completely dissociated gas, each established and equilibrium exists between solvent and original molecule having decomposed into as many sch solution, in the same time, equal amounts of solvent, molecules as there are ions in each molecule of salt must pass in either direction through the membrane So far as these limiting conditions go, the facts su and the impacts of the solvent molecules on the mem- | port the hypothesis. In all cases the observed osmo: brane will then be equal and opposed on either side, and pressure is either equal to one or other of the limits, en therefore negligible. On this reasoning the pressure lies between them. A closer scrutiny leads, nevertheles, recorded on the manometer is taken to be that exerted to apparent discrepancy. It is evident that a measured by the substance in solution.
the amount of dissociation can be obtained from osmeOn examining the magnitude of the pressure thus at- | pressure observations. For if we divide the observed tributed to the dissolved substance, in the case of a solu- osmotic pressure by the corresponding pressure of tion of sugar van't Hoff next showed that it bore the dissociated gas we have obviously, if the preceda closest resemblance to the pressure of a gas. Indeed, if hypotheses are valid, the ratio of the actual number ** we calculate the pressure of a gas which at the same bombarding molecules to the theoretical number had temperature contains as many molecules per unit volume dissociation occurred. The ratio of these two numbes as there are molecules of sugar per unit volume of solu- is denoted by the letter “i," a factor first used by vas? tion, then the pressure of the gas and the osmotic pres- | Hoff. Now, on the new theory, the value of ";" can
btained by measurements of other properties of salt been put forward in favour of the gaseous analogy. solutions, the electric conductivity, the depression of Several physicists, starting from entirely different points he freezing point, &c., and the theory is compared with of view, have arrived at the result that in a dilute solupractice by seeing if the values of “i," as determined, say, tion the dissolved substance should obey laws similar to rom freezing point observations, agree with those de those which hold for gases. At present the attitude of the luced from the osmotic pressure. The comparison shows prominent upholders of the new theory is one of indifhat in some cases, some half-a-dozen in all, the two sets ference as to the exact mechanism of osmotic pressure. of values correspond ; in others, and in by far the majority, The numerical agreement between the measurements on 10 such correspondences exist. In these latter instances solutions and those on gases is regarded as ample justit is argued, and with a certain amount of experimental fication for considering dissolved substances to be in a vidence, that the salts were not without action on the pseudo-gaseous condition. nembrane employed, and that, therefore, diosmose Whatever the ultimate explanation of the facts may be, eally took place, the membrane was not truly semi there can be no doubt that the existing speculations on vermeable. In this way the discordant observations the nature of osmotic pressure and allied phenomena lave been put out of court.
have infused new life into the study of solutions. Indeed, It is thus apparent that the leading hypotheses of the as instigators to fresh inquiry these hypotheses must jew theory do not receive confirmation of the weightiest take rank as the most fruitful of recent times. cind from observations on osmotic pressure. Indeed,
J. W..RODGER. were they supported by such measurements alone, they would hardly be entertained. Their mainstay, however, les in the mass of experimental work on many other
A SANITARIAN'S TRAVELS. properties - evidence which it is much easier to obtain M R. ROBERT BOYLE has travelled round the world than the difficult measurements on osmotic pressure
no fewer than four times for the purpose of studywhich has been correlated and explained by their use. ling sanitary science and preparing the way for the intro
It is only fair to add that both hypotheses, from physi- | duction of the ventilating and sanitary appliances he has al as well as chemical standpoints, have met with a invented. An interesting account of his fourth j.nrne is leasure of adverse criticism. The rôle played by the given in a little book entitled “A Sanitary Crusalie lembrane has also been questioned. It has been sug through the East and Australasia," consisting of a series ested that it is not really semi-permeable, allowing sol of papers reprinted from the Building News. In the ant only to pass, but just as a porous plug behaves course of this “crusade” Mr. Boyle visited Burmah, the wards a mixture of gases, it allows molecules with dif Malay native states, Sumatra, Siam, Borneo, Java, Ausrent momenta to traverse it at different rates. Or, tralia, New Zealand, Samoa, the Sandwich Islands, and ain, its action has been likened to that of a palla America. Of all the facts noted by him as a sanitarian um film towards hydrogen, compounds being formed the most remarkable are those relating to leprosy, a disith the membrane substance on one side, these becom ease which he believes to be spreading to an alarming g diffused and dissociated on the other. If either of extent all over the world. He was particularly struck by ese views be correct the pressures exerted by dissolved the gigantic proportions the evil has assumed in Burmah. bstances have probably never been measured.
The steps of the great Shwedagon pagoda at Rangoon, On the other hand, important theoretical support has the Mecca of the Indo-Chinese Buddhists, he found to be