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minium electrodes, the lower one cup-shaped, and charged , others, as Sirius is acknowledged to be a hotter star than with a salt of calcium.

our sun, is that if it exists at all in the spectrum, it is so In the figure I give exact copies of the results obtained. faint that it was recognised by Dr. Huggins in the first Ic will be seen that with the lowest temperature only the instance. single line (2) and with the highest temperature only the In Sirius, indeed, the Hline due to one molecular two more refrangible lines (6) are recorded on the plate. grouping of calcium is as thick as are the hydrogen lines as

This proved that the intensity of the vibrations was mapped by Secchi, while the K line, due to another mole. quite changed in the two experiments.

cular grouping, which is equally thick in the spectrum of Perhaps it may not be superfluous here to state the the sun, has not yet made its appearance. reasons which induced me to search for further evidence In the sun, where it is as thick as H, the hydrogen lines i. the stars.

have vastly thinned. It is abundantly clear that if the so-called elements, or, While this paper has been in preparation, Dr. Huggins more properly speaking, their finest atoms-those that has been good enough to communicate to me the results give us line spectra-are really compounds, the compounds of his most important observations, and I have also had must have been formed at a very high temperature. It is an opportunity of inspecting several of the photographs easy to imagine that there may be no superior limit to which he has recently taken. The result of the recent temperature, and therefore no superior limit beyond which work has been to show that H and h are of about the same such combinations are possible, because the atoms which breadth in Sirius. In a Aquilæ while the relation of H to have the power of combining together at these transcen- h is not greatly changed, a distinct approach to the solar dental stages of heat do not exist as such, or rather they condition is observed. K being now unmistakably present, exist combined with other atoms, like or unlike, at although its breadth is small as compared with that of H. all lower temperatures. Hence association will be a com- I must express my obligations to Dr. Huggins for granting bination of more complex molecules as temperature is re- me permission to enrich my paper with reference to these duced, and of dissociation, therefore, with increased tem- unpublished observations. His letter, which I have perperature there may be no end,

mission to quote, is as follows:That is the first point.

" It may be gratifying to you to learn that in a photo. The second is this:

graph I have recently taken of the spectrum of a Aquila We are justified in supposing that our "calcium,” once there is a line corresponding to the more refrangible of the formed, is a distinct entity whether it be an element or solar H lines [that is K], but about half the breadth of not, and therefore, by working at it alone, we should never the line corresponding to the first H lines. know whether the temperature produces a single simpler " In the spectra of a Lyræ and Sirius the second line form or more atomic condition of the same thing, or is absent.” whether we actually break it up into X + Y, because Prof. Young's observations of the chromospheric lines, neither X nor Y will ever vary.

tu which I shall afterwards refer, give important evidence But if calcium be a product of a condition of relatively regarding the presence of calcium in the chromosphere of lower temperature, then in the stars, hot enough to enable the sun. He finds that the H and K lines of calcium are its constituents to exist uncompounded, we may expect strongly reversed in every important spot, and that in solar these constituents to vary in quantity; there may be more storms H has been observed injected into the chromo. of X in one star and more of Y in another; and if this be sphere seventy-five times, and K fisty times, while the so, then the H and K lines will vary in thickness, and the blue line at W. L. 4226'3, the all-important line at the extremest limit of variation will be that we shall only arc-temperature, was only injected thrice. have H representing, say X in one star, and only have K Further, in the eclipse observed in Siam in 1875, the H representing say Y in another. Intermediately between and K lines left the strongest record in the spectrum of these extreme conditions we may have cases in which, though both H and K are visible, H is thicker in some

Fig. 4.—THE MOLECULAR GROUPINGS OF Calcium. and K is thicker in others. Prof. Stokes was good enough to add largely to the


value of my paper as it appeared in the Proceedings by
appending a note pointing out that “When a solid body

SIRIUS, such as a platinum wire, traversed by a voltaic current, is heated to incandescence, we know that as the temperature increases not only does the radiatioo of each par

SUNI ticular refrangibility absolutely increase, but the propor. tion of the radiations of the different refrangibilities is

ARC, changed, the proportion of the higher to the lower increasing with the temperature. It would be in accordance with analogy to suppose that as a rule the same would

FLAME take place in an incandescent surface, though in this case the spectrum would be discontinuous instead of continuous. ' Thus is A, B, C, D, E denote conspicuous bright the chromosphere, while the line near G in a photographic lines of increasing refrangibility, in the spectrum of the va- region of much greater intensity was not recorded at all. pour, it might very well be that at a comparatively low tem. In the American eclipse of the present year the H and K perature A should be the brightest and the most persistent; lines of calcium were distinály visible at the base of the at a higher temperature, while all were brighter than be- corona, in which for the first time the observers could fore, the relative brightness might be changed, and C might scarcely trace the existence of any hydrogen. be the brightest and the most persistent, and at a still To sum up, then, the facts regarding calcium, we have higher temperature E."

first of all the H-line differentiated from the others by its On these grounds Prof. Stokes, while he regarded the almost solitary existence in Sirius. We have the Kline fas I mentioned as evidence of the high temperature of differentiated from the rest by its birth, so to speak, in a the sun, did not look upon them as conclusive evidence of Aquilæ, and the thickness of its line in the sun, as comthe dissociation of the molecule of calcium.

pared to that in the arc. We have the blue line differen. Since that paper was sent in, however, the appeal to tiated from H and K by its thinness in the solar spectrum the stars to which I referred in it has been made, and while they are thick, and by its thickness in the arc while made with the most admirable results, by Dr. Huggins. they are thin. We have it again differentiated from them

The result of that appeal is that the line which, accord by its absence in solar storms in which they are almost ing to Prof. Stokes's view, should have prevailed over all'universally seen, and finally, by its absence during eclipses,


January 10: ?
Nature of the Elements.

13 while the H and K lines have been the brightest seen or two intensities are assimilated. What might happen photographed. Last stage of all, we have calcium, dis- does happen; the relative intensity of the two triplets in tinguished from its salts by the fact that the blue line is the spark photograph is grandly reversed in the spectrum only visible when a high temperature is employed, each of the sun. The lines barely visible in the spark photosalt having a definite spectrum of its own, in which none graph are among the most prominent in the solar specof the lines to which I have drawn attention appear, so long rum, while the triplet which is strong in that phoas the temperature is kept below a certain point.

tograph is represented by Fraünhofer lines not hali so

thick. Indeed, while the hypothesis that the iron lines Iron.

in the region I have indicated are produced by the vibraWith regard to the iron spectrum I shall limit my tion of one molecule does not include all the facts, the remarks to that portion of it visible on my photographic hypothesis that the vibrations are produced by at least plates between H and G. It may be described as a very three distinct molecules includes all the phenomena in a complicated spectrum, so far as the number of lines is most satisfactory manner. concerned in comparison with such bodies as sodium and potassium, lead, thallium, and the like, but unlike them

Lithium. again it contains no one line which is clearly and unmis- Before the maps of the long and short lines of some takably reversed on all occasions. Compared, however, of the chemical elements compared with the solar spectra, with the spectrum of such bodies as cerium and uranium which were published in the Phil. Trans. for 1873, the spectrum is simplicity itself.

“ Plate IX.,” were communicated to the Society, I very Now among these lines are two triplets, two sets of carefully tested the work of prior observers on the nonthree lines each, giving us beautiful examples of those coincidence of the red and orange lines of that metal repetitions of structure in the spectrum which we meet with the Fraunhofer lines, and found that neither of them with in the spectra of almost all bodies, some of which were strongly if at all regresented in the sun, and this have already been pointed out by Mascart, Cornu, and remark also applies to a line in the blue at wave-length myself. Now the facts indicate that these two triplets 4603. are not due to the vibration of the same molecular The photographic lithium line, however, in the violet grouping which gives rise to most of the other lines. has a strong representative among the Fraunhofer lines. They are as follows:-In many photographs in which Applying, therefore, the previous method of stating iron has been compared with other bodies, and in others the facts, the presence of this line in the sun differen. again in which iron has been photographed as existing tiates it from all the others. For the differentiation of in different degrees of impurity in other bodies, these the red and yellow lines I need only refer to Bunsen's triplets have been seen almost alone, and the relative in. spectral analytical researches, which were translated in tensity of them, as compared with the few remaining the Phil. Mag., December, 1875. lines, is greatly changed. In this these photographs re- In Plate IV. two spectra of the chloride of lithium are semble one I took three years ago, in which a large coil given, one of them showing the red line strong and the and jar were employed instead of the arc, which neces- yellow one feeble, the other showing merely a trace of sitated an exposure of an hour instead of two minutes. the red fine, while the intensity of the yellow one is much In this the triplet near G is very marked, the two adjacent increased, and a line in the blue is indicated. Another lines more refrangible near it, which are seen nearly as notice of the blue line of lithium occurs in a discourse by strong as the triplet itself in some of the arc photographs Prof. Tyndall, reprinted in the CHEMICAL News, and a I possess, are only very faintly visible, while dimmer still letter of Dr. Frankland's to Prof. Tyndall, dated Novemare seen the lines of the triplet between H and h. ber 7, 1861. This letter is so important for my argument

There is another series of facts in another line of work. / that I reprint it entire from the Phil. Mag., vol. xxii., In solar storms, as is well known, the iron fines some. p. 472: times make their appearance in the chromosphere. Now "On throwing the spectrum of lithium on the screen if we were dealing here with one molecular grouping, we yesterday, I was surprised to see a magnificent blue should expect the lines to make their appearance in the band. At first I thought the lithic chloride must be order of their lengths, and we should expect the shortest adulterated with strontium, but on testing i: with Steinlines to occur less frequently than the longest ones. heil's apparatus it yielded normal results without any Now, precisely the opposite is the fact. One of the most trace of a blue band. I am just now reading the report valuable contributions to solar physics that we possess of your discourse in the CHEMICAL News, and I find that is the memoir in which Prof. C. A. Young records his you have noticed the same thing. Whence does this blue observation of the chromospheric lines, made on behalf line arise ? Does it really belong to the lithium, or are of the United States Government, at Sherman, in the the carbon points or ignited air guilty of its production ? Rocky Mountains. The glorious climate and pure air of I find there blue bands with common salt, but they have this region, to which I can personally testify, enabled neither the definiteness nor the brilliancy of the lithium him to record phenomena which it is hopeless to expect band. When lithium wire burns in air it emits a someto see under less favourable conditions. Among these what crimson light ; plunge it into oxygen, and the light were injections of iron vapour into the chromosphere, the changes to bluish white. This seems to indicate that record taking the form of the number of times any one a high temperature is necessary to bring out the blue line was seen during the whole period of observation.

ray." Now two very faint and short lines close to the triplet “ Postscript, Nov. 22, 1861.-I have just made some near G were observed to be injected thirty times, while further experiments on the lithium spectrum, and they one of the lines of the triplet was only injected twice. conclusively prove that the appearance of the blue line

The question next arises, are the triplets produced by depends entirely on the temperature. The spectrum of one molecular grouping or by two ? This question I also lithic chloride, ignited in a Bunsen's burner flame, does think the facts help us to answer. I will' first state by not disclose the faintest trace of the blue line; replace way of reminder that in the spark photograph the more the Bunsen's burner by a jet of hydrogen (the temperarefrangible triplet is barely visible, while the one near G ture of which is higher than that of the Bunsen's burner) is very strong. Now if one molecular grouping alone and the blue line appears faint, it is true, but sharp and were in question this relative intensity would always be quite unmistakable. If oxygen now be slowly turned preserved however much the absolute intensity of the into the jet. the brilliancy of the blue line increases until compound system might vary, but if it is a question of the temperature of the flame rises high enough to fuse two molecules we might expect that in some of the the platinum, and thus put an end to the experiment." regions open to our observation we should get evidence These observations of Profs. Tyndall and Frankland of cases in which the relative intensity is reversed or the differentiate this blue line from those which are observed





relatively mark atemperatureh goes through its Changes, its p. 45We now turn to the last and strongest series of pro

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Nature of the Elements.


January 10, 1879. at low temperatures. The line in the violet to which I was found, as a mean of several measurements to be 3'. have already referred is again differentiated from all the The goniometer was graduated to 15", and owing to the rest by the fact that it is the only line in the spectrum of small dispersive power, and therefore relatively great the sun which is strongly reversed, so far as our present breadth of the slit, the measurement can only be regarded knowledge extends. The various forms of lithium, there- as a first approximation. Turning now again to our fore, may be shown in the following manner.

photographs, and calculating the angular distance between

the first and second ring of protuberances, we find that Fig. 5.-The MOLECULAR GROUPINGS OF LITHIUM. distance to be 3' 15". We conclude, therefore, that this

second ring is due to hydrogen. We, therefore, naturally looked for the third photographic hydrogen line, which is generally called h, but we found no protuberance on our photographs corresponding to that wave-length. Although this line is always weaker than Hy, its absence on the photograph is rather surprising, if it be due to the fact that the line is one which only comes out at a high temperature. This is rendered likely by the researches of

Frankland and Lockyer (Proc. Roy. Soc., vol. xvii., It is in case of this which at ). compounds are broken up at the temperature of the i tuberances shown on our photographs. The distance Bunsen burner. The spectrum, e.g. of the chloride, so between this series and the one we have found reason for far as I know, has never been seen.

identifying with Hy is very little greater than that between

HB and Hy. Assuming the distances equal, we conclude Hydrogen.

that the squares of the inverse wave-lengths of the three All the phenomena of variability and inversion in the series are in arithmetical progression. This is true as a order of intensity presented to us in the case of calcium first approximation. We then calculated the wave-length can be paralleled by reference to the knowledge already of this unknown line, and found it to be approximately acquired regarding the spectrum of hydrogen.

somewhat smaller than 3957 tenth-metres. No great Dr. Frankland and myself were working together on reliance can be placed, of course, on the number, but it the subject in 1869. In that year (Proc., No. 112) we appears that the line must be close to the end of the visible pointed out that the behaviour of the h line was hors ligne, spectrum. and that the whole spectrum could be reduced to one “ In order to decide if possible what this line is due to, line, F.

we endeavoured to find out both by photography and 1., The Fraünhofer line on the solar spectrum, named Auorescence whether hydrogen possesses a line in that h by Angström, which is due to the absorption of hydro- part of the spectrum. We have not at present come to gen, is not visible in the tubes we employ with low battery any definite conclusion. In vacuum tubes prepared by and Leyden-jar power; it may be looked upon, therefore, Geissler containing hydrogen, a strong line more reas an indication of relatively high temperature. As the frangible than H is seen, but these same tubes .show line in question has been reversed by one of us in the between Hy and Hò, other lines known not to belong to spectrum of the chromosphere, it follows that the chromo- hydrogen, and the origin of the ultra violet line is theresphere, when cool enough to absorb, is still of a relatively fore difficult to make out. We have taken the spark in high temperature.

hydrogen at atmospheric pressures, as impurities are * 2. Under certain conditions of temperature and pres- easier to eliminate, but a continuous spearum extends sure, the very complicated spectrum of hydrogen is over the violet and part of the ultra-violet, and prevents reduced in our instrument to one line in the green corre- any observation as to lines. We are going on with exsponding to F in the solar spectrum.”

periments to settle this point. As in the case of calcium also, solar observation affords “Should it turn out that the line is not due to hydrogen, us most precious knowledge. The h line was missing the question will arise what substance it is due to. It is from the protuberances in 1875, as will be shown from the a remarkable fact that the calculated wave-length comes accompanying extract from the Report of the Eclipse very close to H. Young has found that these calcium Expedition of that year :

lines are always reversed in the penumbra and immediate ** During the first part of the eclipse two strong pro- neighbourhood of every important sun-spot, and calcium tuberances close together are noticed ; on the limb must therefore go up high into the chromosphere. We towards the end these are partially covered, while a series draw attention to this coincidence, but our photographs of protuberances came out at the other edge. The strongest do not allow us to draw any certain conclusions. of these protuberances are repeated three times, an effect “At any rate, it seems made out by our photographs of course of the prism, and we shall have to decide if that the photographic light of the protuberances is in possible the wave-lengths corresponding to the images. great part due to an ultra-violet line which does not We expect a priori to find the hydrogen lines represented. certainly belong to hydrogen.. The protuberances as We know three photographic hydrogen lines : F, a line photographed by this ultra-violet ray seem to go up near G, and h. F is just at the limit of the photographic higher than the hydrogen protuberances, but this may be part of the spectrum, and we find indeed images of pro- due to the relative greater length of the line." tuberances towards the less refrangible part at the limit In my remarks upon calcium I have already referred to of photographic effect. For, as we shall show, a con. the fact that the line which our observation led us to tinuous spectrum in the lower parts of the corona has believe was due to calcium in 1875, was traced to that been recorded, and the extent of this continuous spectrum element in this year's eclipse. The observations also show gives us an idea of the part of the spectrum in which the curious connection that, at the time when the hydrogen each protuberance line is placed. We are justified in lines were most brilliant in the corona, the calcium lines assuming, therefore, as a preliminary hypothesis, that the were not detected ; next, when the hydrogen lines, being least refrangible line in the protuberance shown on the still brilliant, the h line was not present (a condition of photograph is due to F, and we shall find support of this things which, in all probability, indicated a reduction of view in the other lines. In order to determine the posi- temperature), calcium began to make itself unmistakably tion of the next line the dispersive power of the prism visible); and finally, when the hydrogen lines are absent, was investigated. The prism was placed on a goniometer H and K become striking objects in the spectrum of the table in minimum deviation for F, and the angular dis. tance between F and the hydrogen line near G, i.e., Hy, To come back to h then, I have shown that Dr. Frank.

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FIG. 7




Nature of the Elements. January 10, 1879. S

15 land and myself, in 1869, found that it only made its ap- , reversed line in the chromosphere and the line at 1474 of pearance when a high tension was employed. We have Kirchhoff's scale, termed the coronal line, are really other seen that it was absent from among the hydrogen lines forms of hydrogen. One of these is possibly more simple during the eclipse of 1875.

than that which gives us h alone, the other mcre comI have now to strengthen this evidence by the remark plex than that which gives us F alone. The evidence on that it is always the shortest line of hydrogen in the this point is of such extreme importance to solar physics, chromosphere.

and ihrows so much light on star structure generally, that I now pass to another line of evidence.

I am now engaged in discussing it and shall reserve it I submit to the Society a photograph of the spectrum for a special communication. of indium, in which, as already recorded by Thalèn. the strongest line is one of the lines of hydrogen (h), the

Fig. 6. other line of hydrogen (near G) being absent. I have h co F observed the C line in the spark produced by the passage of an induced current between indium poles in dry air.

As I am aware how almost impossible it is to render air perfectly dry, I made the following differential experi. ment. A glass tube with two platinum poles about half an inch apart was employed. Through this tube a slow current of air was driven after passing through a U-tube one foot high, containing calcic chloride, and then through sulphuric acid in a Wolff's bottle. The spectrum of the in which I have arranged the various groupings of

In the meantime I content myself by giving a diagram spark passing between the platinum electrodes was then observed, a coil with five Grove cells and a medium-sized hydrogen as they appear to exist, from the regions of jar being employed. Careful notes were made of the highest to those of lowest temperature in our central brilliancy and thickness of the hydrogen lines as com

luminary. pared with those of air. This done, a piece of metallic indium which was placed loose in the tube, was shaken


1474 so that one part of it rested against the base of one

SUN of the poles,' and one of its ends at a distance of a little less than half an inch from the base of the other

CHROMOSPAERE pole. The spark then passed between the indium and

JAR SPARK the platinum. The red and blue lines of hydrogen were then observed both by my friend, Mr. G. W. Hemming,

SPARK WITHOUT JAR Q.C., and myself. Their brilliancy was most markedly increased. This unmistakable indication of the presence of hydrogen, or rather of that form of hydrogen which

COOLER STILL gives us the h line alone associated into that form which gives us the blue and red lines, showed us that in the

Summation of the above Series of Facts. photograph we were not dealing with a physical coinci. I submit that the facts above recorded are easily dence, but that in the arc this special form of hydrogen grouped together, and a perfect continuity of phenomena had really been present; that it had come from the in established on the hypothesis of successive dissociations dium, and that it had registered itself on the photogra- analogous to those observed in the cases of undoubted phic plate, although ordinary hydrogen persistently re- compounds. fuses to do so. Although I was satisfied from former experiments that occluded hydrogen behaves in this

The other Branches of the Inquiry. respect like ordinary hydrogen. I begged my friend, Mr. When we pass to the other possible evolutionary proW. c. Roberts, F.R.S., chemist to the Mint, to charge a cesses to which I have before referred, and which I hope piece of palladium with hydrogen for me. This he at to discuss on a future occasion, the inquiry becomes much once did, and I take this present opportunity to express more complicated by the extreme difficulty of obtaining my obligation to him. I exhibit to the Society a photo pure specimens to work with, although I should remark graph of this palladium and of indium side by side. It that in the working hypothesis now under discussion the will be seen that one form of hydrogen in indium has dis- cause of the constant occurrence of the same substance tinctly recorded itself on the plate, while that in palladium as an impurity in the same conneæion is not far to seek. has not left a trace. I should add that the palladium was I take this opportunity of expressing my obligations to kept in a sealed tube till the moment of making the ex- many friends who have put themselves to great trouble in periment, and that special precautions were taken to pre obtaining specimens of pure chemicals for me during the vent the two pieces between which the arc was taken whole continuance of my researches. Among these I becoming unduly heated.

must mention Dr. Russell, who has given me many speci. To sum up, then, the facts with regard to hydrogen ; mens prepared by the lamented Matthiessen, as well as we have h differentiated from the other lines by its appear. some of cobalt and nickel prepared by himself; Prof. ance alone in indium; by its absence during the eclipse Roscoe, who has supplied me with vanadium and cæsium of 1875, when the other lines were photographed; by its alum ; Mr. Crookes, who has always responded to my existence as a short line only in the chromosphere of the call for thallium; Mr. Roberts, chemist to the Mint, who sun, and by the fact that in the experiments of 1869 a very has supplied me with portions of the gold and silver trial high temperature was needed to cause it to make its plates and some pieces of palladium ; Dr. Hugo Müller, appearance.

who has furnished me with a large supply of electrolitiWith regard to the isolation of the F line I have already cally-deposited copper; Mr. Holtzman, who has provided referred to other experiments in 1869, in which Dr. me with cerium, lanthanum, and didymium prepared by Frankland and myself got it alone." I exhibit to the himself; Mr. George Matthey, of the wellknown metalSociet; a globe containing hydrogen which gives us the lurgical firm of Johnson and Matthey, who has provided F line without either the red or the blue one.

me with magnesium and aluminium of marvellous purity; The accompanying drawing shows how these lines are while to Mr. Valentin, Mr. Mellor, of Salford, and other integrated in the spectrum of the sun.

friends, my thanks are due for other substances. I have other evidence which, if confirmed, leads to the I have already pointed out that a large portion of the conclusion that the substance which gives the non- work done in the last four years has consisted in the eli* See also Plücker, Phil. Trans., 1865, part 1, p. 21.

ination of the effects of impurities. I am therefore aware


Supersatnrated Solutions.


January 10, 1879.

of the great necessity for caution in the spectroscopic for many hours. In this case the rapidity of the examination of various substances. There is, however, a absorption prevented the formation of crystals. number of bodies which permit of the inquiry into their Swedish filter-paper generally gives the normal simple or complex nature being made in such a manner salt in the centre with this solution, whilst any that the presence of impurities will be to a certain extent number of small drops are absorbed, and a great negligable. I have brought this subject before the Royal many little bits of the paper can be put into drops Society at its present stage, in the hope that possibly or into the flask, and are inactive because satuothers may be induced to aid enquiry in a region in which

rated at once. The absorbents seem to act by the work of one individual is as a drop in the ocean. If abstraâing water. This explains the action of there is anything in what I have said, the spectra of all aërial nuclei; they act by causing gentle absorption, the elementary substances will require to be re-mapped which gives the normal salt, or possibly by gentle and re-mapped from a new standpoint; further, the arc evaporation. There is some reason to believe must replace the spark, the photography must replace that gentle evaporation will give the normal salt the eye. A glance at the red end of the spectrum of al- just as the more rapid evaporation of drops exmost any substance incandescent in the voltaic arc in a posed to air gives a salt with less water. Salts spectroscope of large dispersion, and a glance at the maps vary very much in their sensitiveness to absorb. prepared by such eminent observers as Huggins and ents, sodium sulphate being the most so that I Thalén, who have used the coil, will give an idea of the have yet tried. mass of facts which have yet to be recorded and reduced 3. I believe that most cases of sudden crystallisation before much further progress can be made.

on removing cotton wool, or piercing a bladder, In conclusion, I would state that only a small part of or on giving a jerk, as Professor Tomlinson does the work to which I have drawn attention is my own. In

to flatten a lens of oil into a film, or (as has often some cases I have merely, as it were, codified the work happened to me) on simply entering the room, may done by other observers in other countries. With reser. be explained thus :--After heating a solution ence to that done in my own laboratory, I may here repeat convection currents of air may be seen rising and what I have said before on other occasions, that it is carrying drops of the solution. These settle on largely due to the skill, patience, and untiring zeal of those the cover or round the mouth, crystallise there by who have assisted me. The burthen of the final reduc- absorption or evaporation, and are shaken down tion, to which I have before referred, has fallen to Mr. by any sudden jar. When the covers are loose, or Miller, my present assistant; while the mapping of the are gently removed, crystallisation seldom follows. positions and intensities of the lines was done by Messrs. Friswell, Meldola, Ord, and Starling, who have succes.

Turning now to Prof. Tomlinson's paper, I find that sively filled that post.

he has repeated a certain number of my experiments, with I have to thank Corporal Ewings, R.E., for preparing the result, as he believes, of upsetting my theories. As a the various diagrams which I have submitted to the notice matter of fact, I have to thank him for a very pretty set of this Society.

of illustrations of them.

With regard to the first point, for instance—that of ex. posure-he finds as I did that drops of sodium acetate

can be exposed for weeks, the carbonate for days, the ON SUPERSATURATED SOLUTIONS. sulphate for many hours on paper and other surfaces. He

makes a great point of the fact that this happens only in By J. G. GRENFELL,

damp weather; in dry weather he says the drops crystal.

lise. Nearly half of his paper is devoted to experiments IN September, 1877, Professor Tomlinson read a paper which prove this statement. He does not, however, before the Royal Society, the main object of which was attempt to explain on his own theory why this should be to upset the conclusions to which I had been led by my the case, but remarks that laborious investigation is experiments on supersaturated solutions, and thus to

needed to determine the conditions under which bodies do maintain his theory that the salt adheres to a film of any

or do not act as nuclei. I could not have wished for a greasy substance, while the water does not, and thus neater proof that my theory is correct. If the nuclei act separation ensues.

by absorption and evaporation it is obvious that they will Before answering this paper I wished to repeat some

have much less effect in wet weather than in dry. I could of my experiments, and this I have not been able to do till have predicted the result, though I have never paid special quite recently. Those conclusions of mine which are attention to the point. openly or implicitly attached in the paper are as

I should like to know, however, what Prof. Tomlinson follows:

means by saying that drops crystallise in dry weather. 1. Supersaturated solutions can in many cases be ex.

There is some reason to believe that he was satisfied with posed to the air for an indefinite time, can be seeing crystals after the lapse of a certain time, and did rubbed with dust or oil and yet not crystallise. At not try to find out what the crystals were. a meeting of the Chemical and Physical Section of He says that potash alum crystallised into an opaque

This certainly seems to be the case in one instance. the Bristol Naturalists' Society I treated strong solutions of sodium sulphate and acetate in this white mass or into transparent crystals in concentric way, and a good many persous present did the circles round a central pit. Now the alums generally I used ordinary castor oil.

evaporate on glass, giving a more or less transparent 2. Absorbent substances, when not at once saturated

crust with a central depression, underneath which little

is not at all by the solution, and provided the absorption is not botryoidal masses are often formed. too rapid, cause crystallisation, sometimes as the

common for alum to crystallise in concentric rings of normal salt, sometimes as a modified salt. For

separate crystals, and if, as I suppose, Prof. Tomlinson instance, I recently put a considerable quantity alum at all, as it is quite inactive in drops, while the

refers to the ordinary crust, he would find that this is not of a 6 to I solution of sodium carbonate on new sheet of thick blotting-paper. In about a

merest trace of the normal converts it into the opaque minute it gave a cake of the modified 7-atom salt. white normal alum. The crystals were quite inactive in a 2 to i solu.

Be this, however, as it may, it is not hard to account rion. At the same time I put 13 small drops round for the statement that in dry weather the drops crystallise the edge; of these two crystallised at once as the rapidly. It is impossible to succeed in dry weather with7-atom salt, and they and the central mass were soon

* These experiments so were shown at the meeting mentioned quite diy; the rest remained liquid exposed to air above.



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