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facts leading to the conclusion that materials of extraterrestrial origin play a not unimportant part in the accumulations which are taking place on the deepest ocean-floors. We can only call attention, however, to the clear descriptions and admirable plates which illustrate this part of the subject. The exquisite drawings of magnetic spherules and of chondre upon Plate xxiii., enable the reader to judge of the real nature of the evidence relied upon, and an examination of these figures cannot but remove any lingering doubt, as to the true nature of these materials, from the minds of all those who are familiar with the minute structure of meteorites. The last chapter of the work deals with the chemical products which are formed in situ upon the floor of the ocean, and here, perhaps, the interest of the work for the geologist culminates. We can only refer to the numerous and interesting problems connected with the origin of the red clay, the mode of formation of the glauconite-casts, the source of the materials and the chemical processes involved in the formation of the phillipsite and other zeolites, the manganese-nodules, and the phosphatic and other concretions. The 76 pages of text, and the admirable drawings which illustrate this part of the subject, will excite the interest of all students of the subject. They enable the reader to form a clear idea of the forms and structure of the remarkable manganese nodules, and of the ear-bones, teeth, and other objects which, in a more or less phosphatized condition, are strewn over the deepest part of the ocean-floors. In an appendix is given a report on the analysis of the manganese-nodules by Dr. John Gibson, especial attention being directed to the detection of the rarer elements by spectroscopic and other methods. While traces of barium, strontium, lithium, molybdenum, zinc, titanium, vanadium, and thallium were found, cæsium, rubidium, and the metals of the cerium and yttrium groups were sought for in vain. The quantitative analyses, as shown by the tabular statements, would appear to have been executed with every modern refinement, and were carried out, by Prof. Crum Brown's permission, in the Chemical Laboratory of the University of Edinburgh. Another appendix contains Another appendix contains an account of the analyses which have been made of the different varieties of deep-sea deposits.

In conclusion, we may point out that the work is worthy of praise, not only for what it includes, but for what it omits. The time has not yet arrived for a full discussion of the geological bearings of many of the new and interesting facts brought to light by the Challenger Expedition. Theoretical discussions are, therefore, wisely kept, in the monograph before us, within very narrow bounds. It is evident that much of the work was written before the publication by Messrs. Jukes-Browne and Harrison of their interesting memoirs on the geology of Barbadoes, and before the discovery of the Radiolarian-chert of Ayrshire and other districts. These discoveries, it is true, are mentioned in footnotes, but have evidently had but little influence in moulding the views of the authors. Few geologists will be prepared to accept the views of Mr. Murray, when he endorses the conclusion of M. Cayeux that the white chalk should be classed as a terrigenous deposit. But on this and other points the views of the authors are stated with a commendable absence of dogmatism, and a manifest desire to lay before readers of

the work all the facts bearing upon the questions at issue, even when they are manifestly hostile to the conclusions adopted.

We cannot bring this notice to an end without congratulating the editor of the Challenger Reports on the nearly approaching close of his heavy labours. Only by a worker gifted with unrivalled powers of organization, as well as with indomitable energy, could such a task have been brought to a successful termination. The mass of materials was so vast and multifarious, the interests involved in their distribution so wide and often conflicting, while personal considerations could not always be kept from exercising a disturbing influence, that it is less surprising that criticism should sometimes have been provoked, than that results so substantial, and, on the whole, satisfactory, should in the end have been attained.

The present Report forms the last of the series of splendid monographs in which the results of this famous Expedition-one which will be recorded in the history of Science as perhaps the grandest concession to her claims made up to the present time by the British or any other Government-are fully recorded and discussed. The final volume of the Challenger Reports, which, it is stated, will probably be published in the course of the present year, will contain lists of the organisms collected at every observing station, with other details, in the nature of a summary of results. JOHN W. JUDD.

PARASITIC FUNGI AND MOULDS.

British Fungi: Phycomycetes and Ustilaginea. By G. Massee. (London: Reeve and Co., 1891)

T is a somewhat remarkable fact that no one has hitherto written a book on the British Phycomycetes, the common white moulds so often found growing on decaying substances or in water, or as parasites of a most destructive kind in various valuable plants; and the opportunity thus afforded to the writer of the present volume was a good one, of which, it is but fair to say, he has taken considerable advantage. The Ustilaginea of this country had already been treated by Mr. Plowright, but there are sufficient differences between the works of the two authors to make Mr. Massee's book none the less noteworthy on that account.

When we consider the great variety of "white moulds," such as Mucor, that infest all kinds of rotting fruits and other vegetable débris, of parasites such as the Phytophthora of the potato disease, and the Peronosporea which destroy onions, vines, and other valuable vegetable produce, to say nothing of the Saprolegnia of the salmon disease, the Pythium which decimates seedlings of all kinds, and the Empusa which kills our house-flies in autumn, and glues their dead bodies to the window-panes -when we regard these and a host of other extraordinary and important Phycomycetous Fungi, it seems more and more surprising that no one has compiled an intelligible account of these things in this country; yet so it is, and the author of this little book of a couple of hundred of pages of carefully, and, on the whole, pleasantly-written matter, ought certainly to deserve the thanks of botanical readers for undertaking the difficult task, and discharging it as well as he has done.

In reviewing the work there are two parts to be noticed, and two points of view from which to criticize them: the first fifty pages or so are concerned with a general popular account of the morphology of Fungi in the wider sense, while the remainder is devoted to the setting forth of the British genera and species (so far as they have been worked up) of the two groups specially dealt with.

The general account must strike a careful reader as not only exhibiting a good deal of knowledge on the part of a writer who is wishful to put it at the disposal of all who care for it, and in a pleasant style; but also as showing what enormous advances have been made in the popular exposition of these matters within the last few years. When we look back to the systematic books on Fungi of ten to fifteen years ago, they appear hopelessly dry and uninteresting; whereas here we have a compact, neat little volume, with a store of interesting information thrown in as an introduction to the more serious detailed work which follows.

We do not mean to say that this part of the book is without mistakes or slips, either of fact (e.g. the statement on p. 49 regarding mutualism between Fungi and Phanerogams) or judgment (e.g. the reference to "phanerogamic Fungi" on p. 11). Moreover, there are evidences of careless proof-reading, as at the foot of pp. 41 and 42. But it is far more easy to pick small holes in a book like this than to do proper justice to what is good and useful in it; and we prefer to dwell on the more important positive points, than to emphasize the fewer and more trivial drawbacks.

The more purely systematic part of the work shows evidence of careful and conscientious industry, suggesting constant reference on the part of the author to typespecimens and authorities. Of course, it is not so interesting to the general reader, but the diagnoses are so clear, and so simply written, that we think any amateur ought to be able to follow them with the object in hand; as for professional mycologists, they will probably wonder that it could all be put so plainly-at the same time, they will suspect something is wrong with the German reference on p. 162, and will probably remark on the chapter on "Fossil Fungi." They may also inquire why Ustilaginea are taken with Phycomycetes. The author answers this question on p. 160: he follows Brefeld in regarding Protomycetes as linking the two groups. The somewhat antiquated method of obtaining sections, on p. 62, had better have been omitted.

The most interesting points to the systematists will be Mr. Massee's almost consistent alterations of Plowright's authorities for the species of the Ustilaginea, and his addition of one or two new ones-e.g. I stilago salveii (p. 177), Doassansia comari (p. 198), and Protomyces purpureo-tingens (p. 164); they will also notice the fusion of some species kept apart by Plowright-e.g. on pp. 178 and 186-and the separation of the two species of Tubercinia, on pp. 203 and 204.

We note, also, that Massee has altered the name of Trail's Entyloma matricaria to E. Trailii, possibly on good grounds; but we think it a mistake to use such specific names, here and elsewhere, seeing how much Fungi are in need of useful distinctive appellations.

The figures on the six plates are fairly well drawn and selected, and the references to them are useful and

to the point. We have not tested the indices in detail, but they are very well planned, and appear to be accurate. On the whole, and without being blind to its faults, we think this little book should be welcomed as a useful manual on the subject, and should certainly be in the hands of students of botany who wish to know something of British mycology.

us.

OUR BOOK SHELF.

A Treatise on the Geometry of the Circle, and some Extensions to Conic Sections by the Method of Reciprocation. With numerous Examples. By W. J. M'Clelland, M.A. (London: Macmillan, 1891.) THIS is a full book, written on the lines which previous works by Irish mathematicians have made familiar to The author acknowledges his indebtedness to the writings of Mulcahy, Salmon, and Townsend. He has also freely consulted the similar works by Cremona and Catalan, and in his treatment of the recent geometry has in many cases gone to the fountain-head in the memoirs of Brocard, Neuberg, and Tarry. Though in parts proceeding on parallel lines with Casey's "Sequel," there is a good deal of other matter not to be found in that work. The writer's object is to give a concise statement of those propositions which he considers to be of fundamental importance, and to supply numerous illustrative examples. Many of the exercises are worked out in an elegant manner, and to the major part of the others useful hints are given. Chapter i. is introductory; chapter ii., in four sections, is devoted to "Maximum and Minimum"; chapter iii., also in four sections, rapidly touches upon "Recent Geometry"; chapter iv. discusses the general theory of the mean centre of a system of points; and chapter v. treats of collinear points and concurrent lines. points with respect to a circle, poles and polars (with Chapters vi., vii., and viii. are concerned with inverse respect to a circle), and coaxal circles. In these chapters will be found ample food for the student. Chapter ix. gives an account of the theory of similar figures, and here we specially notice the sketch of Neuberg's and Tarry's researches on three similar figures. Circles of similitude and of antisimilitude form the subject of chapter x. Here some interesting problems are solved. Inversion (chapter xi.), general theory of anharmonic section (chapter xii.), involution (chapter xiii.), and double points (chapter xiv.) close what must unhesitatingly be called a varied and ample menu. The work, being conbefore the reader much that is old; there is, however, fessedly to a great extent elementary, of course brings novelty in the treatment and also in the matter.

There

is one feature we have omitted to mention, to which Mr. M'Clelland draws attention, and that is the application of reciprocation to many of the best known theorems by means of which the corresponding properties of the conic are ascertained. To go through all the examples would Occupy more time than we can spare, but we have dipped into all parts and brought up good results. In the text we have noted one slip: p. 60, l. 12 up should be π- - B. No doubt we have omitted to mark other

errata.

The figures, which are white lines on a black block, carry our thoughts back to old Cambridge days, when we turned over the pages of our Miller's "Hydrostatics." The geometer will find much to interest him in Mr. M'Clelland's work.

Kalm's Account of his Visit to England on his Way to America in 1748. Translated by Joseph Lucas. (London: Macmillan and Co., 1892.)

KALM was a well-known Swedish botanist and economist of the eighteenth century. In 1747 he became Professor of Economy at Abo, and in the same year the Swedish

Government and Academy of Sciences commissioned him to go to America, the object being that he should describe the natural productions of that part of the world, and introduce into Sweden any useful North American plants which might be expected to thrive in Northern Europe. Kalm reached England in February 1748, and remained there until August, when he started for America. On his way back, in 1751, he visited this country again, staying about a month. An account of a portion of his travels he afterwards published in three volumes. The part relating to America was translated into English in the eighteenth century by J. Reinhold Forster, but the author's account of England appears now in English for the first time. The work is full of interest, and was well worth translating. Kalm first records his impressions of London and suburbs, and then takes us successively to Woodford, Little Gaddesden, and Gravesend, each of which is made a centre for a number of observations, chiefly in connection with agriculture. To students of the history of agricultural methods the work will be invaluable; but it will also give pleasure to readers with a less serious purpose, for it contains suggestive references to many aspects of Eng; lish life, and the author always writes accurately and with good taste. The translator has accomplished his task with great spirit and intelligence.

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.]

The University of London.

It is always a pleasure to read Mr. Thiselton-Dyer's expressions of opinion on University organization. I have before now joined my word to his in condemnation of Sir George Young's proposed "Albert" or "Gresham" Charter. Nevertheless, I must beg you to grant me space to point out some inaccuracies in Mr. Dyer's letter in your columns of February 25 (p. 392), the purpose of which seems to be to give reason for distrusting, or, at any rate, treating with little confidence, University organizations on the German or professorial model.

Mr. Dyer rightly enough appeals to his own early experience as a teacher and student. It is therefore fair to point out that this experience does not include a German University, and that the conception of it sketched by him, and of a professor's relations to his pupils therein, is entirely erroneous. Mr. Dyer cannot free his mind of the University of London tradition. He regards the German as well as all Universities as organizations for bringing candidates up to a certain pitch of examination-room performance. This is not what a German University attempts. The measure of its success is not what Mr. Dyer would suggest, but is found in the contributions to science, the new knowledge created by the professor and his students, and in the spread of a love for producing such new knowledge. Mr. Dyer attributes to Lord Sherbrooke a strange sayingnamely, that professors who examine their own students are comparable to "tradesmen who sample their own goods.' can hardly credit that Lord Sherbrooke ever said anything so unmeaning. We have all heard the professor-examiner compared to "a merchant who brands his own herrings "--but this sampling of his own goods" is a new charge.

I

Lastly, I must point out that Mr. Dyer, by inadvertence, attributes to me a statement, or rather assent to a statement, before the Royal Commission on the proposed new University for London, which had exactly the opposite significance to that which he gives to it. Mr. Dyer says that I admitted to Sir William Thomson that "a teacher may, with judiciousness of course, and with common-sense in his teaching, teach the best that he knows" under the present University of London system. I am glad to note that Mr. Dyer has looked at the Blue-book. But if he had read more carefully he would have seen that Question 662, by Sir William Thomson, was, "Can an examiner under the London system ask the best that he

knows?" and that my answer was, "Probably not." Then Sir William continued (Question 663): "But, on the other hand, a teacher may, with judiciousness, &c., teach the best that he knows?' to which I answered, "Yes." Then said Sir William (Question 664), "If he is examining his own pupils he may bring into the examination something of the best and the newest?" to which I replied, Certainly.

It is clear enough that Sir William Thomson's proposition, to which I assented, was that, under the London system of external examiners, an examiner cannot put questions involving the best and newest; yet a teacher may and should teach the best and newest; and if, contrary to the principle of the L ›ndon system, the examiner is the teacher, he can introduce with judg

ment into the examination this element of the best and newest. Mr. Dyer has not, it seems to me, yet mastered the distinctive features of the German or professorial University system, and is, therefore, not a trustworthy guide as to its E. RAY LANKESTER. advantages and disadvantages.

Superheated Steam.

A COMMUNICATION from Lord Rayleigh, under the above heading, in NATURE of February 18 (p. 375), draws attention to a misunderstanding which has been pointed out by me on every occasion in the last twelve years when I have been explaining the thetaphi diagram in public, saying that "only the heat which superheated had its efficiency increased, according to the temperatures at which its respective portions were imparted to the working substance." Mr. Willans has also been disseminating correct views regarding this point amongst those who visit his engine-testing laboratory. The diagram given by me in my paper on thetaphi, in 1880, makes this very plain.

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paring together engines with different cycles has been a source of considerable misapprehension, and very probably the language used in the passage in question may be insufficiently guarded. The use of superheated steam on this method of comparison is not a gain, but a considerable loss, for the heat might ideally all have been used at the maximum temperature, and is so used in the standard of comparison.

The practical case in which the boiler pressure is given is, of course, quite different. There is a gain by superheating, but, 800°, B = 600°, S = 1000°. | putting aside cylinder condensation, the gain is small, because such a small percentage of the heat is employed at temperatures above that of the boiler.

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2389 with superheating.

That is, less than 4 per cent. is gained by superheating 200°. So far, I support Lord Rayleigh's view, or, rather, he says what I have been impressing upon engineers for the last twenty years. If this had been all I had to say, I would not have written now; but Lord Rayleigh adds to his statement what is to me an astounding announcement, that, "by the addition of saline matters, such as chloride of calcium or acetate of soda, ... the possible efficiency, according to Carnot, may be increased." I hasten to call this assertion into question, because there are so many people ready to bring engines on new principles into the field of joint-stock bubbles; and I am afraid we may be having, quite apart from Lord Rayleigh, a new field engine syndicated and floated on the strength of this communication and the signature thereto, before its meaning is understood. As I understand thermodynamics there would be no gain from superheating by a saline solution, over the usual method of superheating steam raised from pure water. The saline mixture is not the working substance. Carnot's law refers to the working substance only, and not to anything left in the boiler. The first step in evaporation from the saline mixture is to separate a particle of water from the salt. In the act of separation, the temperature of the water particle falls to the temperature due to the pressure, and at that temperature it is evaporated into steam particles, which immediately become of the same temperature as the saline mixture. These steps are followed by every particle of water, each independently of every other particle. Of course, we cannot practically test those temperatures, as the complete series is run through for each particle in a fraction of the twinkling of an eye, and immersed in a liquid of greatly higher temperature. A thetaphi diagram for this would give, at B A, and extending upwards to temperature S, a very narrow figure 8, whose loops are equal, and drawn, as in a figure 8, one right-hand and the other left-hand. The line for the reception of latent heat would be identically the same line, the horizontal through A, as when the evaporation was from pure water. It is evident, therefore, that, according to my lights, the efficiency will be precisely the same as without the salt in solution.

Some ten years ago this plan was submitted to me for my opinion by an eminent mechanical engineer, Mr. S. Geoghegan, who, I understood, had then patented it. The above is the substance of the opinion I then expressed, and nothing I have learned since induces me to change my view of it now.

The "complete elaboration of this method," hinted at in the last paragraph of Lord Rayleigh's communication, is not clear to my mind; and it is just possible that a few sentences of explanation would show me that I have been hitting away at something that was not intended by the writer. If so, my excuse must be that I have read the statement, as every practical engineer would, to mean that the latent heat is imparted along the isothermal of the superheat. When I get to understand the first sentence of the last paragraph of the communication, I may be able to confirm the anticipation of higher economy. J. MACFARLANE GRAY.

THE passage quoted by Lord Rayleigh from my book on the steam-engine, in some remarks on this subject in your number of the 18th inst. (p. 375), is taken from one of the earlier chapters, which is devoted to engines which receive and reject heat at constant temperature. When such an engine is used as a standard of perfection, by comparison with which some other engine is tried, it appears to me that the maximum and minimum temperatures of the working fluid must in the first instance be adopted as the temperatures of reception and rejection of heat; and in fact, without entering on questions reserved for discussion in a later chapter, no lower value than the maximum could well have been adopted. There is no doubt that the practice of com

The process was originally introduced with the object of drying the steam and diminishing cylinder condensation; and now that the practical difficulties attending its use have been in great measure removed (as I am informed), by the employment of mineral oil for lubricating purposes, it may be hoped that it may be revived, and be the means of a considerable economy. The action of superheated steam in a cylinder was explained and its economy experimentally demonstrated by Hirn some fifteen or twenty years ago. I have given the explanation briefly on p. 352 of my book, but I purposely avoided discussing questions relating to it, being of opinion that, in the present state of our knowledge, theoretical investigations are of doubtful value. I am certainly, however, under the impression that the true nature of the economy obtained by its use has for a long period been very generally recognized, though some writers in dealing with the theory of heat engines may have expressed themselves incautiously. It would, I think, be very desirable, in teaching the subject, to introduce as early as possible the idea of a mean temperature of supply. I have dwelt on the importance of this conception in the latter part of my book, and I am sure its introduction would remove many difficulties. Greenwich, February 24.

100

JAMES H. COTTERILL,

LORD RAYLEIGH's interesting communication on superheated steam in your last issue (p. 375) leads me to ask whether it is generally known that solutions can be heated up to temperatures higher than by passing into them steam at 100°. The late the Peter Spence at the Exeter meeting of British Association in 1869 called attention to the fact that by simply passing steam at 100 directly into a strong solution of nitrate of soda (other salts will of course answer) it was possible to raise the liquor to its boiling-point, about 121°. Superheated steam is frequently used for heating up liquors in chemical processes on the large scale, but where a slight dilution is no disadvantage, the simpler operation of heating with ordinary low pressure steam might be adopted more generally than it is. Spence used steam in this way for the purpose of extracting sulphate of alumina from alum shales.

G. H. BAILEY.

The Owens College, Manchester, February 22.

Poincaré's "Thermodynamics." PERMETTEZ-MOI de repondre en quelques mots à l'article que M. Tait a consacré à ma thermodynamique, non que je veuille prendre la défense de mon imprimeur, ou réfuter des reproches généraux, contre lesquels ma préface proteste suffisamment.

J'abuserais ainsi de votre hospitalité et de la patience de vos lecteurs; je me bornerai donc à discuter une seule des critiques de M. Tait, et je choisirai celle que ce savant paraît regarder comme la plus importante et qu'il a formulée avec le plus de précision. Je commence par en reproduire le texte :"Even the elaborate thermo-electric experiments of Sir W. Thomson, Magnus, &c., are altogether ignored. What else can we gather from passages like the following?—

666

Si l'effet Thomson a pu être mis en évidence par l'expérience, on n'a pu jusqu'ici constater l'existence des forces électromotrices qui lui donnent naissance.

Rappelons d'abord que, dans l'étude des phénomènes électriques et thermiques qui se produisent au contact de deux métaux, il faut soigneusement distinguer trois choses :

(1) Le phénomène calorifique connu sous le nom d'effet Peltier. Dans le cas d'un métal unique mais inégalement chauffé, le phénomène correspondant s'appelle effet Thomson et se manifeste par un transport de chaleur.

(2) La différence de potentiel vraie ou force électromotrice de

contact.

(3) La force électromotrice apparente ou différence de potentiel entre les couches d'air voisines de la surface de deux métaux.

L'effet Thomson a été mis en évidence par l'expérience. M. Tait croit qu'il en est de même de la différence de potentiel vraie.

Ou la phrase que j'ai citée plus haut n'a aucun sens, ou elle signifie qu'il me blâme d'avoir dit le contraire.

Or cette manière de voir ne soutient pas un instant d'examen. Nous n'avons aucun moyen de mesurer la différence de potentiel vraie.

Les méthodes électrostatiques ne nous font connaître que la différence de potentiel apparente; les méthodes électrodynamiques ne nous font connaître que la somme des forces electromotrices vraies dans un circuit fermé.

Enfin les méthodes indirectes, fondées sur l'écoulement ou sur les phénomènes électrocapillaires, ne sont pas applicables dans le cas qui nous occupe. H. POINCARE.

The Theory of Solutions.

Ir seems that, unfortunately, the period of misconceptions, whose victim the theory of solutions is, has not yet ended. For, after an explanation from my side of the theory of solutions as I understand it, Mr. J. W. Rodger, my critic, asserts (NATURE, p. 342) that it cannot be admitted that a number of exact relationships constitutes a theory." From his further remarks, it must be concluded that he designates by the name theory what I would name a hypothesis, and that, according to him, van 't Hoff's application of the "gaseous laws" to solutions involves the hypothesis that there exists no interaction between the solvent and the dissolved substance.

=

It was therefore in vain that I stated in my letter, in italics, that many properties of the solutions, according to the new theory, "can be treated entirely independently of the question of a possible interaction between the parts of the dissolved substance and the solvent"; it was in vain that I pointed out that all the laws concerning these properties are solely consequences of the one law relating to the volume energy to be gained by making up a solution. This law, whose expression is pu RT, in its various applications to solidification, vaporization, osmosis, &c., of solutions, is the issue of a great many special laws, the whole of which I name the new theory of solutions. Such a complex of laws, grouped around and derived from a main law, is what I call a theory; and if the theory, as in the present case, is everywhere in accordance with experience, the main law is to be regarded as correct. There is nothing of hypothetical nature in this theory, for, if once the main law, pv = RT, is given (by osmotic experiments or otherwise), all the special laws are merely thermodynamical consequences of it. And, I repeat, the main law involves no hypothetical assumption upon the mutual rôle of solvent and dissolved substance, but is solely the condensed expression of a great number of experimental facts Mr. Rodger asks why I did not state clearly in my book that, in my opinion, interactions between solvent and dissolved substance were possible. I can only reply that on suitable occasions I have done so. Besides the sentences quoted by Mr. Rodger himself, I have devoted (pp. 251, 252) half a page to the evidence that considerable interactions take place in salt solutions on dilution. But as the existence of such interactions, as I have shown, is of no consequence in the statement of the general laws, I have treated them as secondary, however interesting they may be as experimental facts, and I am more than ever persuaded by this discussion that I was right in doing so. For I have not written my book for readers prepossessed by some non existing chemical theory of solutions, but for such as wish plainly to learn what is known about solutions.

Similar remarks are to be made as to the definition of solutions as mixtures. Even in the case of interactions, if, e.g., hydrates are formed in a solution, the solution is finally a mixture of the hydrates and the remaining solvent. For the contrary assumption-that the whole of the solvent is combined with the dissolved substance, that, e.g., in a somewhat diluted solution of common salt, there exist compounds, as NaCl + 1000 HOis in such a degree at variance with all known facts that I did not think it worth while to discuss such an idea.

Lastly, Mr. Rodger terms the application of the formula of van der Waals to solutions as in general highly questionable" and as "meaningless," if it is admitted that " something of the nature of a chemical reaction" between solvent and dissolved substance may occur. Mr. Rodger may convince himself from my book that this application is limited to cases in which I do not suppose the occurrence of chemical reactions. The reasons

of his doubts as to the validity of this application I cannot remove, because he has not stated his reasons. But it may be permitted to me to feel some doubts as to the validity of his reasons. For no other than van der Waals himself has taken up this very question, and has discussed (of course much more fully than I was able to do) the application of his formula to solutions, including also the case of interactions between the substances. His papers on this subject are inserted in the Zeitschrift für physikalische Chemie, v. p. 133, and viii. p. 188; and also in the Archives Néerlandaises of 1889 and 1891. Leipzig, February 16. W. OSTWALD.

A Lecture Experiment on Sound.

THE following experiment may be of interest to your readers. A piece of glass tubing is drawn out to a fairly fine point, P, attached by string crosswise to a short lath of wood, w, connected by india-rubber tube to water-tap, and a jet of water directed on to a tambourine, T.

T

W

A tuning fork held in one hand is made to touch the lath held in the other while vibrating, and the whole moved nearer to or further from the tambourine.

At a certain distance the note of the fork will be produced on the tambourine (this of course is not a new experiment). While this was going on, the lath, jet, and fork were slowly moved towards the tambourine, and I was able to sound the octave below.

This showed that at a certain point the vibrations of the fork were not individually capable of separating the fine stream into drops, but that two complete vibrations did so; thus half as many drops per second were set free as there were vibrations from the fork.

The fork gave C = 512; the note on the tambourine was C = 256.

Probably the drops at that stage were of a dumb-bell shapesince at a greater distance the actual note of the fork was produced on the tambourine. REGINALD G. DURRANT. The College, Marlborough, February 13.

The Formation and Erosion of Beaches, &c. As you have more than once permitted me to discuss the problem of sea-waves in your columns, I venture to point out that in your interesting article on Signor Cornaglia's work on sea beaches (p. 362), in your summary of the causes which affect beaches, sand-banks, &c., you have omitted the very important one of wind-raised surface currents. Sea-waves, tidal-currents, and river-currents can be observed, and their effects recorded; but it is the occasional, irregular, and sometimes powerful windraised current, prevalent during storms, which performs such erratic feats, and deludes the unwary observer. For instance, a beach may resist the sea for years, yet in a few hours it may be stripped bare to the solid rock. Shells may be covering the bottom a mile off shore, undisturbed by on-shore gales; a storm, with wind and waves apparently much the same as usual, may sweep them all on shore. One beach will be in kept clear of shells which will be found off 1

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