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ZOOLOGICAL ARTICLES

Contributed to the "Encyclopædia Britannica."

BY

E. RAY LANKESTER,

M.A., LL.D., F.R.S., LINACRE PROFESSOR IN THE UNIVERSITY OF OXFORD.

To which are added Kindred Articles by

W. JOHNSON SOLLAS, LLD, F.R. S.,
Professor of Geology in Trinity Colleg, Dublin.
LUDWIG VON GRAFF, Ph.D.,
Professor of Zoology in the University of Graz, Austria
A. A. W. HUBRECHT, Ph.D., LL.D.,
Professor of Zoology in the University of Utrecht.
A. G. BOURNE, D.Sc.,

Professor of Biology in the Presidency College, Madras
W. A. HERDMAN, D. Sc.

Professor of Natural History in the University College, Liverpool.

THURSDAY, OCTOBER 5, 1893.

THE STUDY OF DIATOMS.

An Introduction to the Study of the Diatomacea. By Frederick Wm. Mills, F.R.M.S. With a Bibliography by Julien Deby, F.R. M.S. (London and Washington: Iliffe and Son, 1893.)

FE

EW forms in the organic world have been the subjects of such close, constant and varied study as the Diatoms. Their minuteness, their exquisite modes of growth, development and multiplication in the living state, and the beautiful refinement of symmetry and delicacy of surface chasing in their dead siliceous remains, have made them the special objects of interest, admiration, and often of serious study and research from certainly the dawn of this century until now. But there are few studies of living objects, at least of those that are extremely minute, that show more clearly that the real difficulties presented by them are understood only by those who thoroughly study them. It is the expert who knows how little is known concerning this most interesting if lowly group.

ing the last ten years some admirable glimpses at the wonderful architecture of these minute siliceous frustules have been obtained, showing that these silicified cases are not merely formed of two symmetrical valves united to one another by means of two embracing rings which constitute the connecting zone or girdle, and making together an elegantly carved box in which the species may be reproduced, but showing also that the most complete structural principles are embodied in their internal and external construction.

These are certainly not complete studies; but they do exactly what the zealous amateur wants: show the paths along which profitable study may be pursued.

66

This will apply with even greater force to the almost new branch of diatom work done in regard to secondary structure" in the siliceous frustule. To those for whom this Introduction could be alone intended, few things could have a larger interest than this.

The nature of the extremely delicate "markings" of diatoms has been so zealously pursued by amateurs and microscopists generally, that it has brought upon them the frequently merited reproach of "Diatomaniacs." None the less it will be by the study of the perforations and physical constitution of the siliceous frustules that we shall ultimately obtain a true knowledge of their modes of

There can be no serious doubt that much of the value that will attach to it as an "Introduction" is due to the very accessible and useful form in which Mr. Julien Deby's "Bibliography relating to Diatomology" has been presented to the student. The work consists of 240 pages; of these only forty-two are devoted to an exposition of the nature and habits of the Diatoms proper. There are three chapters relating to the collecting, the mounting, and the microscopical examination of these forms; but the forty-two pages are supposed to tell us all of importance that is known concerning these beautiful Algæ. Yet the Bibliography is enormous and includes the work and judgments of some of the leading naturalists of our century.

As this volume only aims at being an "introduction" to the study of these organisms, we have no right to anticipate exhaustive treatment in any branch of the subject; but we do not hesitate to affirm that the aim of its author would have been more efficiently reached had certain parts of his subject received a more liberal treat

ment.

No doubt the Bibliography opens to the amateur and the student almost every channel of knowledge, and will prevent him from attempting to repeat work already done, or from exhausting himself on work that it is at present more or less vain to attempt. But it would have been a great advantage to have seen in a concise form much that has been done in recent years.

Thus we find less than three pages devoted to the "Structure" of Diatoms; what is said is interesting and accurate; but, even remembering the aim of the author, we cannot consider it sufficient. It is quite true that no great generalisation of diatom structure has been arrived at; and we venture to think that much time and patient labour must be spent before it will be ; nevertheless, dur

would hardly have been supposed by those who wholly neglect, or even despise the study of the "markings" of diatoms that the wonderful "secondary structure" now demonstrated in many of these frustules had any existence. It may now, however, be taken for granted that every efficient manipulator possessed of a good microscope has demonstrated that, e.g., Coscinodiscus asteromphalus is not only covered on its valves with the beautiful areolæ so long and so well known, but that these areola are in their turn delicately areolated. The coarse areolations so long familiar to us are for the most part approximately circular in outline; but inside these is a most delicately perforated membrane; and that this is related to the functions of the diatom there can be but little doubt.

Again it may be stated that these studies are incomplete; that is so; and, moreover, they require good instruments, and good manipulation of them, for satisfactory results; but we believe that it is such matters that the leisured amateur and the young student are most desirous of knowing in order to find suitable lines for profitable study.

It is true that the very remarkable work of Dr. Flögel on diatom sections, and some of his modes of operation are referred to, but these represent a far higher and more unusual class of research. The most elementary student should know something concerning them, and they are wisely referred to in this volume; but they do not compensate for the absence of efficient reference to the class of work we allude to.

The movement of diatoms receives careful treatment in this treatise; we believe, nevertheless, that more recent results might with profit have been referred to. The subject is in many senses one of the most difficult in the range of Biology. The three principal explanations, viz. endosmatic and exosmatic currents, the pre

sence of cilia, and the existence of a pseudopodic extrusion of hyaline protoplasm, are carefully given. The author wisely inclines to the last. It is certain that one of the results of the use of apochromatic objectives during the last three or four years has been to enable us to demonstrate that not only are there perforations in the siliceous tests of the diatoms, but that in the raphe of some Naviculæ and kindred forms, there is a "great" perforation, which runs tube-like from the apices of the frustule to the central nodule; and this may be readily seen to lend itself to the pseudopodic extrusion and withdrawal of protoplasm; and we commend the study of the possibility of this to microscopists. Delicate stains may be used that will not immediately destroy the organism, and that will tend to make the "hyaline protoplasm" at least more manifest. But in this connection the work of Bütschli and Lauterborn cannot be neglected,

Making Pinnularia nobilis the subject of research, they specially directed attention to its mode of motion. The motion in diatoms is of a peculiar kind, being frequently a series of jerks which carry forward the frustule in the direction of its length, and often carry it back along the same path. Yet the motion may be smooth and equable.

Bütschli conceived the idea of placing under the thin covering glass, laid upon the top of the water in which he was microscopically studying the Pinnularia, a minute drop of Indian ink. This in its ultimate particles is, of course, not soluble. Its extremely fine granulation was therefore of great value, for by means of the enormous multitude of these black granules he affirms that he was able to see an extremely fine thread, which was directed backwards. This, he contends, was a protoplasmic filament, but so fine, and, as we apprehend, so near in its refractive index to that of water, that it is otherwise invisible.

This filament, it is stated, is formed by jerks, and the diatom was simultaneously moved in the opposite direction; while at times the filament appears to be retracted.

That these results are of value, there can be no doubt, and they open a line of study that may be most profitable.

Mr. Mills has adopted the method of classification for the Diatomacea which for the present may fairly be considered the best; but we can but fervently hope that a series of detailed discoveries will at no very distant date make such generalisation possible as will superinduce a great simplification in this direction.

There is a very useful chapter on Mounting Diatoms, and some excellent teaching on the microscopic examination of these forms; and the whole is rendered complete by a chapter that will greatly aid the beginner, on "How to Photograph Diatoms."

We welcome this book; it will occupy a distinct place in the literature of the subject in our language at present, and will, we hope, make the way for a greatly enlarged and amplified second edition. There is much to praise in the volume, and what we have endeavoured to point out as deficiencies we do not treat as defects. The subject is so large that an author may well pause and wonder at what point an "Introduction" to such a subject should

halt in details. But we think that what has been given will open the way for very much more, and hope that Mr. Mills may be called upon and induced to provide it. We note some printer's errors in the book. It will suffice to call attention to page 6, where a period at the end of the second line destroys the sense; to the word rhizopodo" for "rhizopodia" on page 13; to the wrong spelling of an author's name, as in the foot-note on page 5. and to a reference to northern microscopic for "northern microscopist" on page 159. D.

THE PROPAGATION OF ELECTRIC ENERGY. Untersuchungen über die Ausbreitung der Electrisches Kraft. Von Dr. Heinrich Hertz. Pp. 295. (Leipzig: Johann Barth.)

A DISCOVERER'S own account of his work is always of interest, and when it is an epochmaking work and the account so clear and well described as to be intelligible to all, it deserves the most carefu attention, and should be studied by all who feel any interest in the subject. Dr. Hertz's account of his discovery of the propagation of electric energy is eminently a work of this kind. The subject is of immense importance; the work described is of the highest order of experimental investigation; the results attained have contributed more than any other recent results to revolutionise the view taken by the majority of scientific workers as to the nature of electromagnetic actions. It is to be hoped that a translation of this account of one of the greatest advances in our knowledge of nature will soon be in the hands of all who care to learn how the functions of the ether have been raised from obscurity into light, from being in the opinion of many a pioss belief to be the momentous question of the hour. Prof Hertz gives in his introduction an interesting account of the steps by which Maxwell's theory may be connected with the older theories. These latter supposed action at a distance pure and simple, and postulated two fluids, &c., &c. They neglected the intervening medium. The second step was to introduce the medium as performing some function when it was a material medium, but still to retain the positive and negative electricities acting across the space from molecule to molecule. This was practically Mossotti's theory as to the properties of the dielectric founded on Poisson's theory of magnetic induction. M Poincaré seems to have got to about this stage, or per haps a little further. The third stage was to transfer the molecular action to the ether, but still to consider it as due to electrical fluids attracting and repelling one another, producing the etherial stresses. The fourth stage was to see that these attractions and repulsions of electrical fluids are quite superfluous, and to attribute the whole phenomenon to stresses in the ether set up by straining it. In this last stage there is no room for an electrical fluid with attracting and repelling properties, and accordingly it is suppressed. What the structure of the ether may be which is strained, and thereby electromagnetic stresses produced, is still unknown, and consequently the nature of the strain is unknown. It certainly differs from the ordinary straining of a solid in two im

portant respects. In the first place, the mechanical stresses are proportional to the squares of the quantities that represent the strains; and in the second place, they depend on the absolute strain, and not on the relative displacement of the parts of the medium. Solid structures can be invented that have laws of this kind. The change of longitudinal stress in a stretched string is proportional to the square of the transverse displacement, and, if the ends of the string are fixed, this stress depends on the absolute value of the displacement. Upon a foundation of a somewhat similar kind a theory as to the structure of the ether being like a solid in tension may be founded, which gets over many of the difficulties of the simple elastic solid theory of the ether. We are, however, still a good way off any really satisfactory theory as to the structure of the ether, but the leading idea of Maxwell's theory, that electromagnetic attractions and repulsions are due to some sort of strain in the ether, is the direction in which scientific men are at present seeking for a dynamical explanation of electromagnetism and for a structure of the ether. Prof. Hertz, however, seems content to look upon Maxwell's theory as the series of Maxwell's equations. This is hardly fair. Maxwell has done much more than produce a series of equations that represent electromagnetic actions. Weber and Clausius went very close to that without revolutionising our ideas as to the nature of these actions. Any exposition of Maxwell's theory which does not clearly put before the reader that energy is stored in the ether by stresses working on strains, is a very incomplete representation of Maxwell's theory. The bulk of Prof. Hertz's work is, however, not concerned with any theory, but with the practical study of electromagnetic propagation along conducting wires and throughout space.

This

is the work for which Prof. Hertz is so justly famous, and on account of which Hertzian oscillators, Hertzian receivers, Hertzian waves have become in the few years since 1888 the objects of universal attention. No physical experiments since those by which Joule founded the theory of the conservation of energy have produced as great an effect on science as these experiments here described by their author. The subject is brought down to last year, and the experiments of others are mentioned and discussed. In this connection it may be worth while remarking that the observation that the waves emitted by a Hertzian oscillator are of all sorts of wave-lengths was clearly stated by Prof. Hertz himself when he explained how rapidly they died out. For what is a rapidly dying out oscillation except a Fourier series of all sorts of waves? There is consequently no essential difference between these two statements. The first states more than the second, for it explains the character of what in the other statement is described by the vague term, "all sorts of

waves."

The whole work is most interesting, and well deserves the best attention of all interested in the greatest scientific advance of the last quarter of the nineteenth century, a century that has seen thermodynamics founded by Carnot and Clausius, conservation of energy by Joule, bacteriology by Pasteur, the origin of species. by Darwin, and the functions of the ether by Faraday, Maxwell, and Hertz.

OUR BOOK SHELF.

Helps to the Study of the Bible. By Henry Frowde. (London, 1893)

THE publisher of this useful volume of Helps is to be congratulated on the production of a work which is far in advance of any other book of the same kind published in England. It consists of six parts, which comprise a brief history of the Bible and its most ancient versions, including terse remarks on its canon and authenticity; a summary of the contents of the books of the Old and New Testaments; an account of the Apocrypha, together with historical and chronological notices of the period; a series of chapters on the history, geography, geology, botany, zoology and ornithology of the country of Palestine, on the Jewish Calendar, weights, measures, money and time, and on the musical instruments of the Bible; and a concordance, atlas, list of obsolete English words, glossary of antiquities and customs, &c., referred to in the Bible. The book represents the collected learning of many eminent specialists and scholars, arranged in a handy form and most convenient for reference. The evidence relating to Bible history which may be derived from the recently established sciences of Assyriology and Egyptology, is illustrated by a series of beautiful plates, which cannot fail to be appreciated by every thoughtful reader of the Bible, and are worth more for purposes of explanation than many dissertations could ever have been. In the first plate the connection of the Hebrew alphabet with the hieratic writing of Egypt is shown, and from this we are led to the Latin and Greek alphabets and to the Rosetta and Moabite Stones. Facsimiles of the oldest Hebrew and Syriac MSS. of the Bible are next given, together with specimens of the text of the Vaticanus, Sinaiticus and Alexandrinus codices. The funeral customs of the Egyptians are explained by reproductions from bas-reliefs, papyri, &c., and from the monuments of Assyria and Babylonia a large number of important illustrations have been selected to throw light upon the various occasions upon which the Israelites came in contact with referred to in the New Testament, and the Temple of the "great king." The busts of the Roman emperors Diana, are the subjects of the plates inserted to illustrate the New Testament. At the foot of each plate is a brief description, which, we must hope, may in some cases be lengthened in future editions of this excellent book.

Differential Calculus for Beginners. By Joseph Edwards, M.A. (Macmillan and Co., 1893.) MR. EDWARDS has put together in a handy form for schoolboys the elementary parts of his large treatise on the Differential Calculus. The subject is here presented in a clear and interesting manner for beginners, and it is to be hoped that the book will be useful in leading to a more general study of this indispensable subject than has hitherto been customary in this country.

The French schoolboy learns the elementary ideas as part of his Algebra, but with us it has been thought right that "calculus dodging" should precede the study of the calculus itself, under a mistaken application of the proverb-Principiis enim cognitis, multo facilius extrema intelligetis.

Geometrical applications are very judiciously introduced at an early stage, but considering that the first differential coefficient invented was for the expression of a Velocity, these applications would be rendered more instructive by the introduction of the notion of Time as the primary independent variable.

But "this is Dynamics" the schoolmaster will say, and so must be kept separate by a sort of water-tight bulkhead. G.

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 Thieving of Assyrian Antiquities.

I. HAD I known that after having dissected my reply to the article entitled "Thieving of Assyrian Antiquities," which appeared in NATURE of the 10th ultimo, you had intended to add further objectionable remarks to it, I should have certainly declined to have had it published.

2. You seem, even now, to ignore the judgment of the High Court of Justice in the slander case of "Rassam v. Budge,' and volunteer your own version of the story with which you have been supplied.

3. May I ask where you have found it reported about the evidence of the British Museum accountant and Sir Henry Rawlinson's deposition regarding the fragments of the national collection? If you have obtained your information from the latter's deposition that was certainly not revealed in the Press, and if it was supplied you by men who had no business to do so, then in fairness you ought to have quoted the other parts of the evidence. As for the "accountant, no paper reported what the Principal Librarian wanted him to say, and that was for a very good reason, because the Judge did not consider his evidence of any use, seeing that no one had disputed the purchase by the authorities of the British Museum, of Babylonian antiquities before I began my researches in Southern Mesopotamia, at the time when I was there and afterwards.

4. With regard to the cock-and-bull story about the basreliefs which are alleged to be at "Comford Hall," if you had said in your article, above referred to, that they existed in a private house in England, instead of asserting that they were obtained by purchase, I would have surprised you with further revelations that such "slabs" do exist in other houses in England and in different parts of Europe and America. Even half of the sculptures I had discovered in Assur-beni-pal's palace in 1853, belonging legitimately to the national collection, have been squandered, and part of them are now in the bottom of the Tigris.

5. As you seem to have allowed yourself to be imposed upon by malicious men who are not brave enough to put their names to the information with which they have supplied you, I must now close my correspondence, as it seems to me that your journal is not a proper channel through which justice can be obtained.

H. RASSAM.

6, Gloucester-walk, Kensington, W. September 23. [THE above letter calls for some additional "remarks." We trust Mr. Rassam will find them less "objectionable" than the former ones.

1. The dissection to which reference is made consisted only of omissions of personal attacks, not even courteously worded, which moreover had nothing to do with the question of importance to the public.

2. Mr. Rassam is not happy here in his expressions. Nothing was stated in our article which was not openly stated in Court.

3. He is still less happy here. In his last letter he wished to make our readers believe that Sir H. Rawlinson's opinion on the "rubbish Mr. Rassam had sent home was not stated in Court, and had been obtained by us in some improper way from the British Museum. In our "objectionable remarks" charitably suggested that he had forgotten Sir H. Rawlinson's deposition containing this opinion was read in Court. It now seems that Mr. Rassam had not forgotten it in the least.

we

With regard to the accountant; the counsel for the defendant did say what the accountant was to prove, and the Editor does not see what the Principal Librarian had to do with it.

4. Why does Mr. Rassam take the trouble to misquote us by writing "Comford" instead of "Canford," and then to put his misquotation in inverted commas? The "story of a cock and bull," which we took from one edition of Murray's Guide is re

the more reason there is for the inquiry to which we hire pointed.

5. Requires no comment except that not a single maceracy on our part has been established.-ED. NATURE.]

Vectors and Quaternions.

I WISH to make some observations in reply to the letter of Prof. Knott which appeared in NATURE (June 15, p. 145 For my part I have nowhere condemned the system of Ham.. ton and Tait as unnatural" and "weak"; on the contrary, I have always spoken of it with respect and admiration. T appreciate its value and high place in analysis it is not rece whether my work is mere innovation and a recasting of qua sary to be blind to its imperfections and limitations. As in ternion investigations, I leave to the judgment of those who read my papers. I wish merely to remark that Prof. Knott says nothing about exponentials, and that he has not pointed out what quaternion investigations are recast in my paper on 'The Fundamental Theorems of Analysis Generalised for Space." It is the duty of a critic to state correctly and fully the principles which he criticises; this has not been done; my position has been misrepresented. It may aid the scientific discussion of this matter if I state briefly the principal positions I have taken, and the replies that have been given.

I have said that the quaternion notation can be improved. As regards notation, Hamilton himself was an innovator, 1 in his writings he apologises for the introduction of the strange symbols S, V, T, K, U, I, &c. My aim has been to generalise as much as possible the notation of ordinary analysis, as it is desirable to have one harmonious algebra, with easy transition from line algebra to plane algebra, and from plane algebra to space algebra. Prof. Tait himself has said in one of the prefaces to his treatise that a revolution in the matter of notation must ultimately come; but I infer from the ecstacy of his ai miration, that Prof. Knott considers it part of the original brightness of the Archangel.

I have said that the quaternion definitions are not all that can be wished for; I have pointed out what appear to be defects, and I have attempted to remove them. According to Prof Knott, "the quaternion originally defined as the quotient of two vectors, can also be represented as the product of two quadrantal versors." I reply that what is wanted is not an origina or temporary definition of " 'quaternion," but one that wi stand throughout; that in strains we have a quotient of twa vectors which is not a quaternion, but a dyad; that we do not ask for a representation, but a definition; and that the repre sentation indicated involves the idea of a versor, which, leaving out a mere multiplier, is the very thing to be defined. Farther the following questions may be asked: If by a quaternion meant the quotient of two vectors, how can the product of tw vectors be a quaternion? We have also the nice distinction that a quaternion may be represented by the product, but n by the quotient, of two quadrantal versors. It is certain tha the product and the quotient of two quadrantal versors are quantities of the same kind; if the one is a quaternion, 20 a

the other.

"

I have said that some of the fundamental principles of qua ternions require to be corrected, especially the one whics identifies versors with vectors. I have said that if a denote a unit-vector, then a2 1, not = 1. It is not a bare assertion that to my mind it appears so; a reason is given. Let a body of mass, m, have at any time a linear velocity whose rectangular components are a along the axis of i, 6 along, and calong; the kinetic energy of the body is (ai + b) + ek -, that is, m (a+b+c), not as quaternionists would have k - m{a2 + b2 + c). The convention involved is one that per vades the whole of analysis, namely, that the product of two lines having the same direction is positive, while the product of two lines having opposite directions is negative. As kinetic energy is a square, the two lines must always have the same direction.

I have said that if a denote a quadrantal versor, then

not to vectors.

I, and that Hamilton's rules apply to versors, Prof. Knott says that I advocate a system which loses the associative principle and gains nothing but a positive sign and an undesirable complexity in transforming by permutations. Readers of NATURE will be surprised to learn