the latter a signed schedule, the advantage of the professor and not the advancement of the student being the point considered. During the last few years a reaction has been setting in against this perpetual lecturing, and the number required to be attended has been considerably reduced. The University of London deserves the credit

out the metropolis. In a future article we shall suggest what appears to us a desirable and practical scheme for medical education.

THE LITERATURE OF CHEMISTRY HE of the April number of the "Journal of

tem, by requiring attendance on only one or two courses, and this rather as evidence of the student being really engaged in the study of medicine than for any other purpose, leaving him free to acquire his information as best he can, but testing its extent and value by a searching examination.

No doubt many of the posts above alluded to are filled by men of great talent and ability, but their powers are crippled by the small means at their disposal, which prevents many illustrations or experiments from being exhibited which are almost essential for thorough teaching.

As a means of improving the system of education by supplying a better class of lectures on some subjects than those at present given, and at the same time obtaining better remuneration for the lecturers themselves, a scheme has recently been advanced by which it is proposed that certain medical schools in the metropolis should be amalgamated, a reduction in the number of lecturers being thus effected, whilst the pecuniary value of those that remain will undergo considerable augmentation. It is hoped that the value of these posts would then be sufficient to lead to their being accepted not by those who only use them as a steppingstone for advancement, but by gentlemen who have devoted themselves exclusively to the study of the department of science on which they lecture.

At the present moment the lectureships in several of the smaller schools yield such small returns to their holders as would astonish many of their hearers. As a matter of fact we could mention an instance where the proceeds of an entire summer course of lectures has amounted on the average for the past three years to a sum not exceeding 61. Can this for a moment be regarded as in any way proportionate to the intellectual labour, the time, and the money expended in their preparation, illustration, and delivery? It might be considered to be a moderate recompense for one lecture, but as payment for a course it is simply monstrous. Is it surprising that the lectures are often given without animation, and listened to without interest?

By amalgamating several schools, however, such chairs might, it is hoped, be so far increased in value as not only to lead men of high ability, and distinguished for their knowledge in particular branches of science, to accept them, but to provide ample funds to admit of their copious illustration, and for the purchase of expensive apparatus apparatus which the smaller schools now find it difficult or impossible to procure. It would not be difficult, we imagine, to find room for those who at present hold appointments as demonstrators, with lighter but not less important duties than they have hitherto performed. At all events it seems to us that the amalgamation scheme, if fairly carried out, would prove the most splendid example of the Conservation of Force with which we are acquainted, and on that ground alone should receive the cordial support of the medical teachers through

of a new era in English Chemical Literature, containing, as it does, besides the papers which have been read before the Society, the first instalment of the promised "abstracts." The papers selected for this purpose by the accomplished editor are ninety-one in number, comprising every branch of Chemical Science, Technology included, and are classified under six various headings, as "Physical Chemistry," "Inorganic Chemistry," &c. The abstracts themselves, made by the gentlemen whose names appear on the wrapper of the journal, are naturally of different degrees of literary merit, but seem to be carefully and conscientiously done; all the points of essential importance in the original papers being retained. The reader will thus not only have a good general notion of the extent of the researches made by any particular author, but also be able to repeat any of the experiments, or prepare any of the substances from the directions given. These abstracts are therefore really what they profess to be, and not merely notices of a few lines in length, from which but little more information can be gleaned than from the title of the paper.

The Council of the Chemical Society is to be congratulated on the energetic way in which it has endeavoured to supply a great defect in our scientific literature, by affording us the means of obtaining a general view of the progress of Chemistry both here and on the Continent. Chemists have hitherto had to depend chiefly on Will's "Jahresbericht," which, although useful in its way, has the double disadvantage incident upon its method of arrangement, first, in not being published until long after the end of the year, and, secondly, of being rather a résumé of the chemical work done, than a condensed account of particular researches. There is no doubt that these abstracts, if furnished with a full and comprehensive index, both of the subject-matter and the names of the authors, will become a standard work of reference, not only here but on the Continent.

It is to be hoped that other Scientific Societies will be induced to follow the example of the Chemical Society, and, by publishing abstracts of all papers connected with their particular branch of science, give an impetus to its cultivation, and render a knowledge of its general progress easily attainable. The value of such abstracts is greater than might at first sight appear; for the study of Science, both for its own sake, and in its application to the Arts, is extending so rapidly that it requires a considerable expenditure of time to acquire a knowledge of the numerous researches and discoveries which are now being made in any particular science, and leaves but little for the study of the sciences allied to it. If, then, each of the learned societies were to publish abstracts similar to those of the Chemical Society, it would render it comparatively easy for the workers in any one department of science to acquire something more than a superficial knowledge of the discoveries made in the others.

the invitation "Try Lapland" fails to stimulate the jaded nerves of the zealous explorer of "fresh fields and pastures new." In the realms of air, however, there is still plenty of new ground, if we may be allowed the Hibernicism. Mr. Glaisher and the illustrious French trio can claim this field as almost exclusively their own, though, doubtless, they will not long be left in undisturbed possession of it. After a brief history of the rise and progress of aërostatics in England, Mr. Glaisher here recounts to us the particulars of ten of his most remarkable ascents; and the Frenchmen then follow suit. The volume is got

up in drawing-room style, as a veritable livre de luxe; we wish we could transfer to our pages some of the beautiful chromo-lithographs by which it is illustrated, in particular, the wonderful mirage and luminous aureole which serves as frontispiece, and the falling stars as observed from the balloon, at p. 262. We must, however, content ourselves with two or three of the scarcely less effective woodcuts.

The scientific information contained in the volume is important, though rather as showing how little we know

at present of even the fundamental principles of Meteorology, than as establishing any new laws. With regard to temperature, Mr. Glaisher remarks that the decrease as we ascend is far from constant, and we must entirely abandon the theory of a decline of 1° of temperature for every increase of 300 ft. of elevation. With reference to the colour of the sky, he states that, as viewed from above the clouds, it presents a deep blue colour, which deepens in intensity with increase of elevation regularly from the earth if the sky be free from clouds, or

with the increase of elevation above the clouds if they be present, and on this subject he gives the following laws:

"The azure colour of the sky, though resembling the blue of the first order when the sky is viewed from the earth's surface, becomes an exceedingly deep Prussian blue as we ascend, and, when viewed from the height of six or seven miles, is a deep blue of the second or third order. 2. The maximum polarising angle of the atmosphere, 45°, is the same as that of air, and not of water, which is 53. 3. At the greatest height to which I have ascended, namely, at the height of five, six, and seven miles, where the blue is the brightest, the air is almost deprived of moisture. Hence it follows that the exceedingly deep Prussian blue cannot be produced by vesicles of water, but must be caused by reflection from the air, whose polarising angle is 45°. The faint blue which the sky exhibits at the earth's surface is therefore not the blue of the first order, but merely the blue of the second or third order rendered paler by the light reflected from the aqueous vapour in the lower regions of the atmosphere.

Humorous incidents occur here and there; as when the whole apparatus is taken by the French peasantry for "le diable" himself, or when the travellers approaching the earth are required by too zealous gensdarmes to show their passports! And the adventures are not without their serious attendant dangers. More than once the diminished pressure and the intense cold produced so great a numbness and tendency to sleep that it has required the greatest presence of mind for all control over the balloon not to be lost-and for ever. Life and limb were also not unfrequently endangered by the too sudden descents, sometimes to escape the imminent peril of an involuntary dip into the sea. Fig. 2 depicts the manner in which the "Swallow," having Tissandier and de Fonvielle on board as passengers, was dragged along the ground by a furious gale, and both those eminent aëronauts were considerably hurt and in danger of losing their lives.

There is not much contribution in the volume to the mechanics of aërostation, and that mostly from the French

To appreciate all the beauty of cloud scenery when the air is loaded with moisture, an aërial voyage must be made on an autumn morning before sunrise, when the atmosphere is charged with the vapours of night."

Clouds were frequently met with to a height of 20,000ft. or nearly four miles, and heavy rain at almost as great an altitude; and on one occasion, while descending, rain fell on the balloon at a height of three miles, and then for the next 5,000ft. lower, it passed through a beautiful snowy scene; there were no flakes in the air, the snow was entirely composed of spiculæ of ice, of cross spiculæ at angles of 60°, and of an innumerable number of snow crystals, small in size, but of distinct and well-known forms easily recognisable as they fell and remained on the coat. The drawings show many a beautiful scenesunrise from a balloon, moonlight effects, a lunar halo, the shadow of a balloon on the clouds, sometimes surrounded by an aureole, though, perhaps, none more remarkable than the mirage represented in our first illustration.

" BALLOON

F.G. 3-THE VALVE OF THE "ENTREPRENANT contributors. We have drawings of the weighing machine and pulleys of the great Captive balloon of Chelsea, and Fig. 3 represents the valve of the "Entreprenant" balloon from which M. de Fonvielle attempted to take photographs of an eclipse of the moon.. The book is one which will doubtless find a large circle of readers, and will greatly increase the public interest in aërostatics.

OUR BOOK SHELF

A Text-Book of Elementary Chemistry, Theoretical and Inorganic. By George F. Barker, M.D. (New Hayen. Conn. C. C. Chatfield and Co., 1870, pp. 336.) THIS little book is evidently the result of much labour on the part of the author, and cannot fail to be of much value to students of chemistry. In the preface a list of books is given of which the author has made free use; chemists are to be found in the book; but though it canconsequently the peculiarities of the systems of many not be said that any school has been followed, yet all are more or less represented. The prevailing ideas are that

each element has a definite combining power or equivalence, and that the arrangement of atoms in compounds is of as much importance as their kind or number. This work is remarkable for the conciseness of its definitions; one of the first is on chemical and physical changes, in which it is said that "physical changes in matter are those which take place outside the molecule; they do not affect the molecule itself, and therefore do not alter the identity of the matter operated on. Chemical changes take place within the molecule, and hence cause a change in the matter itself." Some of the definitions would not, however, find general acceptance; thus, an acid molecule is said to be "one which consists of one or more negative atoms united by oxygen to hydrogen ;"-a definition which excludes hydrochloric acid and its analogues. And a saline molecule is defined to be one containing a "positive atom or group of atoms, united by oxygen to a negative atom or group of atoms," which removes sodic chloride from the list of salts. The term base is confined to the hydrates of positive elements or groups of elements, and the hydrates of the metals calcium, zinc, and iron are sometimes called calcic base, zincic base, ferrous base, and ferric base. The nomenclature of the acids is systematised, but peculiar names are the result: an ortho-acid is one containing as many atoms of oxygen and hydrogen as is equal to the equivalence of the negative atom or group; and a meta-acid is derived from an orthoacid by the subtraction of molecules of water, thus orthophosphoric acid would be P (OH), metaphosphoric acid (PO)" (OH), and dimetaphosphoric acid (PO)' (OH).

These names and those of most other acids are liable to some misunderstanding, as the compounds they represent have long been known by other designations. The theoretical part of the book contains chapters on elemental molecules and atoms, compound molecules, volume relations of molecules, and stoichiometry. The part on inorganic chemistry is divided into eleven chapters, on hydrogen, the negative monads, dyads, triads, boron, negative tetrads, the iron group, positive tetrads, triads, dyads and monads, thus treating of the elements according to their electro-chemical characters, commencing with the most negative. Each chapter is divided into sections containing the history, occurrence, preparation, and properties of the elements, and is followed by a series of questions intended as exercises for the students, a method now much adopted, and found to be of great assistance to teachers. This book is another of the evidences of the rapid progress of pure science in America.

Czermak's Electric Double Lever. (Der Electrische Doppelhebel, von F. N. Czermak.) (Leipzig: Engelmann. 1871. London: Williams and Norgate.) A DESCRIPTION of a most ingenious little contrivance for marking the exact moment in which a movement begins or changes its direction. The old arrangement, by which a lever, forming part of a circuit, comes, when set in motion, in contact with a fixed point connected with the other part of the same circuit, and so closes the circuit and makes a signal, is modified by Prof. Czermak as follows. The fixed contact point is replaced by a secondary lever, whose axis of revolution is the same as that of the primary lever. This secondary lever bears at one end a contact point. The primary lever touches in its swing this contact point, and so closes the circuit; it then pushes the secondary lever before it, but having reached the limit of its oscillation, leaves the secondary lever at rest in a position marking the farthest point of the excursion. A counter contact-point, however, on the other arm of the primary lever (where the lever is a double-arm one; with single arm levers, a special arrangement is introduced), as the primary lever is returning into position gives to the secondary lever a movement in the same direction. Thus the two levers are continually following each other, making and breaking contact. The instrument is in this way

made capable of being used for signalling all manner of movements. It is impossible fully to explain its construction in a few lines, and we therefore refer the reader to the pamphlet itself, which, we should say, is published in celebration of the Jubilee of the great Leipzig Professor, Ernst Heinrich Weber. By the invention of his delightful "Rabbit Holder," Czermak has endeared himself to every physiologist, and we may well share his hope that this new double lever will be found no less useful. M. FOSTER

LETTERS TO THE EDITOR

[The Editor does not hold himself responsible for opinions expressed by his Correspondents. No notice is taken of anonymous communications.]

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Pangenesis

NATURE, that the views contradicted by my experiments, pubIT appears from Mr. Darwin's letter to you in last week's lished in the recent number of the "Proceedings of the Royal Society," differ from those he entertained. Nevertheless, I think they are what his published account of Pangenesis (Animals, &c., under Domestication, ii. 374, 379) are most likely to convey to the mind of a reader. The ambiguity is due to an inappropriate use of three separate words in the only two sentences which imply (for there are none which tell us anything definite about) the habitat of the Pangenetic gemmules; the words are "circulate," "freely," and "diffused." The proper meaning of circulation is evident enough-it is a re-entering movement. Nothing can justly be said to circulate which does not return, after a while, to a former position. In a circulating library, books return and are re-issued. Coin is said to circulate, because it comes back into the same hands in the interchange of business. A story circulates, when a person hears it repeated over and over again in society. Blood has an undoubted claim to be called a circulating fluid, and when that phrase is used, blood is always meant. I understood Mr. Darwin to speak of blood when he used the phrases "circulating freely," and "the steady circulation of fluids," especially as the other words "freely" and "diffusion" encouraged the idea. But it now that by circulation he meant "dispersion," which is a totally different conception. Probably he used the word with some allueddying, not necessarily circulating, currents. sion to the fact of the dispersion having been carried on by Next, as to the word "freely." Mr. Darwin says in his letter that he supposes the gemmules to pass through the solid walls of the tissues and cells; this is incompatible with the phrase "circulate freely." Freely means "without retardation" as we might say that small fish can swim freely through the larger meshes of a net ; now, it is impossible to suppose gemmules to pass through solid

seems

tissue without any retardation. "Freely would be strictly applicable to gemmules drifting along with the stream of the blood, and it was in that sense I interpreted it. Lastly, I find fault with the use of the word "diffused," which applies to movement in or with fluids, and is inappropriate to the action I

have just described of solid boring its way through solid. If Mr. Darwin had given in his work an additional paragraph or two to a description of the whereabouts of the gemmules which, I must remark, is a cardinal point of his theory, my misapprehension of his meaning could hardly have occurred without more hesitancy than I experienced, but I certainly felt and endeavoured to express in my memoir some shade of doubt; as in the phrase, p. 404, "that the doctrine of Pangenesis, pure and simple, as Í have interpreted it, is incorrect."

As I now understand Mr. Darwin's meaning, the first passage minute (ii. 374), which misled me, and which stands: " which circulate freely throughout the system " granules should be understood as "minute granules which are dispersed thoroughly and are in continual movement throughout the system;" and the second passage (ii. 379), which now stands: "The gemmules in each organism must be thoroughly diffused ; nor does this seem improbable, considering the steady circulation of fluids throughout the body," should be understood as follows: "The gemmules in each organism must be dispersed all over it, in thorough intermixture; nor does this seem impro bable, considering ... the steady circulation of the blood, the continuous movement, and the ready diffusion of other fluids, * NATURE, vol. iii. p. 502.

and the fact that the contents of each pollen grain have to pass through the coats, both of the pollen tube and of the embryonic sack. (I extract these latter addenda from Mr. Darwin's letter.) I do not much complain of having been sent on a false quest by ambiguous language, for I know how conscientious Mr. Darwin is in all he writes, how difficult it is to put thoughts into accurate speech, and, again, how words have conveyed false impressions on the simplest matters from the earliest times. Nay, even in that idyllic scene which Mr. Darwin has sketched of the first invention of language, awkward blunders must of necessity have often occurred. I refer to the passage in which he supposes some unusually wise, ape-like animal to have first thought of imitating the growl of a beast of prey so as to indicate to his fellow monkeys the nature of expected danger. For my part, I feel as if I had just been assisting at such a scene. As if, having heard my trusted leader utter a cry, not particularly well articulated, but to my ears more like that of a hyena than any other animal, and seeing none of my companions stir a step, I had, like a loyal member of the flock, dashed down a path of which I had happily caught sight, into the plain below, followed by the approving nods and kindly grunts of my wise and most-respected chief. And I now feel, after returning from my hard expedition, full of information that the suspected danger was a mistake, for there was no sign of a hyena anywhere in the neighbourhood. I am given to understand for the first time that my leader's cry had no reference to a hyena down in the plain, but to a leopard somewhere up in the trees; his throat had been a little out of order -that was all. Well, my labour has not been in vain; it is something to have established the fact that there are no hyenas in the plain, and I think I see my way to a good position for a look out for leopards among the branches of the trees. In the meantime, Vive Pangenesis. FRANCIS GALTON

The Hylobates Ape and Mankind

THE readers of Mr. Mivart's communication in NATURE for April 20, on the affinity of the Hylobates genus of ape to the human species, may be interested to learn that the fact was well known to the author of the Ramayana, the earliest Sanscrit epic, probably contemporaneous with the Iliad. In this poem the demigod Rama subdues the demon Ravana, and regains his ravished bride Sita by the assistance of a host of apes, which may be identified with Hylobates Hoolook. The human characteristics of these semi-apes, their gentleness, affection, good humour, sagacity, self-importance, impressionability, and proneness to melancholy, are portrayed with the most vivid strokes, and evidently from careful observation. See Miss Frederika Richardson's charming volume, "The Iliad of the East," a selection of legends drawn from the Ramayana. (Macmillan and Co., 1870.) April 27 R. G.

Tables of Prime Numbers

WHEN a number is given, and it is required, without the aid of tables, to find its factors, there is not, I believe, any other method known except the simple but laborious one of dividing it by every odd number until one is found that measures it, and if the number should be prime, this can only be proved by showing that it is not divisible by any odd number less than its square root. Thus to prove that 6966c07 is prime, it would be necessary to divide it by every odd number less than 2639, and even if a table of primes less than 2639 were at hand, about 380 divisions would be requisite.

On the other hand, there are few tables which are more easily constructed than tables of divisors, and it is the extreme facility of a systematic tabulation compared to the labour of isolated determinations, which has led to the construction of such elaborate tables on the subject as have been produced.

The principal tables are Chernac's, which give the factors of numbers from unity to a million; Burckhardt's, which extend as far as three millions, and Dase's, which form a continuation of Burckhardt's, and extend to ten millions.

The mode of formation of these tables was extremely simple. By successive additions, the multiples of 3, 5, 7, 11, 13, 17. were formed up to the limit to which the table was intended to extend; this gave all the numbers having these numbers for factors, and the primes were recognised from the fact of their not occurring as multiples of another prime less than themselves.

Practically the work was rendered even simpler by mechanical means; thus, forms were printed containing, say, a thousand

squares, and in these were written consecutive thousands of odd numbers in order; one number in each square, room being left for its divisors, if any, in the square. A pair of compasses was then taken and opened a distance corresponding to the prime whose multiples were to be obtained; for example, in marking the multiples of seven, the compasses were opened the width of seven squares, and then " stepped" along the lines starting from 7, thereby marking the numbers 7, 21, 35 and the number

7 was written in each of the squares in which a leg of the com. passes fell. When the factor was large it was more convenient to form a separate table of its multiples, and enter it in the square corresponding to the latter. Many simplifications were introduced in the details of the construction; for instance, Burckhardt had a copper plate engraved with 77 (=7x 11) squares one way and 80 the other; by this arrangement the multiples 7 and II, which were of the most frequent occurrence (for all multiples of 2, 3, and 5 were rejected from the tables), occupied the same place on each sheet, and he was thus enabled to engrave the numbers 7 and 11 on the plate, so that these numbers were printed in all the squares containing the numbers they measured.

Dase, who originally applied himself to the construction of the tables at the suggestion of Gauss, left behind him in manuscript at the time of his death, in 1862, the seventh and part of the eighth million complete, besides a considerable portion of the ninth and tenth millions. The seventh, eighth, and ninth millions were completed by Dr. Rosenberg, and published by a committee at Hamburgh. In the preface to the ninth million (1865), which is the last I have seen, it is stated that the tenth million, which was nearly ready, was the last the committee intended to publish.

My object in writing this letter is not only to call attention to a most valuable series of tables, which seem to have scarcely excited so much interest as they deserve, but also to ask if any of your readers can inform me if the work is being continued, or if there is any chance of its continuation. It is not often that tables are so indispensable as in the present case, or that a want so pressing can be supplied with such comparative ease; and the cessation of the tables would be a real calamity. The tenth million has, I presume, been published.

66

At the British Association Meeting at Dundee in 1867, a list of 5.500 large prime numbers was communicated to Section A by Mr. Barrett Davis. A short discussion took place on the reading" of the paper, in the course of which it was stated that Mr. Davis's table was unaccompanied by any explanation of how the numbers had been obtained, or on what grounds they were asserted to be prime; it was also asserted that Mr. Davis wished to keep his method secret.

Perhaps some reader of NATURE can say whether Mr. Davis's numbers have been printed. If they exceed Dase's limit, their publication (if they have not yet been published) is very desirable; and even supposing they are given in Dase's tables, it would be valuable to know how far the latter have been verified by them. The statement about Mr. Davis's method being secret was probably founded on some mistake, and no doubt Mr. Davis would not object to explain it. J. W. L. GLAISHER Trinity College, Cambridge, April 29

Units of Force and Energy

There

THE best root for the name of a unit of force is dúvaus. is, therefore, no ground for Mr. Muir's complaint (NATURE, vol. iii. p. 426), and I now venture to propose that the name dyne be given to that force which, acting on a gramme for a second, generates a velocity of a metre per second. A thousand dynes to make one kilodyne, and a million dynes one megadyne.

Borrowing a hint from Mr. Muir, I would point out that the kilodyne may also be defined as the force which, acting on a kilogramme for a second, generates the velocity of a metre per second, or, as the force which, acting on a gramme for a second, generates a velocity of a kilometre per second.

The kinit, or pound-foot-second unit of force, is about 1381 dynes. Very roughly expressed in terrestrial gravitation measure, the kinit is the gravitating force of half an ounce, the dyne of about 1 grains, the kilodyne of about 4 of a pound, and the megadyne of 2 cwt., the approximation being much closer in this last case than in the otheis, so that within one part in 4CO we have 10 megadynes the force of terrestrial gravity on a ton.

=

I have often felt the want of a name for an absolute unit of energy, or, what amounts to the same thing, an absolute unit of