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mediate stages; systematic and argumentative in the later stages. The descriptive stage includes story-telling, the awakening of the child's imagination and interest, and the introduction of simple map and out-of-door work. In the transition stage, the pupil is chiefly concerned with the investigation of geographical principles rather than with the acquisition of geographical facts. In the final stage, the pupil has to collect his own data, to construct his own maps and diagrams, and to reason out his own conclusions, that is, he has to learn the methods and principles of geographical argument. Most teachers will agree with these divisions, and be prepared to follow Mr. Wallis in his treatment of the first and second stages; the ideas and methods of the third stage are more debatable, but, from the author's point of view, they are well stated and reasoned.

It is difficult, in a short space, to mention even a fraction of the valuable things the book contains. There are practical exercises for in and out of doors; plans and suggestions for the ideal geography-room and for apparatus; arguments on the relation between geography and history and other subjects, with sensible ideas on correlation;

and there are even a few lessons for examiners on how to set examination questions.

Everywhere full details are given in a style attractive, lucid, and often of considerable literary merit. In many places there is geographical information of great value, information such as a teacher cannot always find in the text-books at his disposal.

Full approval may be given to the recommendations to teachers of geography to avoid straying into attractive historical or geological byways, interesting as they may be, for they are not geography, and there is plenty of work to be done without going out of the proper field. It is wise, also, to insist that the geography teacher is not to wait for the science master to explain the working of a barometer before the results of reading the barometer are utilised, nor to waste his own precious time in explaining what falls within the province of another. Sometimes the academic treatment of a subject has to give way to a common-sense one, but it is not every author who writes a book who is willing to state this truth.

We commend the book to all teachers of geography, because it is so eminently sensible, practical, and stimulating. They need not adopt all the conclusions of Mr. Wallis, but they will find it difficult to disprove their truth, or to resist their attractiveness.

MUSICAL FORM AND DEVELOPMENT. The Musical Faculty: Its Origins and Processes. By W. Wallace. Pp. vi+228. (London: Macmillan and Co., Ltd., 1914.) Price 5s. net.

TH

HE systematic and scientific study of the psychological processes involved in the creation and production of music is, from the nature of the case, exceedingly difficult, and the author is fully justified in claiming that there is room for further literature on the subject. How far he has been successful in dealing with these problems in the present book is open to some doubt.

Speaking generally, the first four chapters deal mainly with the development of music pure and simple, the remainder with musicians and their characteristics. Now in treating of individuals, there is no lack of statistical and historical evidence on which to base conclusions. Whether the subject-matter be the existence of musical prodigies, the part played by heredity, the influence on health, both mental and physical, of great musical genius, or such functional characters as mental audition, tonal memory, sense of tempo, power of detecting differences of tone quality, inhibition of sound perception, the data from which inferences may be drawn are of a fairly definite character, and, on the whole, Mr. Wallace's treatment may be regarded as satisfactory. But the present reviewer totally disagrees with what he says about music itself in the chapters entitled "A Readjustment of Values," "Historical Bearings," and "Individual Development."

Mr. Wallace may prefer modern French cacophonies to Haydn's and Mozart's delightful quartets and symphonies, but he surely cannot seriously wish us to believe that those early composers were lacking in originality or individuality simply because they could obtain what they wanted with the use of simple chords and melodies. Neither can we agree with the statement that

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Music, as we understand it, has not yet established her eternal verities," and when he states that "it would be no feat for a composer to write another Orfeo to-day," we can only say we should like to see anyone try to do it!

Admittedly, both the character of musical form and the recognised standard of excellence are changing, and the changes are the result of a process of evolution similar to that which exists everywhere else. But evolution does not always represent a change to a higher standard of perfection. fection. It sometimes stands for degeneration, and "a readjustment of values" may spell barbarism, as is evidenced in the use of poisonous

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Surface Tension and Surface Energy and Their Influence on Chemical Phenomena. By Dr. R. S. Willows and E. Hatschek. Pp. viii+ 80. (London: J. and A. Churchill, 1915.) Price 2s. 6d. net.

THE appearance of this book, following close on that of a similar work by Michaelis, is a welcome sign of the increasing interest now taken in this subject. The distribution of matter and energy at any interface, although of the first importance, has only come into prominence since the development of colloid chemistry.

The scope and character of the book may be indicated by a reference to the chief subjects discussed. They are:-the fundamental ideas of surface tension; intrinsic pressure; Gibbs's surface excess formula; recent experimental work on interfacial concentrations; electrical phenomena at interfaces, with special attention to the Lippmann electrometer and the dropping electrode; and, in conclusion, such matters as condensation on gas ions, effect of electrification on the vapour pressure of drops, and "waterfall electricity.'

It is scarcely surprising that the authors have found it no easy task to co-ordinate these varied subjects; and to this difficulty is doubtless due a want of clearness in a few places. There is one notable omission; one would have expected to find the development of the surface excess equation, either as given as given by Gibbs himself, or by Thomson or Milner. Curiously, no mention is made of Milner's experimental work, the first attempt to test the formula.

In spite of these defects, the work must prove helpful to advanced students and research workers, biological and technical, who have a practical interest in adsorption and allied There is a useful index, but it phenomena. seems a pity that no references to original papers are given. W. W. T. Fire Tests with Glass. "Red Books," Nos. 196 and 197. (London: British Fire Prevention Committee.) Price 2s. 6d. each. THESE two small books embody the British Fire Prevention Committee's Report on Fire Tests made respectively with skylight openings and windows filled in with "wired glass" manu

factured in our own country. The skylights were five in number, each 2 ft. square, and arranged The glazing horizontally in a single straight row. was subjected to fire for an hour, followed by water from a steam fire-engine applied at close range for two minutes on the fire side. No fire passed through the glazing, but more or less water found its way through three of the five. Details of the tests are given, with illustrations showing the effects of the fire. The three vertical windows were subjected to a precisely similar test with In the case of two of much the same results. the windows neither fire nor water passed through the glazing, but in the third, though no fire passed through, the application of water caused perforation and some water got through. The temperatures reached before the application of water were not less than 1500° F. (or 8155° C.). The maximum size of the vertical glazing tested was four feet by one foot.

The results of the tests clearly indicate that British wired glazing, when suitably fixed, can effectually check the spread of fire in a manner comparable with fire-resisting partitions and doors of much greater thickness and weight. The subject is well worth the serious attention of those interested in the limitation of damage by fire, especially in cases where the admission of light is desirable. J. A. A.

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 Okapi.

IN my letter published in NATURE of May 27 (vol. XCV., p. 342) dealing mainly with the "Supposed Horn-Sheaths of an Okapi," I stated that "it is only when extremely young that the backward slope of the back is very noticeable." It is perhaps the only statement I have made regarding the okapi which was no based on my own observations, and it appears to be

erroneous.

The impression was derived from a photograph reproduced in M. Fraipont's "Monograph on the Okapi," of a very young one captured by natives and brought into one of the Uele stations. I have since seen a photograph of the same animal from another source which shows that there was very littl backward slope. At maturity the height of the okapi at the shoulders is only 2 to 3 in. at the most mere than that above the hindquarters. The following measurements taken from three animals lying as they fell, one by Mr. A. E. H. Reid, and two by myself, bring this out quite clearly :

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Mr. Reid has given me the following additional measurements for his specimen :-Girth of neck in front of shoulders 2 ft. 11 in., and girth of chest behind shoulders 5 ft. 7 in.

The okapi has no "hump" at the withers, as might be supposed by the length of the spinous processes of the dorsal vertebræ and as represented in many mounted specimens. The back is as straight as that of any horse or antelope. The neck is deep at the base and tapers to comparatively small dimensions immediately behind the head without any curve. The body is rotund, and the limbs remarkably clean and sleek. The animal as I have seen it has distinctly graceful proportions, and does not give one the impression of being all angles as in the giraffe. When standing on the alert it holds the neck fairly high, slightly above the line of the back, with the head poised at an angle, and the ears well forward.

The animal is probably a surviving primitive form of a family closely allied to the ancestors of the giraffes, which, as the forest once covering all tropical and probably most of subtropical Africa gradually disappeared by the agency of man, have become modified, bulkier animals under the influence of sunlight and freedom in the open scrub country, where to browse on the young shoots of the thorny acacia trees, grown tall and table-topped owing to annually recurring grass fires, has necessitated the development of a longer neck.

The giraffe, now restricted to Africa, at one time, during the latter portion of the Tertiary period, I think, roamed far and wide over southern Europe and throughout a large extent of Asia. Similarly the okapi's area of distribution before the destruction of the forest was probably much wider than at present. There is, in fact, evidence that it inhabited the once forest-covered regions of the Upper Nile Valley. It was pointed out in 1902 that among the twelfthdynasty paintings from Beni-Hasan, Egypt, there exists a picture, long known to archæologists, which portrays a creature termed "Sche," said to have a resemblance to the present-day okapi, except that the upper lip is somewhat protruded and proboscis-like, and there are no zebra-like markings, the body and limbs being of a uniformly reddish colour. It is possible that this picture represents a form of the okapi known to the ancient Egyptians. C. CHRISTY.

July 5.

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Testing Respirators.

THE chief difficulty in the determination of the rela- | tive efficiency of these safeguards which are so freely offered to the public, and also utilised for the purposes of war, is the want of a standard method of testing the same.

As the result of a few preliminary experiments, I would suggest in the testing of different fabrics impregnated with chemical substances that the conditions of working must be standardised, and the following conditions met. A suitable container for the mixture of air and the poisonous gas is required, and this must lead directly to the surface of the fabric which is exposed to the action of the mixed gases as they pass through its substance.

The rate of flow of the gas through the cloth must slightly exceed that of the inflow of air under actual breathing conditions, and this must be standardised. The reduction in the amount of the added gas is observed by actual analysis, and the test only made after the air has been passing for a stated period, which must not be less than five minutes. The composition of the air being tested must correspond with

Comet Mellish, June 6, 1915. Exposure, 9om.

the comet was then R.A. 22h. 35m., declination 70° 18' south.

The proximity of the comet to the pole is well shown, even over the small region of the photograph, by the change in the direction of the star trails. H. E. WOOD (Chief Assistant). Union Observatory, Johannesburg, June 10.

Bird Migration.

A SMALL item which may be of interest to those watching bird migration was noted on a recent voyage to America.

On s.s. St. Louis, of the American Line, about 6.45 p.m. (ship time) on May 3 a swallow came on board, evidently very tired, white breast feathers rather dirty, and, settling down, was caught by one of the passengers. It took some water but died during the night. There was no identification band on either leg.

The point of interest is that the ship was then about 49° N. lat. and 2320 W. long., which would put her some 560 miles west of Cape Clear-about W. by S.and some 680 miles N.W. from Cape Finisterre. The wind had been fairly steady from E.S.E. for the previous thirty hours, blowing 20-30 miles an hour.

The first swallows had been noticed at the place where I now am on Saturday, May 1, and no doubt the bird which reached the St. Louis had got separated from the general migration.

The bird seemed in fairly good condition, its brown throat and indigo head were sleek and glossy; the soiled breast feathers may have been due to the steamer's smoke as the bird came in from the lee side. ED. WILDING. Dunedin, Jordanstown, Co. Antrim, June 23.

Mercury Ripples showing Interference.

THE accompanying print of a photograph of mercury ripples showing interference, made in this laboratory, exhibits singularly well the circular waves from the two sources as well as the interference pattern pro!uced. The two points of disturbance are maintained

by a forked pointer attached to the prong of a fork of frequency 50. With daylight illumination from a window, and a rotating sector to render the effect stroboscopic, a good natural picture of the surface is obtained. S. G. STARLING. Physical Laboratory, Municipal Technical Institute, West Ham, E.

Man's True Thermal Environment.

I FULLY agree with Mr. Grabham (NATURE, June 24, p. 451) as to the unsuitability of the constanttemperature (37° C.) psuchrainometer for many parts of the earth's surface. It is for this very reason, combined with the advantage of its much greater simplicity, that I am experimenting with the constant-energy form of instrument mentioned at the end of my former letter (NATURE, May 6, p. 260).

The effect of moisture can be brought into play with any type of psuchrainometer by providing its

exposed surface with a water-wetted muslin cover, and no doubt in this condition the apparatus approximates more closely to the human body.

My interest in the matter, however, is physical rather than physiological. My immediate aim is to study the extent to which "atmospheric cooling" can be predicted from the readings of the existing meteoro logical instruments. It seems best, therefore, to begin with the simplest case of cooling, namely, that which is free from the thermal complications accompanying evaporation. JAMES ROBERT MILNE. Physical Laboratory, University of Edinburgh, July 1.

HIGH EXPLOSIVES.

MR. LLOYD GEORGE'S recent speech in

the House of Commons, as Minister of Munitions, emphasised the very important part played by high explosives in the present war. is essential at the outset to distinguish clearly between a propellent charge, which forces the projectile or shell through the bore of the gun, and the high explosive charge filling the shell itself and causing it to burst, through the intervention of a time or percussion fuse. Modern military propellants consist of gelatinised guncotton (nitro-cellulose), either alone or mixed with varying proportions of nitro-glycerine, pressed into any required shape. The finished explosive is of a colloidal, horny nature, and a piece of it held in the fingers, whilst burning, can be blown out quite easily. A charge lit in the enclosed space of the chamber of a gun can discharge a projec tile with a velocity of about 1000 yards per second, developing in the chamber a pressure of, perhaps, twenty tons on the square inch. If the same quantity of the ungelatinised material were ignited in the gun-chamber it would detonate and blow the gun to pieces.

There is thus a wide difference between the effects produced by the burning of a propellant in the open, and in the chamber of the gun. In the latter case, before the projectile begins to move, the gases evolved produce pressure in the chamber, thus greatly accelerating the velocity of the explosive reaction.

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The forces at work in the gun, however, are insignificant in comparison with those brought into play when a high explosive detonates. in the open, without any containing envelope other than a thin cylinder of paper, the writer has obtained with a high explosive a velocity of detonation of the explosion wave of some seven miles per second. When the high explosive is in an enclosed space, such as a shell, the velocity of the detonation wave is greatly accelerated, and in an almost infinitesimal period of time the explosive is converted into gases. The volume of gases produced varies according to the nature of the explosive, but, generally, for those used in shells, it may be taken that, at the temperature of explosion, the volume of gas evolved occupies from 15,000 to 20,000 times the volume of the original explosive. This is the reason for the enormous destructive and shattering effect of a high explosive.

The nitro-glycerine high explosives used in mining are unsuitable for shell filling, owing to the sensitiveness of nitro-glycerine to shock, which would cause premature detonation in the bore of the gun. The ammonium nitrate group of high explosives, also used in mining, which contain nitro-hydrocarbons, and in some instances aluminium, have been advantageously adapted for shells. Although the hygroscopic nature of ammonium nitrate is detrimental, this may be successfully overcome.

Abel, in 1865, first proposed the use in mines of compressed wet gun-cotton fired by means of a dry gun-cotton primer; this was later used for filling torpedoes, but it has the disadvantage of low charge density. Gun-cotton cannot be compressed to a greater density than 1*25. In other words, a torpedo head which would hold 125 lb. of compressed gun-cotton could hold from 160 to 180 lb. of the denser trinitrotoluene or picric acid, with a corresponding increase of destructive power.

Sprengel, in 1875, first showed that picric acid could be detonated, and in 1881, Turpin, in France, demonstrated the practical possibility of using it for filling shells. The idea was rapidly taken up by other countries.

The methods of manufacturing nitro-hydrocarbons suitable for shell filling are very similar to those in use for producing nitro-glycerine. A mixture of sulphuric acid and nitric acid is used, and large quantities, very frequently as much as a ton or even more, are made in one operation. To obtain the highest yields of pure products very great attention must be paid to the composition of the acids, to the efficiency of agitation, and to the temperature, which is regulated by internal heating or cooling coils.

It is

Picric acid, discovered in 1771 by Woulfe, of London, when used for filling shells has a different name in each country. It is called Mélinite in France, Lyddite in England, Pertite in Italy, Shimose powder in Japan, Granatfüllung 88 in Germany, and Ecrasite in Austria. not always employed in the pure state, there being occasional addition of crésylite (trinitrocresol) or a salt of that substance, the object of which is to reduce the temperature of fusion. It is manufactured from phenol (carbolic acid, obtained from the distillation of coal-tar) by first dissolving in sulphuric acid and then treating the resulting phenol-sulphonic acid with nitric acid in excess. It forms yellow crystals with an intensely bitter taste. It has a specific gravity of 1777 and melts at 1225° C. Picric acid, if heated gradually, takes fire without explosion, giving rise to dense black smoke, but the application of a red-hot rod will cause it to detonate, as will also the explosion of a capsule of fulminate of mercury. Owing to the readiness with which it forms certain unstable metallic salts, the use of picric acid is not free from danger, and it is largely on this account that it is being rapidly replaced by the somewhat less energetic but much safer trinitrotoluene.

Trinol, trotyl, trilite, tritolo, or T.N.T., as trinitrotoluene is variously called, is made from

toluene (obtained from coal-tar naphtha), and was first proposed for use in shells by Haüssermann, in 1891. The result of the first nitration of toluene is a mixture of mono-nitro-toluenes, which are then treated with a mixture of strong nitric acid and sulphuric acid, and raised to the third or trinitro degree of nitration in one stage. Trinitrotoluene, when pure, forms brownish-yellow crystals with a melting point of 81° C. It is very stable, not igniting below 300°, but when it explodes it does so with great violence. Its density when melted varies between 157 and 160. Neither picric acid nor trinitrotoluene can be detonated with certainty by fulminate of mercury, and a small quantity of an intermediate priming charge is employed. In the case of trinitrotoluene, the use of tetranitromethylaniline has been found suitable. Tetranitraniline is a very powerful explosive, and has a higher density than either trinitrotoluene or picric acid.

The nitro-hydrocarbon high explosives used for the shell bursting are, in the molten state, poured into the cavity of the shell, in which they solidify, sufficient room being left for the priming charge and the detonating fuse. All the above nitrocompounds can only be obtained in a state of purity by re-crystallisation from various solvents.

Fulminate of mercury has been mentioned several times as a detonator, or initiator of explosion. Lead azide has been used, in conjunction with fulminate of mercury and tetranitromethylaniline, as a detonator for high explosives. It will be noticed that the majority of the high explosives referred to are derived from coal-tar products, and it is therefore evident that Mr. Lloyd George's statement, "If there were a shortage in the coal supply for any reason, the consequences would be very calamitous,' which must be taken very seriously.

is one

GEORGE W. MACDONALD.

SCIENCE IN THE SERVICE OF THE STATE. MODERN war is an affair of applied science

military, engineering, chemical, medical, and economic. Its successful prosecution requires more than an extremely high efficiency on the part of the officers in their professional work. Everything that chemical, physical, and engineering science can suggest must be pressed into service. The scientific men of the country have been keenly aware of this necessity from the beginning of the war, and many of them have individually done a great deal of important work for the Government and the various Services. The Royal Society has formed a War Committee, to which the Government has confided the solution of many pressing scientific problems arising out of the war. public thanks of the country have been given to the Royal Society by Mr. Asquith. We note also with pleasure the issue, by the Council of the Chemical Society, of the letter (see p. 523) announcing that the Council has constituted itself a consultative body to consider, organise, and utilise all suggestions and inventions which may be communicated to it.

The

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