270 



NA TURE 



[July 17, 1890 



attach valency values to the metals is carried out in all 

 cases with the exception of iron, to which no value is 

 affixed. The reason for this omission is not obvious, as 

 the author does not hesitate to call lead a dyad, antimony 

 a triad, &c. Amongst minor points the use of potassium 

 antimony tartrate for potassium antimonyl tartrate, of 

 arsenic acid for arsenic pentoxide, may be noticed. 



The book may go some way to fulfil the author's 

 expectation that it will give the student " some acquaint- 

 ance with the art of test-tubing," but that it will 

 materially increase his knowledge of the principles of 

 practical chemistry, or sharpen his appreciation of the 

 raison d'etre of a chemical process, is another matter. 



Manual of Pharmaceutical Testing. By Barnard S. 

 Proctor, F.I.C. Pp. vii., 176. (London : The Chemist 

 and Druggist, 1890.) 



This book is a collection of tests suitable for ascer- 

 taining the purity of the chemicals of the British 

 Pharmacopoeia, &c. The tests described are the simplest 

 possible, and can be carried out with the apparatus and 

 chemicals in use at the dispensing counter. They apply 

 more especially to the impurities of manufacture than 

 to adulterations. In many cases they are simply those 

 recommended by the British Pharmacopoeia for deter- 

 mining if the purity of a material falls short of the re- 

 quired standard. As a rule they are qualitative, and 

 sufficiently accurate for the purpose in view, although 

 quantitative methods, more especially in determinations 

 of solubility, or fixed residues of volatile liquids, are 

 employed. The book contains a chapter on manipulation, 

 which includes the method of weighing precipitates, and 

 an index, and will be found a handy volume to the 

 pharmacist. 



The Encyclopcedia of Photography. By Walter E. Wood- 

 bury. (London: Iliffe and Son, 1890.) 



This work, which will be concluded in about twelve parts, 

 is written on the same lines as other photographic ency- 

 clopaedias, but treats especially of the sciences of optics 

 and chemistry. The art of photography being so largely 

 practised nowadays, it is curious what a small percentage 

 of those who have taken it up know anything about optics 

 or chemistry, which form the basis of the whole subject. 



Throughout the book the author has borne this well in 

 mind, and has spared no pains to place before the reader, 

 in a simple and clear manner, the principles underlying 

 the formation of images, the construction of lenses, chro- 

 matic and spherical aberration, the theory of atoms and 

 molecules, and many other very important points relating 

 to optics and chemistry. 



The illustrations, which will be about 200 in number, 

 consisting of explanatory sketches and diagrams, will be 

 found, if up to the standard maintained in this first part, 

 to serve their purpose well. 



For amateurs this encyclopaedia should be very useful, 

 as it is written especially for beginners, and some of the 

 most complicated terms likely to lead to confusion are 

 avoided as much as possible. 



Dy?tamics for Begiimers. By the Rev. J. B. Lock, M.A. 

 Third Edition, stereotyped. (London : Macmillan and 

 Co., 1890.) 



The author has fully succeeded in supplying the want 

 that has been long felt, of a book which should explain 

 the elementary principles of dynamics, illustrating them 

 by easy examples in a manner suitable for use in schools 

 with boys of ordinary mathematical attainments. 



Section L deals with rectilinear dynamics, in which the 



fundamental principles are explained. The words " velo " 



and " celo," abbreviations for unit velocity and unit 



acceleration respectively, are here used, and the author 



NO. 1 08 1, VOL. 42] 



says in the preface, " Of their value for the pur- 

 poses of teaching and explanation I have received the 

 very strongest testimony from those best qualified to 

 judge." 



Sections IL and IIL treat of " Direction " and " Illus- 

 trations," the former dealing with the parallelograms of 

 distances, velocities, and accelerations, chords of quickest 

 descent, &c., the latter with projectiles, oblique impact, 

 relative motion, hodograph, &c. 



Work, energy, power, are discussed in Section IV., 

 and there is a chapter on the indestructibility of 

 matter. 



An excellent set of examples is collected at the 

 end, and a series of examination papers is added, 

 taken from the various examinations held from time to 

 time at Oxford and Cambridge. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed 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 Climates of Past Ages." 



I FEEL somewhat disappointed not to see a flood of corre- 

 spondence in your pages arising out of Dr. Neumayr's very 

 interesting lecture on the climates of past ages. The subject is 

 difficult and complex, and the factors of the problem are no 

 doubt various and of different kinds. I wish to make a few 

 remarks on some of these. 



It seems impossible to doubt that the sun is losing heat ; and, 

 consequently, that the quantity of heat annually received by the 

 earth from the sun is less than it once was. Now, one of the 

 most remarkable of the facts before us is the evidence, from fossil 

 vegetation, of comparatively warm climates in the polar regions. 

 There is no similar evidence respecting the equatorial regions ; 

 but it is probably impossible that such evidence should be pre- 

 served, so that its absence proves nothing as to the equatorial 

 climate of the same period ; but it is worth noticing that, if we 

 suppose the force of solar radiation increased, the increase of 

 terrestrial temperatures will be greater in high than in low lati- 

 tudes, because, with the increased quantity of heat received into 

 the atmosphere, an increased quantity will become latent by 

 evaporation in the lower latitudes, and will be carried to the 

 higher latitudes by vapour-bearing winds. 



If our planet had neither atmosphere nor ocean, the tempera- 

 ture of the higher latitudes could be raised only as a direct result 

 of increased solar radiation. If it had an atmosphere but no 

 ocean, the increase of the temperature of the higher latitudes 

 would be assisted by heat-bearing winds ; and this would be at 

 the expense of the temperature of the lower latitudes, which 

 would be lowered by the heat so carried away. In the actual 

 case of our earth, with both atmosphere and ocean, this action 

 will be greatly increased by the power of vapour-bearing winds 

 to carry heat in the latent form, which again becomes sensible 

 heat on the condensation of the vapour. This appears to show 

 that a considerable increase of temperature might be produced 

 in the higher latitudes by a comparatively small increase in the 

 force of solar radiation. 



Dr. Neumayr says that the cause of the glacial climate is 

 quhe unknown ; and at the same time he asserts that both 

 hemispheres— the northern and the southern — were glaciated at 

 the same time. I dispute both of these opinions. I think Mr. 

 Croli has shown the direction in which the explanation of the 

 glacial climate is to be sought ; and if this is so, the two hemi- 

 spheres were not glaciated at the same time, but alternately. 



If, during a glacial period, the northern and the southern 

 hemispheres were each alternately glaciated for geologically 

 short periods, this would account for the fact mentioned by Dr. 

 Neumayr, that the glacial period appears not to have been 

 continuous, but interrupted by periods of milder climate. 

 Croll's theory accounts for this. His theory is, that glacial 

 periods occur at those astronomical epochs when the eccentricity 

 of the earth's orbit is at its greatest ; and a glacial climate is 

 produced in the two hemispheres alternately, according as the 



