;42 



NATURE 



[February ii, 



racter of which is said to be somewhat similar to that of 

 the Bulletin. Reviews follow of a work on the " Photo- 

 chronograph (Hagen and Fargis, of the Georgetown Col- 

 lege Observatory), and of Dr. Craig's " Treatise on Linear 

 Differential Equations, vol. i. (by J. C. Fields). Besides 

 there is a note on " Nomenclature of Mechanics" (our 

 readers are familiar with the discussion raised by Prof. 

 Greenhill, anent the same matter, the equation W = M_if;. 

 The "Notes" (in both numbers) give information re- 

 specting the Society and its doings. One property of 

 numbers, out of many given, we give here — 



45 _|_ 50 4. 65 -I- 75 + 95 -1- Ii5 = 12^. 



In No. 3 Dr. Fiske prints a rcsutuc' oi a lecture, before 

 the Society, " On the Doubly Infinite Products," which 

 bristles with references to papers on the subject. Prof. 

 Hathaway then, in a very interesting note on the 

 " Early History of the Potential," sums up, in correc- 

 tion of an error that occurs in Todhunter's " History of 

 the Theories of Attraction" (vol. ii. § 789, 1007, and 1 138), 

 " the evidence in favour of assigning to Lagrange" (as 

 against Laplace) " the honour of the introduction of the 

 potential into dynamics." Mr. J. E. Davies contributes 

 a favourable review of Preston's "The Theory of Light." 



To each number is appended a long list of new publica- 

 tions. This Bulletin, it will be seen, breaks new ground, 

 and presents several points of interest to mathematicians. 



Guide to the Examinations in Chemistry. By W. Jerome 



Harrison, F.G S. Pp. 56. (London : Blackie and 



Son.) 

 The greater portion of this little book consists of answers 

 to the questions which have been set in elementary 

 inorganic chemistry in the examinations held by the 

 Science and Art Department during the period 1884 to 

 1 891. The rest of the book contains general information 

 regarding the Department and its examinations, and also 

 supplies hints for the successful working of the papers. 



The answers are but moderately satisfactory ; it may 

 be taken that the author has frequently underrated the 

 difficulty of expressing concisely, and at the same time 

 clearly, the meaning which he wishes to convey. The 

 following extracts may be taken as instances : — 



" Gunpowder .... depends for its energy upon the 

 suddenness with which the nitre parts with its oxygen." 



" The terminations -ide, -ite, and -ate are given to the 

 names of the acid-forming portions of salts." 



" Nitrous water [oxide ?] dissolves in water equally, and 

 as a whole. Air dissolves unequally in water, the oxygen 

 being more soluble than the nitrogen." 



The book is intended to be a companion to Sexton's 

 " Chemistry, Theoretical and Practical." 



Manipulation of the Microscope. By Edward Bausch. 



(New York : Bausch and Lomb Optical Company, 



1891.) 

 This little treatise on the microscope, which is now in its 

 second edition, is sure to find favour with workers with 

 this instrument, as it forms a good introduction to books 

 of a more advanced nature. The subject is not treated 

 extensively, but just so far as to enable a beginner to 

 know the whys and the wherefores of the various mani- 

 pulations. 



The first two chapters deal with the simple and com- 

 pound microscopes, describing their adjustments, &c. 

 Under " Objectives and Eye-pieces," which forms the 

 heading of the next chaper, we fiind short but good de- 

 scriptions relating to achromatism, resolving power, 

 flatness of field, magnifying power, &c. In the chapters 

 on "Requisites for Work," "How to Work," and 

 "Advanced Manipulation," the beginner is shown how 

 to set up his instrument, to illuminate the field properly, 

 to use the high- power objectives, and, among other 

 things, receives instruction in the dry and immersion 

 adjustable objectives. 



NO. I 163, VOL. 45] 



The selection of an instrument is always an important 

 item to be thought of, and the author here gives some 

 good sound advice both about it and the choice of its 

 accessories, and about the care which should be bestowed 

 on it to keep it in the best working condition. The 

 appendix contains some considerations in the testing of 

 objectives. 



The work is one which all beginners with the micro- 

 scope should read, while many a hint might be gathered 

 by an advanced student. 



Harrow Birds. By G. E. H. Barrett- Hamilton. 



(Harrow : Sold for the Harrow School Scientific 



Society by J. C. Willbee, 1892.) 

 This little volume ought to be of good service to the 

 Harrow School Scientific Society, for whose benefit it 

 has been prepared. The author was a member of Harrow 

 School from 1885 to 1890, and evidently made excellent 

 use of his opportunities for ornithological study. For 

 facts which have not come within his own observation he 

 has had recourse to the best authorities, and various 

 gentlemen, whose names he gives, have contributed notes 

 on the birds observed about Harrow during their school- 

 life. The district covered in the list is contained within 

 a radius of about five miles around Harrow. The list 

 includes 197 species, of which 55 are partially or wholly 

 resident, 27 are regular summer visitors, about 22 appear 

 annually on migration or in winter, and the remaining 94 

 are visitors of rare or accidental occurrence. The species 

 which breed regularly number 82. 



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 ^/Nature, 

 No notice is taken of anonymous communications. ] 

 The Theory of Solutions. 

 In Nature, vol. xlv. p. 293, appears a letter by Prof. Ost- 

 wald, in which he replies to a portion of my review of his book 

 on solutions (Nature, vol. xlv. p. 193). 



Prof. Ostwald finds his main cause for objection in my con- 

 clusion that he is a supporter of the "physical" as distinguished 

 from the "chemical" theory of solution. To .such a statement 

 he objects on the ground that he " cannot at all admit the exist- 

 ence " of a " contrast " between the two theories; and further 

 that he intentionally neither set up nor attempted to answer the 

 question— Is solution a physical or a chemical process ?— because 

 he holds it to be " unclear and therefore very harmful." In the 

 rest of the letter he concerns himself mainly with expounding 

 what he prefers to name the " new theory " of solution, and 

 seeks to show that between it and the hydrate theory there is 

 no antagonism or rivalry. 



The first point to consider as bearing on the question at issue 

 is the definition of the "new theory" which may be gathered 

 from extracts such as the following : — 



" The theory of solutions which I represent and defend 

 consists" " of a certain number of laws, i.e. of exact relations 

 between measurable quantities." 



" The presentation of laws of solutions, as known up to the 

 present, . . . forms the subject of my book." 



But surely it cannot be admitted that a number of exact 

 relationships constitute a theory ; for theory is concerned with 

 saying why such relationships should exist, wUh supplying ideas 

 to connect them together. Now, contrary to the apparerit 

 meaning of the last quotation given above, Prof. Ostwald's 

 book contained much of the nature of a true theory. Indeed, 

 the ideas which seemed to determine the general treatment of 

 the subject, and which formed the only justification for the free 

 use made in the book of gaseous laws, were the hypothetical 

 functions ascribed to the solvent and the dissolved substance. 

 The hypothesis here involved, in conformity with what has been 

 the usual custom in this country I termed the "physical 

 theory," and I am at a loss to see how any reasoning based on 

 the definition of the "new theory" affects the use of this term. 

 For the theoretical matter given in the book evidently refers 



