October 28, 1897] 



NATURE 



611 



with "unverified deductions," the interesting case of the 

 electrical organ of certain fish is considered in some 

 detail, and the treatment should have been enriched by 

 some account of Prof. Gotch's researches on the subject 

 continued through so many years. 



We regret to see the reference to Garner (on p. 191) in 

 a work of such high merit ; although the author wisely 

 hesitates to accept any conclusions as yet made public. 



We cannot do better than conclude this notice of Mr. 

 Cramer's admirable and well-written little book, in his 

 own well-chosen words : — 



" Whatever may be the future of the particular con- 

 clusions which Charles Darwin reached, the general 

 method which he employed and the general drift of his 

 conclusions will have a permanent value. All his efforts 

 tended towards the unification of knowledge. All his 

 inductions became corollaries of one great theory ; all 

 his deductions had to do with efforts to test and prove the 

 truth of that theory. The subordination of all the devices 

 of the intellect to one great comprehensive purpose has 

 given a unity of aim to all the great works of his life, 

 and has made his general method conspicuously lucid, 

 and has knit the products of his intellect into one great 

 logical whole." E. B. P. 



A THEORY OF PHYSICS. 

 Theory of Physics. By Joseph S. Ames, Ph.D. Pp. 

 xviii -I- 513. (New York : Harper, 1897.) 



MR. AMES has written a very interesting book, and 

 one which to many students will be of great 

 value. At the same time it is extremely condensed. 

 To cover the whole range of physics, beginning with 

 mechanics and properties of matter, including also 

 sound, heat, light, electricity and magnetism in about 

 500 pages, is no easy task. Nor is it made less easy by 

 the fact that Mr. Ames is not content with dealing only 

 with the elementary parts of each subject, but carries 

 his readers far into the region of modern theories. 

 Thus Book ii., on heat, contains a chapter on the kinetic 

 theory of matter ; while the introduction to Book iv., 

 electricity and magnetism, deals with the properties of 

 the ether, and in the section on light we have chapters 

 on double refraction and polarisation. The book is 

 intended to be studied in an academic year of nine 

 months by " students who have had no previous training 

 in physics, or at the most only an elementary course.'' 

 A large majority of these, we fear, will find beyond 

 them the task of assimilating in so short a time all the 

 nourishment it contains ; the minority who succeed in 

 the attempt will have a very good knowledge of physics, 

 and all who read the book intelligently will benefit by 

 its study. 



There is a freshness about the style and about the 

 manner in which the laws and facts of physics are pre- 

 sented, which is very invigorating, and which adds 

 greatly to the interest of the book. 



The book opens with a chapter on kinematics ; this, 

 on the whole, is clear and precise, but the proof given 

 in § 18 of the formula connecting the space passed 

 over by a body moving with uniform acceleration with 

 its central velocity and the time of motion is incomplete. 



The chapter on dynamics, dealing with motion and 

 force, might usefully be amplified. The author starts 

 NO. 1 46 I, VOL. 56] 



from the law of the conservation of linear momentum for 

 a series of bodies "entirely free from all external in- 

 flences " as an experimental fact, which, so far as it has 

 been tested, seems absolutely true and without exception. 

 Inertia is defined as a property of matter which 

 becomes evident to our muscular senses when we try to 

 change the motion of matter, and two bodies are said to 

 have the same mass when they have the same inertia ; 

 the masses of two bodies are supposed to be compared 

 by observing the velocities which a given spiral spring 

 can confer on them when compressed to a definite extent 

 and allowed to act on each in turn. 



Knowing the masses, the momenta of a series of bodies 

 having motions of translation only can be calculated, 

 and the law of the conservation of momentum tested. 



The rate of change of the linear momentum of a body 

 with respect to the time is then defined as the " force 

 acting on it," and Newton's laws of motion are explained. 

 Such an explanation of dynamics has many claims lo our 

 attention, and is certainly more logical than that adopted 

 in many modern books ; at the same time, it is more 

 difficult for the ordinary student to grasp ; he knows 

 force as muscular exertion. The schoolboy who illustrated 

 his reply to his teacher's question. What is force ? by 

 doubling his fist and striking his classmate is typical, and 

 the difficulties of introducing him in his first study of 

 dynamics, to " force " as a name for rate of change of 

 momentum are considerable. The book is, the author 

 implies, intended for students working in a class, seeing 

 demonstrations and doing experiments for themselves ; 

 thus opportunities for explanation and amplification will 

 naturally arise ; still we wish the author had found space 

 to develop this part of his book more fully. 



Matter in rotation is dealt with in § 41, and the ele- 

 ments of rigid dynamics are based on the law of the 

 conservation of angular momentum. This law is 

 j stated as independent of the law of the conservation 

 I of linear momentum ; it is usually deduced from the 

 I latter. The treatment of the section on liquids intro- 

 j duces fewer novelties than that dealing with solids ; the 

 • important idea of pressure at a point, however, deserves, 

 I we think, a fuller explanation than is accorded to it ; the 

 I sections on capillarity are clear, and the principal pheno- 

 mena of osmosis are described. The parts of the book 

 on sound and heat do not call for any very detailed 

 notice ; the main criticism, to which the book is open 

 throughout, applies here also : advanced portions of the 

 subject are somewhat frequently referred to in so brief a 

 j manner that the student cannot, without further assist- 

 I ance, obtain really clear ideas on the subjects dealt 



with. 



i Book iv., electricity and magnetism, commences with a 



I reference to the ether, with which, we are told, electric 



! and magnetic effects are intimately connected. But the 



j earlier sections of the book describe the production of 



j electric charges and the ordinary phenomena of electro- 



I statics in the usual manner. It is stated, § 215, that the 



property of attraction possessed by an electrified body 



"will be shown in Article 221 to be due to changes in 



: the strain of the medium round the charged body" ; but 



the demonstration given in § 221 can hardly be called 



a proof. 



i The difficult questions involved in the theory of primary 



