586 



NATURE 



{Oct. 1 8, 



electrification should be recognized, as it follows that the 

 speed with which electrical oscillations are propagated 

 across any medium will be diminished by the presence 

 of conductors moving about in it. "Thus, if the electro- 

 magnetic theory of light is true, the result we have been 

 discussing has an important bearing on the effect of the 

 molecules of matter on the rate of propagation of light." 



It would take too long to follow the author in detail 

 through the interesting discussion which is pursued on 

 these lines. Another example must therefore suffice. 

 The specific inductive capacity of a dielectric depends 

 upon the strain, and it follows that the distribution of stress 

 which Maxwell supposes to exist in the electric field is 

 supplemented by another, which is due to the relation 

 between inductive capacity and strain. Maxwell's dis- 

 tribution will be the same for all dielectrics, but Quincke 

 has shown that though most dielectrics expand when 

 placed in an electric field, the fatty oils contract. In 

 these cases the effects of what may be called the sup- 

 plementary distribution are contrary to, and greater than, 

 those produced by Maxwell's stress. 



Phenomena which depend on temperature are specially 

 discussed, and an interesting conclusion with respect to 

 thermo-electricity may be noted. It is that from the 

 heat developed by a current at a junction of two dissimilar 

 metals we can derive information as to that part only of 

 the electromotive force which depends upon the tempera- 

 ture. Hence the Peltier effect can throw no light upon 

 the absolute difference of potential between two different 

 metals. 



A chapter is devoted to the calculation of " the 

 Lagrangian function in the simplest case, when the body 

 is in a steady state, when it is free from all strain except 

 that inseparable from the body at the temperature we 

 are considering, and when it is neither electrified nor 

 magnetized." Two forms are found, which hold for the 

 gaseous and the liquid or solid states respectively. The 

 general principle is also laid down, that, "when the 

 physical environment of a system is slightly changed, 

 and the consequent change in the mean Lagrangian func- 

 tion increases as any physical process goes on, then this 

 process will have to go on further in the changed system 

 before equilibrium is reached than in the unchanged one, 

 while if the change in the mean Lagrangian function 

 diminishes as the process goes on it will not have to 

 proceed so far." 



As an example of this we may take the effect of a 

 charge of electricity on the vapour-pressure of a liquid. 

 If a spherical drop, of radius a, surrounded by a medium 

 of specific inductive capacity K, is charged with e units of 

 electricity, its potential energy is increased by e 2 /2Ka, and 

 thus electrification changes the mean Lagrangian function 

 by the amount -e 2 /2Ka. Prof. Thomson quotes experi- 

 ments by Blake to prove that when an electrified liquid 

 evaporates the vapour is not electrified, so that the charge 

 e is unaffected by evaporation, while the radius a of course 

 diminishes. On the whole, then, evaporation algebraically 

 diminishes the term — e 2 /2Ka, and therefore it will not 

 proceed so far as before the liquid was electrified. Thus 

 electrification diminishes the vapour-tension by an amount 

 which is limited by the insulating power of the air. The 

 maximum effect is about equal in magnitude, though 

 opposite in sign, to^that due to a curvature of a quarter of 



a centimetre. The suggestion is made that we should 

 therefore expect an electrified drop of rain to be larger 

 than an unelectrified one, so that this effect may help to 

 produce the large drops of rain which fall in thunder- 

 storms. The principle also leads to the conclusion that 

 the density of saturated aqueous vapour in the presence 

 of air is greater than if no other gas is present, and thus, 

 apart from other causes, rain- drops would form more 

 easily when the barometer is failing than when it is 

 rising. 



The properties of dilute solutions are discussed at 

 length, and the Lagrangian function is calculated in 

 accordance with the views of Van 't Hoff on the assump- 

 tion that the molecules of a salt in a dilute solution 

 behave as though they were in the gaseous state. 



The results obtained cannot be considered favourable 

 to the view that the effects of solution are capable of 

 being stated in such simple terms. Rontgen and 

 Schneider's experiments on the compressibility of saline 

 solutions prove that the decrease in the compressibility is 

 sometimes more than a hundred times greater than that 

 calculated on the above assumptions. The author also 

 points out that the rise in the osmometer, which is 

 explained as due to the pressure of the dissolved salt, may 

 be capable of other interpretations, and that at present 

 the indications of the instrument must be considered 

 ambiguous. 



Enough has perhaps been said to give an idea of the 

 method and scope of Prof. J. J. Thomson's work. 



It is possible that some of the experimental results 

 which are quoted require fuller confirmation than they 

 have as yet received, but if the work is regarded as a 

 text-book of mathematical physics this is a very minor 

 defect. The author has developed a method of wide 

 scope, and it is important that its applications should be 

 fully illustrated, even if the data assumed are not in all 

 cases unexceptionable. 



The book literally bristles with novel suggestions and 

 points of interest. An explanation of the fact recently 

 discovered by Mr. Shelford Bidwell, that iron becomes 

 shorter when the magnetizing force is very great ; the 

 effect of surface-tension on electromotive force ; chemical 

 action in thin films ; the effect of a neutral gas on dissocia- 

 tion — these are some of the subjects, in addition to those 

 which have already been mentioned, upon which we light 

 on turning over the pages haphazard. 



That it will make the study of physics and chemistry 

 easier is only in one sense true. Nihil tetigit qaorf noti 

 ornavit may, as applied to Prof. J. J. Thomson, be freely 

 translated, that he hardly mentions any law of physics 

 except to complicate it with correction terms. 



From a more serious point of view, however, it is 

 difficult to over-estimate the value of the establishment of 

 the less obvious connections between phenomena. 



On many points, such as Quincke's and Bidwell's obser- 

 vations on the changes of magnitude produced in the 

 electric and magnetic fields respectively, experiment 

 needed the support of theory, and Prof. J. J. Thomson 

 points out causes to which the observed effects may be 

 due. Almost daily, conscientious experimentalists are 

 spending time and ability in the detailed examination of 

 facts which they cannot explain, and which they can only 

 hope to explain by the most minute investigations. In the 



