NATURE 



505 



THURSDAY, APRIL 2, 190S. 



ELECTRICITY AND MATTER. 



The Corptiscidar llieory of Matter. By Prof. J. J. 

 Thomson, F.R.S. Pp. vii+172. (London: Archibald 

 Constable and Co., Ltd., 1907.) Price ys. 6d. net. 



THF; present volume is an expansion of six lectures 

 delivered by Prof. J. J. Thom.son in his capacity 

 of professor at the Royal Institution. It is a simple 

 and clear account of the development of the corpus- 

 cular, or, as some prefer to call it, the electronic, 

 theory of matter to explain the passage of electricity 

 through metals and gases. The last two chapters are 

 devoted to consideration of the properties of model 

 atoms built up of corpuscles, and the evidence in 

 favour of the view that the number of the corpuscles 

 in an atom is about the same as its atomic weight in 

 terms of hydrogen. 



The proof of the independent existence in matter of 

 .-lectrons of mass small compared with the atoms has 

 supplied a great stimulus to the attack of that most 

 important problem of physics, the connection between 

 electricity and matter. This attack has been con- 

 ducted both on experimental and theoretical lines, and 

 while only a beginning has been made, yet the results 

 already obtained have been instrumental in giving a 

 much clearer and deeper insight into the conditions of 

 the problem, and afford considerable justification for 

 the hope of still greater advances in the immediate 

 future. There has been a tendency in some quarters 

 to view with alarm, if not with distrust, the philo- 

 sophic speculations of the physicist, more particu- 

 larly when dealing with the question of the constitu- 

 tion of the chemist's atom. It is apparently considered 

 indelicate to pry too deeply into the mysteries of 

 atomic structure, especially if mathematical analysis is 

 the instrument of investigation. This attitude appears 

 somewhat unreasonable to the average physicist, and 

 arises largely from a misunderstanding of the relative 

 place of theory and experiment in physical science. A 

 student of the history of physical science cannot fail to 

 be impressed by the notable part played by mathe- 

 matical physics in the development of the subject, and 

 there is no obvious reason why the cooperation 

 between the two branches of the subject should not be 

 as fruitful in the future. The physicist from his train- 

 ing is seldom content merely to describe phenomena, 

 but seeks for some form of theory that will serve to 

 give a general explanation of the facts and to show 

 their relation with other branches of the subject. In 

 dealing with such a complicated and intangible 

 problem as the constitution of the atom, it is essential 

 that theory should go hand in hand with experiment, 

 for without some kind of theory the experimenter 

 is in most cases as helpless as a ship without a 

 rudder. 



This attitude of the physicist is very well expressed 

 by Prof. J. J. Thomson in the opening chapter. After 

 NO. 2005, VOL. 77] 



mentioning the postulates on which the corpuscula 

 theory of matter is based, he proceeds : — 



" From the point of view of the physicist, a theory 

 of matter is a policy rather than a creed ; its object is 

 to connect or coordinate apparently diverse phenomena, 

 and above all to suggest, stimulate and direct ex- 

 periment. It ought to furnish a compass which, if 

 followed, w'il lead the observer further and further 

 into previously unexplored regions. Whether these 

 regions will be barren or fertile experience alone will 

 decide ; but at any rate, one who is guided in this 

 way will travel onward in a definite direction, and will 

 not wander aimlessly to and fro." 



The working out of the logical consequences of a 

 simple theory and the comparison of the deductions 

 with experiment is eminently scientific, and of great 

 importance to the specialist who is able to form a 

 critical estimate of the adequacy of the theory. The 

 danger of too free a use of hypothesis is not so much 

 for the specialist as for the general reader who, from 

 lack of expert knowledge or of time, is unable to form 

 a critical judgment on the matter. In such a case 

 there is a tendency to assume that a theory which may 

 be admittedly tentative in character represents the 

 final, accepted views on the subject. 



Two of the most interesting chapters of the book 

 are devoted to the application of the corpuscular theory 

 to explain the passage of electricity through metals. 

 In one chapter the theory developed is similar in 

 general outlines to that originally advanced by the 

 author and the late Prof. Drude. The corpuscles 

 which are responsible for the passage of electricity 

 through a conductoi are supposed to be free from the 

 molecules for a time sufficiently long for them to be 

 in temperature equilibrium with the molecules of the 

 metal. This implies that the corpuscles behave like a 

 gas, and that temperature equilibrium is reached when 

 the mean kinetic energy of the corpuscle has become 

 equal to that of a molecule of a gas at the same tem- 

 perature. The passage of the current is then supposed 

 to result from the drift of these free charged cor- 

 puscles, brought about by the action of the external 

 electric field applied to the conductor. This theory is 

 shown to account in a satisfactory way for the con- 

 nection between thermal and electric conductivities of 

 metal, and with minor assumptions for the Peltier and 

 Thomson effects. Prof. Thomson points out that this 

 form of theory suffers from one very serious defect. 

 In order to account for the conductivities observed in 

 metals, it is necessary to assume the presence of such 

 a large number of free corpuscles in the metal that 

 the specific heat of these alone, quite independently of 

 the atoms of the metal itself, is about ten times 

 greater than that experimentally observed. The 

 author in the next chapter develops another form of 

 the theory which is free from this objection, and at 

 the same time fits in with the facts to be explained 

 equally well as the first theory. The second method 

 supposes that the corpuscles are not free in the metal 

 except for the time required to pass from 

 one atom to another. They are pulled out of 

 the atoms of the metals bv the action of the 



