NA TURE 



THURSDAY, MAY 26, 1904. 



STEPS TOWARDS A. NEW PRINCIPIA. 

 Electricity and Matter. By Prof. J. J. Thomson. Pp. 

 162 ; with diagrams. (Westminster : Archibald 

 Constable and Co., Ltd., 1904.) Price 55. net. 



IT is an interesting fact that the British Association 

 is going so soon to meet, under distinguished 

 presidency, at Cambridge, for there of late years has 

 the most splendid work in pure physical science been 

 done, and there it seems to me an erection comparable 

 in some respects to the Principia is being raised. One 

 of the foundation stones was laid in 1881 by J. J. 

 Thomson in a mathematical paper on the motion of a 

 charged sphere, a paper in which the idea of real 

 rlectric inertia took tangible and tractable form. 

 The building has been growing ever since, and is 

 now in full process of construction, though I hope it 

 will be long before its development shall cease or the 

 work be regarded as finished and ready to be left to 

 the admiration of future generations. 



The little book which constitutes the subject-matter 

 of this review embodies a set of six lectures delivered 

 on the other side of the Atlantic, at the invitation of 

 Yale University, last spring, by the Cavendish pro- 

 fessor of physics at Cambridge, England. 



They are not exactly popular, for although no 

 mathematics is introduced beyond what ought to be 

 (but usually is not) familiar to all who have been in 

 ihe sixth form at a public school, yet the ideas are 

 definite and quantitative, and are briefly expressed 

 because they are addressed to persons with some know- 

 ledge of physics ; moreover, they are such as can hardly 

 be made so childishly simple as to be apprehended of 

 the general average of so-called educated men in this 

 country, whose sense-perceptions in the direction of 

 great and comprehensive ideas have not been de- 

 veloped. 



To students of physics and higher chemistry this 

 book serves as a very readable digest and summary 

 of what is to be found worked out in more detail in 

 other writings by the same author ; and those who 

 have studied his most recent papers can hardly avoid 

 reading back into this little volume, which can be 

 skimmed at a sitting, much that has really been 

 elaborated since, and some things which still await 

 elaboration, on lines which are merely suggested here. 



It is difficult to exaggerate the suggestiveness of 

 the wealth of theory which is now being lavished upon 

 us in the domain of atomic structure and the mathe- 

 matics of chemistry; it appears likely to lead to a 

 definite microcosmic astronomy, based upon the known 

 properties of electric lines of force, akin to the weld- 

 ing together of the observed facts of the heavens by 

 a single comprehensive law, and forming the basis of 

 a real chemical " Principia." 



The discovery on which everything depends is the 



! recognition of the atom of electricity, a discovery to 

 which no one man can lay claim, and superposed upon 

 I this a detection of the extraordinary properties belong- 

 ! ing to an electric line of force, at rest, in motion, and 

 NO. 1804, VOL. 70] 



under acceleration, which is again a development to 

 which several have contributed. But it is safe to say 

 that a great bulk of the treatment of the subject, both 

 on its e.xperimental and mathematical side, emanates, 

 with a few important exceptions, in some form or 

 other from Cambridge, and it is difficult to over- 

 estimate the force and suggestiveness with which ideas 

 connected with the most recent and still nascent steps 

 in theory are presented in this small book. 



The book is in six chapters, corresponding apparently 

 with six lectures. 



Chapter iv. , on the atomic structure of electricity, 

 gives the customary account of the evidence for elec- 

 tric atoms, and for considering the charge on a 

 corpuscle to be identical with the ionic charge of a 

 hydrogen atom ; the experiments by which these con- 

 clusions were reached in the Cavendish Laboratory 

 being summarised and explained on the usual lines. 

 First, it was shown that the conductivity of a gas de- 

 pended on something that could be filtered out of it ; 

 next, the aggregate charge of the ions in a given 

 volume of gas was determined by the method of the 

 saturation current ; then these ions were counted by the 

 highly ingenious " cloud " method, and thus the 

 charge on each determined. The value, it may be 

 noted in passing, is rather higher than what used to 

 be roughly estimated for this fundamental electric 

 unit. Whereas it used to be guessed as about 10- " 

 electrostatic units, measurement seems to show that 

 it is more nearly 3.4x10-'°; and', in order to make 

 this quantity equal to the charge on a monad ion in 

 electrolysis, the number of molecules in a cubic centi- 

 metre of gas, at standard temperature and pressure, 

 must be rather fewer than used to be estimated as 

 probable, and also rather definite, being about 3.6 x 10", 

 which means 8 x 10-^ atoms of hydrogen per gram. 



A new confirmatory method applied by Dr. H. A. 

 Wilson is described, in which the settling of a cloud 

 under gravity is opposed by a measured electric field ; 

 and on this plan the same result is obtained. .'Mso 

 Prof. Townsend and Dr. H. A. Wilson, by applying 

 electrolytic considerations and experiments to ionised 

 air, were able to show directly that the ionic charges 

 in all cases are really identical, and are the same as 

 that familiar in electrolysis. 



A rapid summary of the now well known methods 

 by which the mass of the carriers for negative and 

 for positive electricity respectively has been determined 

 is also given, and it is pointed out how striking is 

 the resemblance of the result to Franklin's one-fluid 

 theory of electricity : — 



" The ' electric fluid ' of Franklin corresponds to an 

 assemblage of corpuscles, negative electrification 

 being a collection of these corpuscles. The trans- 

 ference of electrification from one place to another is 

 effected by the motion of corpuscles from the place 

 where there is a gain of positive electrification to the 

 place where there is a gain of negative. A positively 

 electrified body is one that has lost some of its 

 corpuscles. We have seen that the mass and charge 

 of the corpuscles have been determined directlv by 

 experiment. We in fact know more about the ' electric 

 fluid ' than we know about such fluids as air or water. " 



