222 



SCIENCE 



[N. S. Vol. XXXI. Xo. 



the density, the chemical constitution or tlie 

 crystalline state of substances, we can not be 

 satisfied with simply introducing for each sub- 

 stance these coefficients, whose values are to be 

 determined by experiment; we shall be obliged to 

 have recourse to some hypothesis about the mech- 

 anism that is at the bottom of the phenomena. 

 It is by this necessity that we have been led to 

 the conception of electrons, i. e., of extremely 

 small particles, charged with electricity, which 

 are present in immense numbers in all ponderable 

 bodies, and by whose distribution and motions 

 we endeavor to explain all electric and optical 

 phenomena which are not confined to the free 

 ether. 



After the development of the fundamental 

 equations, the first chapter is chiefly devoted 

 to the general properties of free electrons. 

 Use is made of the quantity named by Abra- 

 ham the electromagnetic momentum and em- 

 ployed by him in his " Prinzipien der Dy- 

 namik des Electrons." The interesting ques- 

 tion of the electromagnetic mass of the 

 electron receives comprehensive treatment, in 

 which the necessary distinction between 

 " longitudinal " and " transverse " mass is 

 very clearly brought out. With a view to 

 their subsequent application in connection 

 with the influence of the earth's motion upon 

 optical phenomena the fundamental equa- 

 tions for a moving system are derived. The 

 chapter closes with a brief review of Drude's 

 theory of the conduction of electricity in 

 metals, and of a revised form of this theory, 

 proposed by the author, and considered by 

 him to be somewhat more rigorously developed 

 than that of Drude. 



In the second chapter the subject of emis- 

 sion and absorption of heat is discussed from 

 the standpoint of electron theorj', with the 

 view of indicating how far this theory may 

 lead toward the elucidation of the mechanism 

 involved in the phenomena. 



Eeference is made to the classical work of 

 Kirchhoff, Boltzmann and Wien in connec- 

 tion with black body radiation, and it is re- 

 marked that the results obtained by Boltz- 

 mann and Wien represent all that could be 

 expected from the methods of thermodynam- 

 ics and general electromagnetic theory, and 

 that these results afford small clue to the dis- 



covery of the real nature of the mechanism 

 of emission and absorption. 



Planck's theory of radiation is then discussed. 

 As is well known, this theory is based on the 

 assumption that every ponderable body con- 

 tains a very large number of electromagnetic 

 resonators. Different resonators may have dif- 

 ferent natural frequencies. In order to arrive 

 at his weU-known radiation formula. Professor 

 Planck assumes that each resonator possesses 

 the peculiar property of being able to receive or 

 give up energy in definite finite amounts only, 

 and not gradually. Many who have attempted 

 to follow Professor Planck's arguments in the 

 development of his theory have found their 

 chief difficulty in this assumption. In view of 

 this fact, the concluding remarks of the au- 

 thor are of particular interest. Referring to 

 Planck's theory, he says: 



Yet, we can not say that the mechanism of the 

 phenomena has been unveiled by it, and it must 

 be admitted that it is difficult to see a reason for 

 this partition of energy by finite portions, which 

 are not even equal to each other, but vary from 

 one resonator to another. 



Professor Larmor in the Bakerian lecture 

 of November 18, 1909, referring to Planck's 

 theory, also calls attention to the same diffi- 

 culty. 



The author goes on to develop an electron 

 theory of radiation for metals, and arrives at 

 a formula, valid for long waves, which is in 

 agreement with Planck's for this case. 



Finally, Jeans's theory of radiation is briefly 

 reviewed. This theory, as is well known, is 

 based on the assumption that the mechanical 

 theorem of equipartition of energy is appli- 

 cable to modes of vibration in the ether, and it 

 furnishes a radiation formula which for long 

 waves also agrees with Planck's for this case. 

 The author's concluding remark is again of 

 much interest: 



I shall conclude by observing that the law of 

 equipartition which, for systems of molecules, can 

 be deduced from the principles of statistical me- 

 chanics, can not as yet be considered to have been 

 proved for systems containing ether. 



Professor Larmor in the lecture referred to 

 above refers to the well-known controversy 

 concerning this matter. 



