Strain along the Lines of Force. 679 



greater. A variation of such magnitude due solely to 

 differences in the composition of the glass is out of all propor- 

 tion, and certainly if electrifying glass may more than double 

 its ordinary elasticity, such an elaborate method of experimen- 

 tation is unnecessary. Lastly, a variation in elasticity is 

 directly contrary to theory. Lippmann, and his formulas are 

 shown by Sacerdote to be essentially in agreement with the 

 other writers on the theory of this subject, states : — " That the 

 dilatation produced electrically must be due to a direct action 

 of electricity and cannot be caused by a variation of the 

 coefficient of elasticity, or, the coefficient of elasticity is indepen- 

 dent of the electrification ; on the other hand, the dielectric 

 constant varies with the electrification." 



Some years ago I published a paper in the Philosophical 

 Magazine containing experiments which did not confirm the 

 conclusions just cited, and in fact made me doubt the existence 

 of eiectrostriction. It was criticised by Sacerdote and 

 Can tone on the ground that my method was faulty ; they 

 claimed I had employed a mechanical instead of an optical 

 device for magnifying the elongations, and that it was not 

 sufficiently delicate to observe this minute effect. The 

 criticism was easy to meet, and in a letter to the Philosophical 

 Magazine I demonstrated that my apparatus was an optical 

 system, as sensitive or even a little more sensitive than 

 interference methods, and that their criticism of its lack of 

 sensibility and accuracy was founded on a misinterpretation. 

 Later work has confirmed my opinions. 



Maxwell's formula has been discussed by Korteweg *, 

 Lorberg t-. Kirchhoff J, Lippmann §, Sacerdote || and others. 

 Their methods may be divided into two categories : first, the 

 determination of the attractions and repulsions of electrified 

 conducting particles immersed in a non-conducting medium, 

 or in other words the results of a polarization of the dielectric; 

 and secondly the application of the thermodynamic equation 

 and the interdependence of the laws of conservation of energy 

 of electricity and matter. Sacerdote has made a careful 

 synopsis of all the articles published on this subject, with a 

 critical discussion of methods and results. He finds, barring 

 small differences, that both general methods lead to the same 

 conclusion, which may be expressed by the formula 



KV 2 

 P = (Kl + a) SS3*' 



* Korteweg, Wied. Ann. Bd. ix. p. 48. 



f Lorberg, Wied. Ann. Bd. xxi. p. 300. 



X Kirchhoff, Wied. Ann. Bd. xxiv. p. 52. 



§ Lippmann, Anna!, de Chim. et de Phys. [5] t. xxi v. p 159. 



II Sacerdote, Thp*e. 



