155 



an elastic solid.* — (See the A/males de Chimie, torn. xliv. 

 p. 432). The remark, however, did not meet with much 

 attention from mathematicians, who were, perhaps, not dis- 

 posed to scrutinize too closely any hypothesis which gave 

 transversal vibrations as a result. Besides, the hypothesis 

 appeared to go much further, as it offered prima facie 

 explanations of a great variety of phenomena ; it was one 

 to which calculation could be readily applied, and thei'e- 

 fore it naturally found favour with the calculator; and as 

 to M. Poisson's objection, it was easily removed by a change 

 of terms, for when the elastic solid was called an " elastic 

 system" there was no longer anything startling in the an- 

 nouncement that the motions of the ether are those of such 

 a system. The hypothesis was therefore embraced by a great 

 number of writers in every part of Europe, who reproduced, 

 each in his own way, the results of M. Cauchy, though some- 

 times with considerable modifications. Every day saw some 

 new investigation pui'ely analytical — some new mathematical 

 research uncontrolled by a single physical conception — put 

 forward as a "mechanical theory" of double refraction, of 

 circular polarization, of dispersion, of absorption ; until at 

 length the Journals of Science and Transactions of Societies 

 were filled with a great mass of unmeaning formulas. This 

 state of things was partly occasioned by the great number of 

 " disposable" constants entering into the differential equations 

 of M. Cauchy and their integrals; for it was easy to introduce, 

 among the constants, such relations as would lead to any de- 

 sired conclusion ; and this method was frequently adopted by 

 M. Cauchy himself. Thus, in his theory of double (or rather 

 triple) refraction, given in the works already cited (p. 145), he 

 supposes three out of his nineconstants to vanish, and assumes, 



* As the theory of M. Cauchy {Mem. de VInstitut, torn, x.) had been communi- 

 cated to the Academy of Sciences some months before the period (October, 1830) 

 at which M. Poisson wrote, there can be no doubt that M. Poisson's remark was 

 directed against that theory, though he did not expressly mention it. 

 VOL. II. O 



