KINETIC OR MECHANICAL VIEW OF NATURE. 31 



He must have fully realised the difficulty of imagining a 

 substance more subtle than air and yet endowed with the 

 property of rigidity, known to us only in solid bodies. 

 The elaboration of the theory of light pressed upon 

 physicists and mathematicians a more careful study of 

 the different states in which matter can exist. The 

 different properties which this hypothetical substance 

 called ether must possess had to be mathematically de- 

 fined; and, further, it had to be shown whether it 

 would be physically possible for a body, subject to the 

 empirical laws of motion, to possess certain of the pro- 

 perties of what we term solids, and yet to be in other 

 respects the very opposite of a solid. The solution of the 

 first problem was a purely mathematical performance, in 

 which many eminent mathematicians, such as Cauchy, 

 Neumann, Green, M'Cullagh, and Stokes, 1 have been 



21. 



The pro- 

 perties of 

 the ether. 



complished abroad " (vol. i. p. 105). 

 " It is difficult to picture the re- 

 markable scientific ignorance of 

 practical men in England in the 

 first quarter of the century. One 

 can only trust that there may be a 

 closer union of practice and theory 

 in our own day " (p. 106). This 

 passage was probably written in the 

 'seventies. 



According to Todhunter, the true 

 theory of elasticity was founded 

 in France between the years 1820 

 and 1830, by Navier, Poisson, and 

 Cauchy on the one side ; by the ex- 

 perimental work of Savart on the 

 other. It had been allied with 

 theoretical acoustics since Euler's 

 time. Chladni in Germany fur- 

 thered that branch of the subject in 

 three celebrated works : ' Theorie 

 des Klanges' (1787), ' Akustik ' 

 (1802), 'Beitrage zur Akustik' 

 (1817). Chladni influenced the 



brothers Weber, whose ' Wellenlehre 

 auf Experimente gegriindet' ap- 

 peared in 1825. In it wave-motion, 

 such as the theories of sound and 

 light had made specially interesting 

 and important, was experimentally 

 examined and illustrated. The 

 theory of elasticity now received a 

 new ally, viz., the elastic theory 

 of light or of the ether. Though 

 suggested by Fresnel, its real 

 founder was Cauchy. 



1 The natural philosopher to whom 

 we are most indebted for bringing 

 clearness and definiteness into our 

 ideas and our language in these 

 very intricate subjects is Sir George 

 Stokes. In two papers, published 

 respectively in 1845 and 1849 (see 

 ' Mathematical and Physical Papers,' 

 vol. i. pp. 75-129, and vol. ii. pp. 

 8-13), he has done more than any 

 other writer to fix for nearly half 

 a century the conceptions and the 



