255 



and near the sun it eould be no less than 



lib, 

 17x1015 

 If the earth's velocity (being about xoiirtk ^^ *^® velocity of light) 

 be admitted as not too great for the maximum velocity of vibration 

 of plane polarized light, the mass of the luminiferous medium within 

 a sphere concentric with the sun, with radius equal to that of the 

 earth's orbit, might be not more than y^^go ^^ *^® earth's mass, 

 since the mechanical value of light within that space is about ysVir 

 of that of the earth's motion. 



4. Account of Experimental Investigations to answer ques- 



tions originating in the Mechanical Theory of Thermo- 

 Electric Currents. By Professor W. Thomson. 



In this communication the mode of experimenting was described 

 by which the experimental results quoted in the theoretical paper 

 were obtained ; and the principal parts of the special apparatus 

 which had been constructed and used in the investigation, were laid 

 before the Royal Society. 



5. Dynamical Theory of Heat, Part VI. continued. A Me- 



chanical Theory of Thermo-electric Currents in Crystal- 

 line Solids. By Professor W. Thomson. 



In this paper the Mechanical Theory of Thermo-electric Currents 

 in linear conductors of non-crystalline substance, first communicated 

 to the Royal Society December 15, 1851, is extended to solids of 

 any form and of crystalline substance. 



It is first proved, that if a solid be such that bars cut from it in 

 different directions have different thermo-electric powers relatively to 

 one another, or to other linear conductors, forming part of a circuit, 

 there must, for every bar cut from it, except in certain particular 

 directions (principal thermo-electric axes), be a new thermo-electric 

 quality, of a kind quite distinct from any hitherto known ; giving 

 rise to a reciprocal thermo-dynamic action, which consists of a differ- 

 ence in temperature at the sides of the bar causing a current to flow 

 longitudinally^ when the tiuo ends, being at the same temperature, 

 are connected by a uniformly heated conductor; and a current 

 through the bar causing an absorption and evolution of heat at its 

 two sides, when these are kept at the same temperature. 



VOL. III. X 



