896 Intelligence and Miscellaneous Articles. 



duced independently, and, electrically considered, would act^in'the 

 same direction, the one answering to positive, and the other to 

 negative, electricity. Hence, as the amount of compression may be 

 supposed to be equal to that of dilatation, the light incident on 

 both surfaces simultaneously would have double the effect of the 

 light incident on either singly. This inference agrees with experi- 

 mental results. 



I have said in the April Number (p. 279) that " the intrinsic 

 molecular forces of the vanes tend continuously to make the atoms 

 return to their neutral positions ; but since at the same time the 

 disturbing force is constantly in action, the result of the antago- 

 nistic forces will be a permanent abnormal condition of the super- 

 ficial atoms, so long as the disturbance continues." I propose now 

 to qualify this view by saying that the tendency of the atoms to 

 take the neutral positions is due to the law of heat-exchanges, which 

 is always in operation, and which may even be said to include the 

 before supposed action of molecular forces. The thermal influences 

 of the enclosing glass case, and of other surrounding substances, 

 which physicists have taken into consideration in proposed ex- 

 planations of the action of the radiometer, may be assumed to be 

 embraced by this general law, and on that account may not require 

 to be specifically determined. 



I am, Gentlemen, 



Your obedient Servant, 



J. Challis. 



Cambridge, April 21, 1877. 



DIATHERMANEITY OF METALS AND OF PAPER. BY M. AYMONNET. 



Conclusions. — My experiments, joined with the observation made 

 by Wiedemann and Frantz* on the rapidity with which equilibrium 

 of temperature establishes itself in the bars whose coefficients of con- 

 ductivity they were seeking, show : — 



1st. That the metals and paper are not athermanous, as is gene- 

 rally believed. 



2nd. That they are more diathermanous for obscure heat emana- 

 ting from metallic bodies raised to a temperature below 100° C. 

 than for the luminous heat-radiations, or those near the red. 



3rd. That they have feebler absorbing-powerst than that of 

 water. 



4th. That it is possible to find a mathematical relation between 

 the absorbing-power of a body and its coefficient of conductivity. — 

 Comptes Rendus de VAcademie des Sciences, Feb. 5, 1877, tome 

 lxxxiv. p. 259. 



* Pogg. Ann. vol. lxxxix. 



t I here call absorbing -power the complement of the inverse of the ratio 

 existing between the quantity of heat which normally penetrates into a 

 body and that which issues from it in the same direction. 



