1861.] on the Physical Basis of Solar Chemistry, 389 



caused to issue from the same slit as that through which the oxygen 

 had passed. No one present could see the gas ; it was quite invisible, 

 the light went through it as freely as through oxygen or air ; but its 

 effect upon the thermal rays emanating from the cube, was what 

 might be expected from a sheet of metal. A quantity so large was 

 cut off, that the needle of the galvanometer, promptly quitting the zero 

 line, moved with energy to its stops : thus the olefiant gas, so light 

 and clear and pervious to luminous rays, was a most potent destroyer 

 of the rays emanating from an obscure source. The reciprocity of 

 action established in the case of oxygen comes out here ; the good 

 radiator is found by this experiment to be the good absorber. 



This result, which was exhibited before a public audience this 

 evening for the first time, was typical of what had been obtained with 

 gases generally. Going through the entire list of gases and vapours 

 in this way, we should find radiation and absorption to be as rigidly 

 associated as positive and negative in electricity, or as north and south 

 polarity in magnetism. The gas which, when heated, is most competent 

 to generate a calorific ray, is precisely that which is most competent 

 to stop such a ray. If the radiation be high, the absorption is high ; 

 if the radiation be moderate, the absorption is moderate ; if the 

 radiation be low, the absorption is low ; so that if we make the 

 number which expresses the absorptive power the numerator of a frac- 

 tion, and that which expresses its radiative power, the denominator, 

 the result would be, that, on account of the numerator and denominator 

 varying in the same proportion, the value of that fraction would 

 always remain the same, whatever might be the gas or vapour ex- 

 perimented with. 



But why should this reciprocity exist ? What is the meaning of 

 absorption? what is the meaning of radiation? When you cast a 

 stone into still water, rings of waves surround the place where it falls ; 

 motion is radiated on all sides from the centre of disturbance. When 

 the hammer strikes a bell, the latter vibrates ; and sound, which is 

 nothing more than an undulatory motion of the air, is radiated in all 

 directions. Modern philosophy reduces light and heat to the same me- 

 chanical category. A luminous body is one with its particles in a state 

 of vibration ; a hot body is one with its particles also vibrating, but at 

 a rate which is incompetent to excite the sense of vision ; and, as a 

 sounding body has the air around it, through which it propagates its 

 vibrations, so also the luminous or heated body has a medium, called 

 ether, which accepts its motions and carries them forward with incon- 

 ceivable velocity. Radiation, then, as regards both light and heat, 

 is the transference of motion from the vibrating body to the ether in 

 which it swings : and, as in the case of sound, the motion imparted 

 to the air is soon transferred to the surrounding objects, against which 

 the aerial undulations strike, the sound being, in technical language, 

 absorbed; so also with regard to light and heat, absorption consists in 

 the transference of motion from the agitated ether to the particles of the 

 absorbing body. 



