350 FRAGMENTS OF SCIENCE 



would be, that on account of the numerator and denom- 

 inator varying in the same proportion, the value of that 

 fraction would always remain the same, whatever might 

 be the gas or vapor experimented 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 a ham- 

 mer 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 mechanical category. A lumi- 

 nous body is one with its atoms in a state of vibration; 

 a hot body is one with its atoms 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 inconceivable 

 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 surrounding objects, against which the aerial undula- 

 tions strike, the sound being, in technical language, ab- 

 sorbed; so also with regard to light and heat, absorption 

 consists in the transference of motion from the agitated 

 ether to the molecules of the absorbing body. 



The simple atoms are found to be bad radiators; the 

 compound atoms good ones: and the higher the degree of 

 complexity in the atomic grouping, the more potent, as a 



