332 FRAGMENTS OP SCIENCE. 



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; mo- 

 tion is radiated on all sides from the centre of disturb- 

 ance. When a hammer strikes a bell, the latter vi- 

 brates; and sound, which is nothing more than an 

 undulatory motion of the air, is radiated in all direc- 

 tions. Modern philosophy reduces light and heat to 

 the same mechanical category. A luminous 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 inconceiv- 

 able velocity. Eadiation, then, as regards both light 

 and heat, is the transference of motion from the vibrat- 

 ing 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 undulations strike, the sound being, in tech- 

 nical language, absorbed; 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 general rule, is the radiation and absorp- 

 tion. Let us get definite ideas here, however gross, and 

 purify them afterwards by the process of abstraction. 

 Imagine our simple atoms swinging like single spheres 

 in the ether; they cannot create the swell which a 



