384 FKAGMENTS OF 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; motion 

 is radiated on all sides from the centre of disturbance. 

 W r hen a 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 philo- 

 sophy 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 aether, which accepts its motions and 

 carries them forward with inconceivable velocity. 

 Eadiation, then, as regards both light and heat, is the 

 transference of motion from the vibrating body to the 

 aether 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 undu- 

 lations strike, the sound being, in technical language, 

 absorbed ; so also with regard to light and heat, absorp- 

 tion consists in the transference of motion from the 

 agitated aether 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 absorption. 

 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 

 aether ; they cannot create the swell which a group of 

 them united to form a system can produce. An oar 



