PHYSICAL BASTS OF SOL AH CI1EM TSTIl Y 253 



meaning of absorption? what is the meaning of radiation? 

 When you cast a stone into still water, rings of waves sur- 

 round the place where it falls; motion is radiated ou all 

 sides from the center of disturbance. When a hammer 

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

 ing 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 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 sound- 

 ing 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. Radia- 

 tion, 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 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 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 after- 

 ward by the process of abstraction. Imagine our simple 

 atoms swinging like single spheres in the ether; they can- 

 not create the swell which a group of them united to form 

 a system can produce. An oar runs freely edgeways through 

 the water, and imparts far less of its motion to the water 

 than when its broad flat side is brought to bear upon it. 

 In our present language the oar, broad side vertical, is a 

 good radiator; broad side horizontal, it is a bad radiator. 

 Conversely the waves of water, impinging upon the flat 

 face of the oar-blade, will impart a greater amount of mo- 

 tion to it than when impinging upon the edge. In the 

 position in which the oar radiates well, it also absorbs 

 well. Simple atoms glide through the ether without much 

 resistance; compound ones encounter resistance, and hence 



