PHYSICAL BASIS OF SOLAR CHEMISTRY 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 on 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. Kadia- 
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 
