Prof. Tyndall on the Physical Basis of Solar Chemistry. 149 



polarity in magnetism. The gas which, when heated, is most com- 

 petent to generate a calorific ray, is precisely that which is most com- 

 petent to stop such a ray. If the radiation be high, the absorption 

 is high ; if the radiation be moderate, the absorption is moderate ; if 

 the radiation be low, the absorption is low ; so that if we make the 

 number which expresses the absorptive power the numerator of a frac- 

 tion, and that which expresses its radiative power the denominator, 

 the result would be that, on account of the numerator and denomi- 

 nator varying in the same proportion, the value of that fraction would 

 always remain the same, whatever might be the gas or vapour expe- 

 rimented 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 

 the 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 philosophy reduces light and heat to the same 

 mechanical category. A luminous body is one with its particles in 

 a state of vibration ; a hot body is one with its particles 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. 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 the 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 

 cether to the particles 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 runs freely edgeways through the water, and im- 

 parts 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 motion 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 aether without much resistance ; compound ones encounter this, 

 and yield up more speedily their motion to the aether. Mix oxygen 

 and nitrogen mechanically, they absorb and radiate a certain amount. 



