RADIATION. 295 



from nnconnted centres cross, coincide, oppose, and pass tliroiigli each other, 

 witliont confusion or ultimate extinction. Tlie waves from the zenith do not 

 jostk' out of existence those from the horizon, and everj- star is seen across the 

 entanglement of wave motions produced hy all other stars. It is the ceaseless 

 thrill which those distant orbs collectively create in the ctlier which constitutes 

 what we call the temperature of spaee. As the air of a room accommodates itself 

 to the requirements of an orchestra, transmittin<if each vibration of every pipe and 

 string, so docs the inter-stellar ether acconimodate itself to the requirements of 

 light and heat. Its waves mingle in space without disorder, each being endowed 

 ■with an individuality as indestructible as if it alone had disturbed the universal 

 repose. 



All vagueness with regard to the use of the terms radiation and absorptiun 

 will now disappear. Radiation is the communication of vibratory motion to the 

 ether, and when a body is said to be chilled l)y radiation, as, fttr example, the 

 grass of a meadow on a starlight night, the meaning is, that the molecules of 

 the grass have lost a portion of their motion by imparting it to the medium in 

 which they vibrate. On the other hand, the waves of ether once generated, 

 ma}'' so strike against the molecules of a body exposed to their action as to yield 

 np their motion to the latter ; *and in this transfer of the motion fnmi the ether 

 to the molecules consists the absorption of radiant heat. All the phenomena of 

 heat are in this way reducible to interchanges of motion ; and it is j)urely as the 

 recipients or the donors of this motion that we ourselves become conscious of 

 the action of lieat and cold, 



ni. — THE ATOMIC THEORY IN KEFEKEXCE TO THE ETHER. 



The \''ord '' atoms " has been more than once employed in this discourse. 

 Chemists have taught us that all matter is I'educible to certain elementary fonns, 

 to which they give this name. These atoms are endowed with powers of mutual 

 attraction, and under suitable circumstances they coalesce to form compounds. 

 Thus oxygen and hydrogen are elements when separate, or merely mi-xed, but 

 they may be made to combine no as to fonn molecules, each consisting of two 

 atoms of hydrogen and one of oxygen. In this condition they constitute water. 

 So also chlorine and sodium are elements, the former a ])ungent gas, the latter a 

 soft metal; and they unite together to form chloride of sodium or common salt. 

 In the same way the element nitrogen combines with hydrogen, in the propor- 

 tion of one atom of the former to three of the latter, to form anunonia, or spirit 

 of hartshorn. Picturing in imagination the atoms of elementary bodies as little 

 spheres, the molecules of compound bodies must be pictured as groups of such 

 spheres. This is the atomic theory as Dalton conceived it. Now, if this theory 

 have any fotmdation in fact, and if the theory of an ether pervading space, and 

 constituting the vehicle of atomic motion, be founded in fact, w(^ may assuredly 

 expect the vibrations of elementary bodies to be profoundly modified by the act 

 of combination. It is on the face of it almost certain that, both as regards radi- 

 ation and absorption, that is to say, both as regards the communication of motion 

 to the ether and the acceptance of motion from it, the deportment of the uucom- 

 bined will be difl'erent from that of the combined atom^. 



IV. ABSORPTIOX OF RADIANT HEAT BY CASKS. 



We have now to submit these considerations to the only test by which they 

 can tie tried, namely, that of expenment. An experiment is well defined as a 

 question put to Nature ; but to avoid the risk of asking amiss, we ought to purify 

 the (piestion from all adjuncts which do not necessarily lielong to it. Matter 

 has been shown to l)e comjiosed of elementary constituents, by the compoujiding 

 of which all its varieties are produced. But besides the chemical unions which 



