86 ATOMS, MOLECULES, 



the thousand, I have, to my regret, encountered no fact 

 which proves that free aqueous vapour is decomposed by 

 the solar rays, or that the sun is nourished by the re- 

 combination of gases, in the severance of which it had 

 previously sacrificed its heat. 



The memorable investigations of Leslie and Rum- 

 ford, and the subsequent classical researches of Melloni 

 and Knoblauch, dealt, injbhe main, with the properties 

 of radiant heat ; while, in my investigations, radiant 

 heat, instead of being regarded as an end, was employed 

 as a means of exploring molecular condition. On this 

 score little could be said until the gaseous form of 

 matter was brought under the dominion of experiment. 

 This was first effected in 1859, when it was proved that 

 gases and vapours, notwithstanding the open door which 

 the distances between their molecules might be supposed 

 to offer to the heat waves, were, in many cases, able effec- 

 tually to bar their passage. It was then proved that 

 while the elementary gases and their mixtures, including 

 among the latter the earth's atmosphere, were almost as 

 pervious as a vacuum to ordinary radiant heat, the 

 compound gases were one and all absorbers, some of 

 them indeed taking up with intense avidity the motion 

 of the ether waves. 



A single illustration will here suffice. Let a mix- 

 ture of hydrogen and nitrogen in the proportion of three 

 to fourteen by weight be enclosed in a space through 

 which are passing the heat-rays from an ordinary stove. 

 The gaseous mixture offers no measurable impediment 

 to the rays of heat. Let the hydrogen and nitrogen 

 now unite to form the compound ammonia. A magical 

 change instantly occurs. The number of atoms present 

 remains unchanged. The transparency of the com- 

 pound is quite equal to that of the mixture prior to 



