64 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



Melloni, by the use of the electric pile, vastly in- 

 creased our knowledge of the phenomena of radiant 

 heat. His researches were confined to the solid and 

 liquid forms of matter. Tyndall studied the gases in 

 this respect, showing that differences greater than those 

 established by Melloni existed between gases and 

 vapours, both as regards the absorption and radiation 

 of heat. He proved, moreover, that the aqueous vapour 

 of our atmosphere, by checking terrestrial radiation, 

 augments the earth's temperature, and he considers 

 that the existence of tropical vegetation the remains 

 of which now constitute our coal-beds may have been 

 due to the heat retained by the vapours which at that 

 period were diffused in the earth's atmosphere. Indeed, 

 but for the vapour in our atmosphere, a single night 

 would suffice to destroy the whole vegetation of the 

 temperate regions. 



Inspired by a contemplation of Graham Bell's in- 

 genious experiments with intermittent beams on solid 

 bodies, Tyndall took a new and original departure ; 

 and regarding the sounds as due to changes of tempera- 

 ture he concluded that the same method would prove 

 applicable to gases. He thus found himself in posses- 

 sion of a new and independent method of procedure. 

 It need perhaps be hardly added that, when submitted 

 to this new test, his former conclusions on the inter- 

 action of heat and gaseous matter stood their ground. 



The determination of the mechanical equivalent of 

 heat is mainly due to the researches of Mayer and Joule. 

 Mayer, in 1842, pointed out the mechanical equivalent 

 of heat as a fundamental datum to be determined by 

 experiment. Taking the heat produced by the con- 



