Radiation of Heat by Gases and Vapours. 277 



De Saussure, Fourier, M. Pouillet, and Mr. Hopkins regard 

 this interception of the terrestrial rays as exercising the most 

 important influence on climate. Now if, as the above experi- 

 ments indicate, the chief influence be exercised by the aqueous 

 vapour, every variation of this constituent must produce a 

 change of climate. Similar remarks would apply to the car- 

 bonic acid diffused through the air, while an almost inappre- 

 ciable admixture of any of the hydrocarbon vapours would pro- 

 duce great effects on the terrestrial rays and produce correspond- 

 ing changes of climate. It is not, therefore, necessary to assume 

 alterations in the density and height of the atmosphere to account 

 for different amounts of heat being preserved to the earth at dif- 

 ferent times ; a slight change in its variable constituents would 

 suffice for this. Such changes in fact may have produced all the 

 mutations of climate which the researches of geologists reveal. 

 However this may be, the facts above cited remain; they constitute 

 true causes, the extent alone of the operation remaining doubtful. 



The measurements recorded in the foregoing pages consti- 

 tute only a fraction of those actually made; but they fulfil the 

 object of the present portion of the inquiry. They establish the 

 existence of enormous differences among colourless gases and 

 vapours as to their action upon radiant heat; and they also 

 show that, when the quantities are sufficiently small, the absorp- 

 tion in the case of each particular vapour is exactly proportional 

 to the density. 



These experiments furnish us with purer cases of molecular 

 action than have been hitherto attained in experiments of this 

 nature. In both solids and liquids the cohesion of the particles 

 is implicated ; they mutually control and limit each other. A 

 certain action, over and above that which belongs to them sepa- 

 rately, comes into play and embarrasses our conceptions. But 

 in the cases above recorded the molecules are perfectly free, and 

 we fix upon them individually the effects which the experiments 

 exhibit ; thus the mind's eye is directed more firmly than ever 

 on those distinctive physical qualities whereby a ray of heat is 

 stopped by one molecule and unimpeded by another. 



§ 9. Radiation of Heat by Gases. — It is known that the quan- 

 tity of light emitted by a flame depends chiefly on the incan- 

 descence of solid matter, — the brightness of an ignited jet of or- 

 dinary gas, for example, being chiefly due to the solid particles 

 of carbon liberated in the flame. 



Melloni drew a parallel between this action and that of radiant 

 heat. He found the radiation from his alcohol lamp greatly 

 augmented by plunging a spiral of platinum wire into the flame. 

 He also found that a bundle of wire placed in the current of 

 hot air ascending from an argand chimney gave a copious radia- 



