XXIV 



In this paper, Tyndall also studied the radiation of heat by gases, 

 and found that oxygen, hydrogen, and nitrogen are practically in- 

 capable of radiating heat from a source of comparatively low 

 temperature, whilst the radiating power of four compound gases is 

 expressed by the following numbers : 



Carbonic oxide 12 



Carbonic anhydride 18 



Nitrons oxide 29 



Olefiant gas 53 



Their radiative powers follow precisely the same order as their 

 powers of absorption. He then proceeds to discuss the theoretical 

 bearings of his experimental results. He draws attention to the 

 enormous difference in behaviour towards radiant heat exhibited by 

 mechanical mixtures of gases as compared with the same gases chemi- 

 cally combined. Thus hydrogen and nitrogen when mixed together 

 in the proportion of 1 vol. of nitrogen to 3 vols. of hydrogen, produce 

 a scarcely perceptible absorptive effect ; whilst, when chemically 

 united in the form of ammonia, they produce an enormous effect. 

 Again, oxygen and hydrogen which, when mixed in their electrolytic 

 proportion, show scarcely sensible action, when chemically combined 

 in the form of aqueous vapour, exert a powerful action. So also with 

 .oxygen and nitrogen, which, when mixed, as in our atmosphere, both 

 absorb and radiate feebly, when united as nitrous oxide, have their 

 powers vastly augmented. Atmospheric air, freed from moisture and 

 carbonic anhydride, and at a tension of 5 inches, did not effect an 

 absorption equivalent to more than one-fifth of a degree of the differ- 

 ential galvanometer ; whilst nitrons oxide of the same tension effected 

 an absorption equivalent to 51. Hence the absorption by niti-ous 

 oxide at this tension is about 250 times that of air. In like manner 

 the absorption by carbonic oxide of this tension is nearly 100 times 

 that of oxygen alone; the absorption of carbonic anhydride being 

 about 150 times that of oxygen ; whilst the absorption by olefiant gas 

 of this tension is 1000 times that of its constituent hydrogen. But 

 even the enormous action last mentioned was surpassed by the vapour 

 of many volatile liquids possessing greater atomic complexity. 



Tyndall visualised to himself the cause of this enormous difference. 

 He considered that the compound molecules present broad sides to the 

 ether, while the simple or elementary molecules do not ; bnt, in con- 

 sequence of these differences, the ether must swell into billows when 

 the former are moved, while it merely trembles into ripples when the 

 latter are agitated. In the interception of motion also, the former, 

 other things being equal, must be far more influential than the latter. 



Now, besides presenting broader sides to the ether, the association 

 of atoms to form groups must, as a general rule, render their motion 



