Mr. B. Stewart on the Radiation and Absorption of Gases. 221 



these gases with that which belongs to an extremely low tempe- 

 rature, or, in other words, with heat of a very low refrangibility 

 or of very great wave-length. Of course I do not imagine that 

 this hypothesis has been proved, but certainly think there is a 

 probability that some such connexion exists. 



Again, with respect to the kind of heat which is radiated and 

 absorbed by rock-salt, I have endeavoured* to show that this is 

 heat of very great wave-length. It may therefore be to some 

 extent the same description of heat as that which I have supposed 

 to be radiated and absorbed by the three permanent gases, which, 

 in conformity with the above hypothesis, is probably heat of great 

 wave-length. 



I would further remark that, in any arrangement where a 

 source of heat such as boiling water is used, and where the 

 other parts of the apparatus are at a temperature, let us say of 

 60° F., the heat which is operated upon is not the radiation 

 of 212° F., but the radiation of 212° F. minus the radiation of 

 60° F. Now, since the average wave-length of the radiation 

 diminishes as the temperature increases, it is easily seen that the 

 radiation of 212° F. minus the radiation of 60° F. has a less 

 average wave-length than the whole radiation of 212° F. Indeed 

 by such an arrangement we separate from the whole body of heat 

 which, by Prevost's theory, is radiated by boiling water, that 

 portion of it which possesses the smallest wave-length or greatest 

 refrangibility, and upon this heat we make our experiments. 



Let us now consider how atmospheric air is situated with 

 respect to an arrangement in which the source of heat is boiling- 

 water, and the temperature of the thermo-pile 60° F., and in 

 which the vacuum-tube is closed with plates of rock-salt, while 

 it is presumed that there is absolutely no atmospheric air in the 

 arrangement except that which is the subject of experiment. 



1. If it be supposed that the heat which atmospheric air 

 radiates and absorbs is that of a very low refrangibility, it is 

 evident that the total radiation and absorption of such air will 

 be less increased by raising the temperature from 60° F. to 

 212° F. than those of another gas which radiates heat of a high 

 refrangibility— the reason of this being that the rays between 

 60° F. and 212° F. are of a comparatively high refrangibility, 

 and that atmospheric air radiates and absorbs these only to a 

 small extent. 



2. Moreover, part of the radiant heat is stopped by the plate 

 of rock-salt ; but it is possible that many of the rays which rock- 

 salt stops are those which are also stopped by atmospheric air. 

 The rock- salt would therefore in this case sift the radiant heat, 



* Transactions of the Royal Society of Edinburgh, vol. xxii. part 1, p. 63. 



