88 ATOMS, MOLECULES, 



But when we select a radiant with whose waves the 

 atoms of carbonic acid are in accord, the case is entirely 

 altered. Such a radiant is found in a carbonic oxide 

 flame, where the radiating body is really hot carbonic 

 acid. To this special radiation carbonic acid is the 

 most opaque of gases. 



And here we find ourselves face to face with a 

 question of great delicacy and importance. Both as a 

 radiator and as an absorber carbonic acid is, in general, 

 a feeble gas. It is beaten in this respect by chloride of 

 methyl, ethylene, ammonia, sulphurous acid, nitrous 

 oxide, and marsh gas. Compared with some of these 

 gases, its behaviour in fact approaches that of ele- 

 mentary bodies. May it not help to explain their 

 neutrality ? The doctrine is now very generally accepted 

 that atoms of the same kind may, like atoms of different 

 kinds, group themselves to molecules. Affinity exists 

 between hydrogen and hydrogen, and between chlorine 

 and chlorine, as well as between hydrogen and chlorine. 

 We have thus homogeneous molecules as well as hetero- 

 geneous molecules, and the neutrality so strikingly ex- 

 hibited by the elements may be due to a quality of 

 which carbonic acid furnishes a partial illustration. 

 The paired atoms of the elementary molecules may be 

 so out of accord with the periods of the ultra-red waves 

 — the vibrating periods of these atoms may, for ex- 

 ample, be so rapid — as to disqualify them both from 

 emitting those waves, and from accepting their energy. 

 This would practically destroy their power, both as 

 radiators and absorbers. I have reason to know that by 

 a distinguished authority this hypothesis has for some 

 time been entertained. 



We must, however, refresh ourselves by occasional 

 contact with the solid ground of experiment, and an in- 

 teresting problem now lies before us awaiting experi- 



