RADIATION. 67 



olefiant gas absorbs ten times the quantity absorbed by 

 carbonic acid. Of the rays emitted by a carbonic oxide 

 flame, carbonic acid absorbs twice as much as olefiant 

 gas. This wonderful change in the power of the for- 

 mer, as an absorber, is simply due to the fact, that the 

 periods of the hot and cold carbonic acid are identical, 

 and that the waves from the flame freely transfer their 

 motion to the molecules which synchronise with them. 

 Thus it is that the tenth of an atmosphere of carbonic 

 acid, enclosed in a tube four feet long, absorbs 60 per 

 cent, of the radiation from a carbonic oxide flame, while 

 one-thirtieth of an atmosphere absorbs 48 per cent, of 

 the heat from the same source. 



In fact, the presence of the minutest quantity of car- 

 bonic acid may be detected by its action on the rays 

 from the carbonic oxide flame. Carrying, for example, 

 the dried human breath into a tube four feet long, the 

 absorption there effected by the carbonic acid of the 

 breath amounts to 50 per cent, of the entire radiation. 

 Radiant heat may indeed be employed as a means of 

 determining practically the amount of carbonic acid 

 expired from the lungs. My late assistant, Mr. Bar- 

 rett, while under my direction, made this determina- 

 tion. The absorption produced by the breath freed 

 from its moisture, but retaining its carbonic acid, was 

 first determined. Carbonic acid, artificially prepared, 

 was then mixed with dry air in such proportions that 

 the action of the mixture upon the rays of heat was the 

 same as that of the dried breath. The percentage of 

 the former being known, immediately gave that of the 

 latter. The same breath, analysed chemically by Dr. 

 Frankland, and physically by Mr. Barrett, gave the fol- 

 lowing results: 



