RADIATION. 67 



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 won- 

 derful change in the power of the former, 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 tabe 

 four feet long, absorbs 60 per cent, of the radiation from 

 a carbonic oxide flame, while one-thirtieth of an atmo- 

 sphere 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. Barrett, 

 while under my direction, made this determination. 

 The absorption produced by the breath freed from its 

 moisture, but retaining its carbonic acid, was first deter- 

 mined. 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 

 following results : 



