carbonic 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 assistant, Mr Barrett, has, at my request, made this determination. 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 analyzed chemically by Dr Frankland, and physically by Mr Barrett, gave the following results:
| Percentage of Carbonic Acid in the human breath. | ||
| Chemical analysis. | Physical analysis. | |
| 4·66 | | 4·56 |
| 5·33 | 5·22 | |
It is thus proved that in the quantity of ethereal motion which it is competent to take up,
