RADIATION. 300 



One striking example of the influence of period may 1)C here recorded. Car- 

 bonic acid gas is one of tlie lecblest of absorbers of the radiant heat emitted 1)y 

 solid sources. It is, for example, extremely transparent to the rays emitted by 

 the heated copper plate already referred to. There are, however, certain rays, 

 comparatively few in number, emitted by the copper, to which the carbonic acid 

 is impervious; and could we obtain a source of heat emitting such rays only, we 

 shouhl lind carbonic acid more opaque than any other gas to the radiation from 

 that source. Such a source is actually found in the Bamo of carbonic oxide, 

 wliere hot carbonic acid constitutes the main radiating body. Of the rays emitted 

 l)y our heated plate of copper, oleliant gas absorbs ten times the quantity absorlicd 

 by carbonic acid ; of the rays emitted by a carbonic oxi(U3 flame, carlxjuic; acid 

 ai)Sorbs twice as much as olefiant gas. This wonderful change in the power of 

 tlie f()rmer as an alisorber is simply due to the fact that the periods of the hot 

 and cold carbonic acid are identical, and the waves from t\w fhune freely trans- 

 fer their motion to the molecules which synchronize 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 fhinie, while one- 

 thirtieth of an atmosphere absorbs 48 per cent, of the heat h\nn the same origin. 



In fact the presence of the minutest quantity of carbonic acid may be detected 

 by its action on the rays from the carbonic oxide flame. Oarrying, 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 radia- 

 tion. Radiant heat may indeed be employed as a means of determining practi- 

 cally 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 rbixed 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 f(jrmej- being hnowc, 

 iunnediately gave that of the latter. The same breath, analyzed chemically by 

 Dr. Frankland, and physically by Mr. Banx'tt, gave the following results : 



Percentage of carbonic acid in the liuman breath. 



Chemical analysis. Physical analysis. 



4.0(5 4.5() 



b.S.i 5.^2 



It is thus proved that in the (juantity of ethereal motion which it is competent 

 to take np, we have a practical measiue of the carbonic acid of the breath, and 

 hence of the combustion going on in the human lungs. 



Still this question of period, though of the utmost importance, is not comi)e- 

 tent to account for the whole of the (jbserved facts. Tlie ether, as far as we 

 knt)W, accepts vibrations of all periods with the same readiness. To it the oscil- 

 lations of an atom of oxygen are just as acceptable as those; of a molecule of ole- 

 fiant gas; that the vibrating oxygen then stands so far below the oleliant gas 

 in radiant power must be referred not to period, but to some other pe(;uliarily of 

 the respective molecules. The atomic grouj) which constitutes the molecule of 

 oleliant gas produces many thousand times the disturbance caused l)y the oxy- 

 ijeu because the group is able to lay a vastly more powerful hold n\nm the ether 

 than the single atoms can. The cavities and indentations of a m<decule com- 

 posed of spheriwil atoms may be one cause of this augmented hold. Audther, 

 and i)Ossibly very potent one, may Ix;, that the ether itself, condensed and entaii- 

 gle(l among the constituent atoms of a comjjound, virtually increases tlie magni- 

 tude of the group, and hence augments the disturbance. Whatever may be the 

 fate of these attempts to visualize the physics of the process, it will still n-maiu 

 true, that to account for the phenomena of radiation and absorption we must 



