192 LECTURES TO SCIENCE TEACHERS. 



gases were artificially heated. Now he went a step farther. 

 The portion of the tube nearest the pile, instead of being 

 allowed to remain empty, was successively tilled with dif- 

 ferent gases, and it was found that the elementary gases, 

 which are so transcalent to ordinary heat emitted from 

 lamp-black or glowing platinum, are extremely opaque to 

 the radiation emitted from themselves. That is to say, if 

 into this near portion of the tube oxygen gas be admitted, 

 and then oxygen be allowed to stream into the hinder 

 exhausted portion of the tube, the dynamic radiation from 

 this portion of oxygen is completely intercepted by the- 

 absorption of the particles of oxygen in this other portion 

 of the tube. And still more strikingly was this seen 

 when, instead of an elementary gas like oxygen, a com- 

 pound gas like olefiant gas was admitted into this part 

 of the tube. Then a mere trace of a compound gas of 

 the same nature in the front part of the tube com- 

 pletely intercepted the radiation in the posterior por- 

 tion of the tube. Further, if we use here, as a source of 

 heat, the radiation from a hydrogen flame which generates 

 water or aqueous vapour by its combustion, then the radia- 

 tion from that incandescent aqueous vapour is entirely 

 intercepted by a small trace of cool aqueous vapour present 

 in the tube. In like manner, if we use the radiation from a 

 flame of carbonic oxide which produces carbonic acid by its 

 combustion, the radiation from the glowing molecules of 

 carbonic acid is completely intercepted by a small trace of 

 cool carbonic acid in the experimental tube. 



In this way I have found that we may make an extremely 

 delicate analysis of the human breath. By introducing 

 traces of the dry breath into the experimental tube, and 

 using the radiation from a carbonic oxide flame, we have 

 the means of making a physical analysis of the breath, 

 exceeding in delicacy and even rivalling in accuracy the 

 ordinary chemical methods. 



You will perceive that all this is really an illustra- 

 tion of Kirchhoff's familiar law of the reciprocity of radia- 

 tion and absorption. Hence I think you will agree with 

 me that Professor Tyndall, who was led to these conclusions 

 by an independent line of research, if not a pioneer in this 

 direction, like Stokes and Stewart, yet should in truth be 

 regarded as one of those who contributed to lay the broad 



