1881.] Action of an Intermittent Beam of Heat, Sfc. 307 



III. "Action of an Intermittent Beam of Radiant Heat npon 

 Gaseons Matter." By John Tyndall, F.R.S. Received 

 January 3, 1881. 



The Royal Society lias already done me the honour of publishing a 

 long series of memoirs on the interaction of radiant heat and gaseous 

 matter. These memoirs did not escape criticism. Distinguished men, 

 among whom the late Professor Magnus and the late Professor Buff 

 may be more specially mentioned, examined my experiments, and 

 arrived at results different from mine. Living workers of merit have 

 also taken up the question : the latest of whom,* while justly re- 

 cognising the extreme difficulty of the subject, and while verifying, 

 so far as their experiments reach, what I had published regarding 

 dry gases, find me to. have fallen into what they consider grave 

 errors in my treatment of vapours. 



None of these investigators appear to me to have realised the true 

 strength of my position in its relation to the objects I had in view. 

 Occupied for the most part with details, they have failed to recognise 

 the stringency of my work as a whole, and have not taken into account 

 the independent support rendered by the various parts of the investi- 

 gation to each other. They thus ignore verifications, both general 

 and special, which are to me of conclusive force. Nevertheless, 

 thinking it due to them and me to submit the questions at issue to 

 a fresh examination, I resumed, some time ago, the threads of the 

 inquiry. The results shall, in due time, be communicated to the 

 Royal Society; but, meanwhile, I would ask permission to bring to 

 the notice of the Fellows a novel mode of testing the relations of 

 radiant heat to gaseous matter, whereby singularly instructive effects 

 have been obtained. 



After working for some time with the thermopile and galvanometer, 

 it occurred to me several weeks ago that the results thus obtained 

 might be checked by a more direct and simple form of experiment. 

 Placing the gases and vapours in diathermanous bulbs, and exposing 

 the bulbs to the action of radiant heat, the heat absorbed by different 

 gases and vapours ought, I considered, to be rendered evident by 

 ordinary expansion. I devised an apparatus with a view of testing 

 this idea. But, at this point, and before my proposed gas- thermometer 

 was constructed, I became acquainted with the ingenious and original 

 experiments of Mr. Graham Bell, wherein musical sounds are obtained 

 through the action of an intermittent beam of light upon solid bodies. 



* MM. Lecher and Pernter, "Philosophical Magazine," January, 1881. "Sitzb 

 der K. Akad. der Wissensch. in Wien," July, 1880. 



