34 THE 'POPULAR SCIENCE MONTHLY. 



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 rec- 

 ognizing 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 vapors. 



None of these investigators appear to me to have realized 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 recognize 

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

 count the independent support rendered by the various parts of the 

 investigation to each other. They thus ignore verifications, both 

 general and special, which are to me of conclusive force. Never- 

 theless, 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 ra- 

 diant heat to gaseous matter, whereby singularly instructive effects 

 have been obtained. 



After working for some time with the thermopile and galvanome- 

 ter, 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 vapors in diathermanous bulbs, and exposing 

 the bulbs to the action of radiant heat, the heat absorbed by differ- 

 ent gases and vapors 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-thermom- 

 eter 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. 



From the first, I entertained the opinion that these singular sounds 

 were caused by rapid changes of temperature, producing correspond- 

 ing changes of shape and volume in the bodies impinged upon by the 

 beam. But if this be the case, and if gases and vapors really absorb 

 radiant heat, they ought to produce sounds more intense than those 

 obtainable from solids. I pictured every stroke of the beam respond- 

 ed to by a sudden expansion of the absorbent gas, and concluded that, 

 when the pulses thus excited followed each other with sufficient rapid- 

 ity, a musical note must be the result. It seemed plain, moreover, 

 that by this new method many of my previous results might be 

 brought to an independent test. Highly diathermanous bodies, I 



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

 Akad. der Wissench. in Wien," July, 1880. 



