[ 475 ] 



LXIII. On the immediate Transmission of Calorific Rays 

 through Diathermal Bodies. By M. Macedoine Melloni.* 



A T the last meeting of the British Association for the Ad- 

 -**• vancement of Science, Mr. H. Hudson and Mr. Powell 

 furnished several communications on radiant caloric t* After 

 having cited some of my experiments on calorific transmis- 

 sion, these ingenious philosophers endeavoured to explain 

 them by hypotheses which in my opinion can no longer be 

 sustained in the present state of science. I wish to direct 

 inquiry to a subject which by its intimate connexion with the 

 fundamental properties of one of the principal agents of nature, 

 appears to me worthy to engage our attention. 



For a long time the immediate transmission of terrestrial 

 radiant heat by transparent substances, both solid and liquid, 

 has been denied ; and the opinion has become prevalent that 

 we see in experiments of this kind only an effect of the heat 

 absorbed by the body submitted to the calorific radiation. 

 Hence, from the first researches which I undertook upon the 

 immediate transmission of heat, I have endeavoured to render 

 my observations entirely independent of the heating effect 

 proper to the diaphanous plate submitted to experiment ; and 

 I succeeded in this by a very simple arrangement, which con- 

 sists in diminishing as much as possible, in the first instance, 

 the heating effect of the plate, by placing it at a consider- 

 able distance from the source, and then in rendering its action 

 upon the thermoscope wholly insensible^ by removing the in- 

 strument to the requisite distance from the plate itself. But in 

 order to experiment under these circumstances, it is clearly ne- 

 cessary to employ an extremely delicate thermoscope, such as 

 well-constructed thermomultipliers otherwise, the feeble rays 

 of heat, direct or transmitted, which arrive from the distance 

 at which the instrument is fixed, would produce no percep- 

 tible effect. Further, when any one wishes to make experi- 

 ments on the transmission of caloric, he may always assure 

 himself that the condition above mentioned is fulfilled. For 

 that I have given four different proofs : the following is the one 

 which is inserted in the Report on Radiant Heat made by 

 M. Biot to the Academie des Sciences ; it will soon be seen 

 why I have preferred this proof to the others. 



Let us suppose the source of heat, the body, and the ther- 

 momultiplier in the proper positions. The plate of the dia- 



* Communicated by the Author, through Michael Faraday, Esq.,D.C.L., 

 F.R.S. 



t Abstracts of these communications have been .given in pp. 296-298 of 

 our present volume. — Edit. 



3 P2 



