60 M. MELLONI ON THE IMMEDIATE TRANSMISSION 



Let US now proceed to consider the transmission of heat through he- 

 terogeneous screens. The calorific rays emerging from each plate ex- 

 posed to the action of the same source produce a particular elevation of 

 temperature when they fall on the thermoscopic body of our apparatus. 

 Whence we have inferred that the quantity of heat which passes through 

 a given screen varies according to the quality and thickness of the sub- 

 stance. But, it may be a.sked, is this the only difference between ihe 

 rays immediately transmitted through bodies of different kinds? 



For the purpose of answering this question we have made the follow- 

 ing experiments. 



If the rays from a Locatelli lamp be brought to act on a thermoelec- 

 tric pile after having previously passed through a screen of diaphanous 

 matter (such as citric acid) but in a slight degree permeable to radiant 

 heat, the effect obtained in the ordinary case, in which the whole ac- 

 tion is equivalent to 30° of the thermomultiplier, will be very inconsi- 

 derable ; but it may be increased by bringing the source of heat nearer, 

 or by concentrating its rays on the plate with the help of metallic mir- 

 rors or lenses of rock salt. I suppose then that a deviation of 25° or 

 30° of the galvanometer has been produced through a plate of citric 

 acid. I now interpose a plate of alum in such a manner that the rays 

 emerging from the citric acid may be forced to pass thi'ough it before 

 they can reach the thermoscopic body ; the magnetic needle descends 

 only about 3 or i degrees. 



I now recommence the operation on any other diaphanous and colour- 

 less substance different from the citric acid ; that is to say, I vary the 

 distance from the lamp to the pile until I obtain the same galvanome- 

 tric deviation of 25° or 30° by the action of the radiant heat on this ncAv 

 substance also. I then interpose the plate of alum, and the magnetic 

 index, as in the case of the citric acid, descends again not more than 

 about 3 or 4 degrees, but it aj^proaches nearer to zero, and the retro- 



tbat caloric and light bear to one another, we have no means of proving that, as 

 no polarization of beat is produced by the transmission through tbe tom-malines, 

 none can be produced by reflexion at tbe surface of the glass. I am bound also 

 to remark that some very able experimental philosophers having lately tried to 

 polarize light by M. Beravd's process, their efforts proved vniavailing. Mr. Powell 

 informs us that although he had taken the necessary precautions against the 

 heating of the glass and other causes of error he has never been able to discover 

 the least appearance of polarization when operating with nonluminous heat 

 But he Uiinks that when he employed luminous sources he was enabled to ob- 

 serve a small perceptible effect by making the rays previously pass through a 

 screen of glass (Edi.ib. Journal of Science, N. S., vol. vi.) Mr. Lloyd communi- 

 cated at the last meeting of the British Association for the Advancement of 

 Science (Cambridge 183,'5) some new results tending to support the conclusions 

 derived by Mr. Powell from his own experiments. [No communication upon 

 this subject by Professor Lloyd appears in the Report of the British Association 

 for 1833.— Edit.] 



