166 M. MELLONI ON THE POLARIZATION OF HEAT. 



happens when, instead of incandescent platina, a metallic lamina 

 heated to 400° is employed, or simply a vessel filled with boiling 

 water. 



But the heat of these two latter sources being very slightly 

 transmissible by the mica, and consequently unable to traverse 

 piles composed of a great number of laminae, notwithstanding the 

 action of the salt lens by which their parallelism is estabUshed, 

 I receive the parallel rays emerging from the apparatus of polar- 

 ization upon a second lens of rock salt, which collects them all, and 

 concentrates them upon the thermoscopic body. The divergent 

 rays, which proceed from the heating of the posterior pile, must 

 be weakened until they become perfectly insensible, either by re- 

 moving the collecting lens to a suitable distance, or by bringing 

 it very near. In the first case these rays are more and more dis- 

 persed by their natural divergence, and arrive upon the collector- 

 lens M'ith too little intensity to give an appreciable effect. In the 

 second case, the central parts of the last sheets of mica being 

 within the principal focal distance, the greater part of their pro- 

 per rays of heat, instead of being concentrated and consequently 

 mixed with the direct heat, are, on the contrary, dispersed by 

 the lens still more rapidly than is effected by their natural diver- 

 gence, and have no action whatever at a very short distance. 

 Whatever be the means adopted, care should always be taken, 

 after the collector is added, to ensure that the condition of the 

 insensibihty of the thermoscope to the heating of the piles is 

 exactly fulfilled. For this pux-pose the anterior pile is to be re- 

 moved from its frame, and in its place is to be substituted, as in 

 the experiment at page 146, a sheet of paper blackened upon each 

 side, Avhich becomes as much heated as mica, and even more, 

 because it does not immediately transmit radiant heat. If every- 

 thing be well arranged, no appreciable calorific action is obtained. 

 In the apparatus which I possess, the use of the collector about 

 doubles the intensity of the effects, while preserving, according 

 to the method just indicated, the direct rays completely pure, 

 and without mixture with the secondary heat of the pUes. 



*Experiments of polarization may be thus carried, with the 



* It is evident that more might be gained with piles of mica, and a lens of 

 larger dimensions. To attain this amplification of tlie thermoscopic effects, 

 Mr. Forbes, in his second series of experiments upon polarization, employed 

 the conical reflector of brass that M. Gourjon generally uses, in addition to my 

 apparatus of transi.iission ; hut this icflcctor collects at the same time the 

 direct heat of the source, and that proceeding from the heating of the piles, 



