M. MELLOXI ON THE POLARIZATION OF HEAT. 147 



SO, I turned the plane of refraction of the mica leaves perpendi- 

 cularly to the virtual plane of refraction of the blackened paper, 

 without on that account altering the common incUnation of the 

 laminae upon the axis of calorific radiation : no difference was 

 produced in the permanent deviation of the galvanometrical 

 index, which, after a few minutes' oscillation, again stopped be- 

 tween 25° and 26°. The action due to the heating of the piles 

 in experiments of polarization is therefore equal for the two 

 du'ections, parallel and perpendicular, that are given to their 

 plane of refraction *. 



Now the index of polarization of a pair of piles at a given in- 

 clination, being only the difference between the two quantities 

 immediately transmitted in the parallel and perpendicular posi- 

 tions of the planes of refraction, referred to the greater of them, 

 it might be inferred that the action of the proper heating of the 

 mica piles would diminish this index, by adding the same quan- 

 tity to the two terms of relation. But mica becomes more 



tei-ior siu-face a quantity of heat which increases according to the diminution 

 of their thickness. But is the fact the same with regard to diathermanous 

 bodies ? 



If the impossibihty of measuring the elevation of temperatui-e of thin laminas 

 prevents the sokition of this question by direct experiment, the properties ac- 

 tually known of immediate transmission furnish us with a satisfactory reply. In 

 fact, glass, water, alum, and diaphanous substances the most refractory to the 

 passage of calorific rays, admit the passage of notable quantities of heat thrown 

 off by sources of all kinds, when reduced into thin laminpe ; and as their faculties 

 of transmission increase in proportion as the thickness diminishes, it is clear 

 that in this case the quantity of heat retained will follow the contrary propor- 

 tion, that is, the heating of the lamina will be in the direct ratio of its thick- 

 ness. But this latter law requires invariability in the radiating source. It 

 cannot always take place in cases in which the laminje are submitted to rays of 

 different origins, for these rays pass in various proportions by the same lamina, 

 and, consequently, heat it in a degree proportionate to their intransmissibility. 

 Certain sorts of heat that traverse in abundance a thin lamina, may therefore 

 communicate to it a slight elevation of temperature, while others will heat it 

 considerably by virtue of their feeble transmission through the substance of 

 which it is composed; and if two laminae, of the same substance, but of different 

 thickness, be exposed to equal quantities of heat, thrown off by different sources, 

 the thick lamina will become less heated than the thin one, if it receives the 

 heat of the source whose rays are more transmissible. 



According to all analogy, the substance which forms the spider's threads is 

 extremely permeable to calorific radiation ; on the other hand, heat emanating 

 from the sun passes with greater facility through diathermanous bodies in 

 general, than heat emitted from any other source. These two causes combined 

 appear to be sufficient to account for the phgenomenon of incombustibility pre- 

 sented by spiders' threads placed in the focus of lenses under the action of the 

 solar rays. 



* Mr. Forbes arrived at the same conclusion, by substituting for the anterior 

 pile the sloping side of a metallic vessel containing hot water. London and 

 Edinburgh Philosophical Magazine, March 18.'36, p. 218. 



