OF RADIANT HEAT THROUGH DIFFERENT BODIES. 21 



fact, everything leads us to believe that the equal layers which succeed 

 one anotlier in a diaphanous medium, act in the same manner on the 

 rays of light which come in succession to pass through them, and that 

 they consequently absorb or reflect a quantity of light proportional 

 to the intensity of the incident rays ; that is to say, that the loss sus- 

 tained by the luminous radiation at every layer of equal thickness is 

 constant. In the case under consideration, the invariable decrement of 

 the light at each of the layers into which we suppose the screen divided 

 is found to be none at all, or extremely feeble, because of the perfect 

 transparency of the glass ; and yet the caloric rays undergo in their 

 successive passages an absorption, the sum of which is equal to about 

 the half of their whole value ; and the losses at each layer, instead of 

 being constant, as happens to those sustained by the luminous rays, are 

 found to differ enormously from one another, being m the proportion of 

 the numbers 381, 71, 3U and 16. 



The resistance of diaphanous media to the immediate transmission of 

 the rays of heat is therefore of a nature altogether different from that 

 which is presented by the same media to the propagation of light. 



Whatever be the cause of this singular difference, it is highly im- 

 portant to determine with certainty whether it takes place at great di- 

 stances from the surface at which the rays enter ; and this may be done 

 by repeating the experiments on layers of glass much thicker than those 

 which we have been using. 



, With this view I took several pieces of the glass of Saint-Gobain, and 

 caused them to be recast. This operation was not completely success- 

 ful. The matter either formed itself into layers that were too thin, or 

 was slightly striated. From among the thick pieces I selected that which 

 was the purest. It was six inches in length. I divided it into three parts, 

 of one, two, and three inches in thickness. The defects being uniformly 

 distributed over all the points of the mass might probably enough alter 

 the quantity of the caloric rays that would have passed through a per- 

 fectly pure mass of the same matter and thickness ; but it is clear that 

 they could have no influence on the nature of the progression of the 

 losses which these rays might undergo in passing from one layer to an- 

 other. 



The following are the results obtained by exposing these screens to 

 the ordinary radiation of 30° : 



Thickness of the screens ^ , .... 



in millimetres. Galvanometric deviations. 



27 17-105 



54. 13-458 



81 10-702 



By a calculation exactly similar to that already made wc find that, 



