42 



M. MELLONl ON THE IMMEDIATE TRANSMISSION 



being, in those different circumstances, submitted to the same quantity 

 of radiant heat, the different degrees of diminution suffei'ed by this heat 

 in passing through it must evidently be attributed only to the peculiar 

 (juality of each radiation. This reflection will give still greater force 

 to the truth of the consequences which we are about to deduce from the 

 results of our experiments. 



Seven plates of glass of different degrees of thickness submitted to 

 the action of the four sorts of calorific rays in succession have given the 

 following transmissions : 



Although we do not exactly know the degree of heat given by the 

 flame of oil or by platina kept in a state of incandescence by an alcohol 

 lamp, we are nevertheless quite certain that the first of these possesses 

 a higher temperature than the second, and that this again exceeds the 

 390° of the first plate of copper. Now a glance at the table is sufficient 

 to show that the number of rays transmitted by the same plate decreases 

 with the temperature of the calorific source, a fact which confirms the 

 well-known law of Delaroche. But the decrease is more or less rapid 

 in proportion to the greater or less thickness of the plate. 



Let OM, ON, (Plate I. Fig. 1.) be two rectangular axes of the same 

 length; let the first represent the thickness of the screen of 8""" and the 

 second the total quantity of incident heat. Let us divide OM into six parts, 

 0a,0b,0c;0d,0 e, Of, respectively equal to -5^ O M, -^ O M, i O M, 

 i OM, 5- OM,and |- OM; and through the points of division let us 

 draw the perpendiculars aa' =z ^ ON, i 6' = ^ ON, c c' = ^ ON, 



rfd' = i^ON, ee' = i^ON,//' = ,^ON,M^' = ^ON. Thecurve 

 (a' b' c' d' e'f g'} passing through the extremities of these perpendicu- 

 lars will represent the decreasing intensity of the Locatelli lamp at each 

 point of the screen of 8™"^ in thickness. 



