104 KNOBLAUCH ON THE TRANSMISSION 



To ascertain this, I made use of a process which on former 

 occasions enabled me to detect very small differences in the 

 properties of radiant heat. The principle of the procedure 

 consists simply in ascertaining whether the calorific rays whose 

 qualities were to be compared, possessed the same power of 

 transmission through diathermanous substances. 



The experiments commenced, as before, with brown rock- 

 crystal. The deflection produced by the heat which traversed 

 the crystal parallel to its axis, was first observed and found to be 

 34°*7l' A diathermanous body, a plate of blue glass for ex- 

 ample, was then introduced between the crystal and the pile ; a 

 portion of the heat was thus held back, the pile being excited by 

 those rays only whic!i had passed through the blue glass. The 

 deflection became reduced in consequence to 17°' 19. Hence 

 the quantity of heat transmitted through the crystal and falling 

 on the forward surfaces of the glass, is to the quantity which 

 emanates from the latter in the proportion of 34°' 71 : 17^*19? or 

 of 100:51. 



The calorific rays were now permitted to traverse the crystal in 

 a line perpendicular to its axis. Their direct action upon the 

 thermoscope produced a deflection of 33°'58. When the blue 

 glass was introduced the deflection was reduced to 17°*21. Ac- 

 cording to this, the quantity of heat falling on the forward sur- 

 face of the glass, is to the quantity transmitted through it in the 

 ratio of 33*58 : 17*21, or as 100 : 5 1, exactly as in the former case. 



In the same manner the influence of other diathermanous 

 bodies, of yellow, red and green glass, was ascertained. The re- 

 sults of these experiments are contained in the following table ; 

 the quantity of heat transmitted through the rock-crystal and 

 falling upon the diathermanous plate is, as before, expressed by 

 100, and each number is the mean of about six observations. 



In none of these cases does the observed difference exceed 



