Radiant Heat, and its Conversion thereby into Sound. 509 



the other. Two rock-salt plates were to be employed, with the 

 view of allowing the beam free escape from the tube after it 

 had done its work upon the gas or vapour. The warming of 

 the apparatus by the reverberation of the heat would be thus 

 avoided. The point aimed at was to effect the expansion of 

 the gaseous body purely by radiant heat, and undisturbed as 

 far as possible by heat derived from the envelope*. 



A number of manometric tubes of different lengths and 

 materials were constructed on this principle, some being of 

 glass and some of metal. The instrument with which the 

 measurements now to be recorded were executed is represented 

 in fig. 8. T T' is a glass tube 4 inches long and 3 inches in 

 diameter. It is provided with brass flanges at the ends which 

 Fig. 8. 



* Professor Rontgen was, I believe, the first to turn the expansion of 

 gases to account in demonstrating the absorption of radiant heat. The 



