RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
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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 
Eig. 8. 
a glass tube 4 inches long and 3 inches in diameter. It is provided with brass flanges 
at the ends which reduce the diameter to 2'5 inches. Against these flanges, transparent 
plates of rocksalt are fixed air-tight. The tightness of the tube was secured, sometimes 
by india-rubber washers properly greased, and sometimes by cement.t A stop-cock a 
* Professor Rontgen was, I believe, the first to turn the expansion of gases to account in demonstrating 
the absorption of radiant heat. The very day, moreover, on which I made my communication to the 
Society of Telegraph Engineers, viz. the 8th of December, 1880, he forwarded to a scientific journal the 
announcement of Ins having obtained sounds from coal-gas and ammonia (see Wiedemann’s Annalen, 
Jan. 1881). His subsequent experiments with aqueous vapour, &c., agree with mine. 
f The absorption of the vapours by india-rubber—which was in some cases extraordinary—caused the 
washers to be abandoned. 
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