RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 343 
boiling point of 21° C. The figure 5 attached to the elementary gases, and to dry 
air, expresses, not absorption of radiant heat, but expansion, due to contact with the 
slightly warmed apparatus. The nitrous oxide employed was derived from an iron 
bottle in which it was preserved for medical purposes. In some of my experiments 
marsh gas showed itself a better absorber than nitrous oxide. This, for instance, 
was the case in experiments made in the spring of 1880 with the manometer. The 
sample of marsh gas wherewith the foregoing result was obtained was very carefully 
prepared in our chemical laboratory. 
The temperature of 50° C. in the case of humid air was obtained in a wooden shed 
erected in our laboratory. The shed is traversed by two tubes of sheet iron 4 inches 
in diameter, which carry the heated air and products of combustion from two large 
ring-burners. It is 8' 6" long, 4' 3" wide, and 7' feet high. The temperature of 
the air within it can be readily raised to 60° C. In the experiment recorded in 
Table VI. the air was taken from the outside laboratory through a tube passing 
through the wooden wall of the shed. It was caused to bubble through water con¬ 
tained in a large flask which had been permitted to remain for some time in the warm 
shed. The mixed air and vapour entered the manometer tube at a temperature some 
degrees lower than that of the tube itself. Closely examined, all parts of this tube 
were bright and dry when the vapour-laden air was wfithin it. On permitting the 
beam from the lime light (produced by coal-gas and oxygen) to pass through the 
mixture, a prompt rise of 65 millimeters was the consequence. Cutting the beam off 
the column rapidly returned to zero. The double of 65, or 130 millimeters, gives the 
difference of level in the two legs of the U-tube. 
I have done my best to render these determinations correct. They have been 
repeated both by myself and my assistant'" a great number of times. The first 
measurements were made in the early part of last year, and were made known in the 
Eoyal Institution on the 8th of April, 1881. Difficulties were encountered in obtaining 
a powerful, and at the same time constant, source of heat. The mixture of coal-gas 
and oxygen issuing from independent holders w r as finally resorted to. The sounds are 
classified into “very strong,” “strong,” &c., but it is, of course, impossible to say 
where one class ends and another begins. They shade gradually into each other. 
But if the middle members of any class be compared with the corresponding members 
of another class, the difference of sounding power will appear. 
§. 10. Application of Results to Meteorology. 
If it be at length conceded that aqueous vapour exerts upon radiant heat the 
action which I so long ago ascribed to it, I think the knowledge of this action will 
prove of importance to the scientific meteorologist. Meteorology, as connected with 
heat, seems to me to abound in facts which it has hitherto been incompetent to 
* Who has aided me in this investigation with his usual zeal and intelligence. 
