Radiation of Heat by Gases and Vapours. 179 



opacity was observed. To remove the last trace of doubt as to 

 the possible action of the gas on the plates of rock-salt, the tin 

 tube referred to at the commencement was perforated at its 

 centre and a cock inserted into it ; the source of heat was at 

 one end of the tube, and the thermo-electric pile at some distance 

 from the other. The plates of salt were entirely abandoned, the 

 tube being open at its ends and consequently full of air. On 

 allowing the olefiant gas to stream for a second or two into the 

 tube through the central cock, the needle flew off and struck 

 against its stops. It was held steadily for a considerable time 

 between 80° and 90°. 



A slow current of air sent through the tube gradually removed 

 the gas, and the needle returned accurately to zero. 



The gas within the holder being under a pressure of about 12 

 inches of water, the cock attached to the cube was turned quickly 

 on and off; the quantity of gas which entered the tube in this 

 brief interval was sufficient to cause the needle to be driven to 

 the stops, and steadily held between 60° and 70°. 



The gas being again removed, the cock was turned once half 

 round as quickly as possible. The needle was driven in the 

 first instance through an arc of 60°, and was held permanently 

 at 50°. 



The quantity of gas which produced this last effect, on being 

 admitted into a graduated tube, was found not to exceed one- 

 sixth of a cubic inch in volume. 



The tube was now taken away, and both sources of heat 

 allowed to act from some distance on the thermo-electric pile. 

 When the needle was at zero, olefiant gas was allowed to issue 

 from a common argand burner into the air between one of the 

 sources of heat and the pile. The gas was invisible, nothing- 

 was seen in the air, but the needle immediately declared its pre- 

 sence, being driven through an arc of 41°. In the four experi- 

 ments last described, the source of heat was a cube of oil heated 

 to 250° Centigrade, the compensation cube being filled with 

 boiling water*. 



Those who like myself have been taught to regard transparent 

 gases as almost perfectly diathermanous, will probably share the 

 astonishment with which I witnessed the foregoing effects. I 

 was indeed slow to believe it possible that a body so constituted, 

 and so transparent to light as olefiant gas, could be so densely 

 opake to any kind of calorific rays; and to secure myself against 

 error, I made several hundred experiments with this single sub- 

 stance. By citing them at greater length, however, I do not think I 



* With a cube containing boiling water I have since made this experi- 

 ment visible to a large audience. 



