SECT. ii. BY GASES AND VAPOURS. 39 



only dried in this manner, but it was deprived of its 

 carbonic acid by passing over caustic potash, and many 

 other precautions were taken to prevent the possibility 

 of error. 



Under the ordinary pressure of the atmosphere, when 

 the experimental tube was exhausted, the needle of the 

 goniometer stood at zero, but as soon as pure dry 

 atmospheric air was introduced into the tube its absorp- 

 tion caused the needle to move from zero to 1. 



The tube was again exhausted ; the needle stood at 

 zero, but was deflected from zero to 1 as soon as the 

 tube was filled with oxygen. A similar experiment was 

 made with nitrogen and hydrogen with the same result. 

 Thus, dry air and the elementary gases, oxygen, nitrogen, 

 and hydrogen, have the same absorptive power, and 

 consequently they all deflected the needle of the gflxne=- <jf & 

 meter one degree. The whole amount of radiant heat 

 that passed through the exhausted tube produced a 

 deflection of 71 5' ; hence taking as unit of heat the 

 amount that would deflect the needle one degree, the 

 number of units expressed by 71 5' is 308, consequently 

 the absorption of each of these four gases amounts to 

 -3% I? or 0"^ P er cen ^. The most delicate tests could not 

 show any difference between the three first, but Pro- 

 fessor Tyndall had reason to believe that hydrogen has 

 the lowest absorptive power of all gases and vapours, 

 though he was unable to express the amount. The 

 absorptive power of all four is very much less than that 

 of every other gas or vapour, and invariably deflects the 

 needle to 1, which thus becomes the unit of comparison. 

 Olefiant gas, the most luminous of the constituents of 

 coal gas, possesses the highest absorptive power of the 

 permanent gases. When sent into the exhausted tube 

 it deflected the needle of the goniometer from to 

 70 3', which is equivalent to 290 units. The whole 

 heat that passed through the exhausted tube before the 



