56 FRAGMENTS OF SCIENCE. 



the light-giving waves follow as the necessary product 

 of the heat-giving ones. 



11. Absorption of Radiant Heat ly Vapours and 

 Odours. 



"We commenced the demonstrations brought for- 

 ward in this lecture by experiments on permanent gases, 

 and we have now to turn our attention to the vapours of 

 volatile liquids. Here, as in the case of the gases, vast 

 differences have been proved to exist between various 

 kinds of molecules, as regards their power of intercept- 

 ing the calorific waves. While some vapours allow the 

 waves a comparatively free passage, the faintest mixture 

 of other vapours causes a deflection of the magnetic 

 needle. Assuming the absorption effected by air, at a 

 pressure of one atmosphere, to be unity, the following 

 are the absorptions effected by a series of vapours at a 

 pressure of -g^th of an atmosphere: 



Name of vapour Absorption 



Bisulphide of carbon ..... 47 



Iodide of methyl ...... 115 



Benzol ........ 136 



Amylene ........ 321 



Sulphuric ether ...... 440 



Formic ether ........ 548 



Acetic ether ....... 612 



Bisulphide of carbon is the most transparent vapour 

 in this list; and acetic ether the most opaque; ^th of 

 an atmosphere of the former, however, produces 47 

 times the effect of a whole atmosphere of air, while 

 5*5 th of an atmosphere of the latter produces 612 times 

 the effect of a whole atmosphere of air. Reducing dry 

 air to the pressure of the acetic ether here employed, 

 And comparing them then together, the quantity of 



