48 EFFECT OF THE THICKNESS OF A GAS. PART 1. 



bers by a third plate of rock-salt movable in the interior ; 

 the source of heat being at one end and the differential 

 goniometer at the other. 



Carbonic oxide and carbonic acid are pervious to a 

 vast majority of the rays of radiant heat. When the 

 cylinder was filled with carbonic oxide gas and so 

 divided, by moving the internal plate of rock-salt, that 

 a stratum of the gas 8 inches long was next to the 

 source of heat, and that 41 '4 inches long farthest from 

 it, the 8 inches of gas intercepted 6 per cent, of the 

 whole radiation. But when the plate of rock-salt was 

 moved till the column 41 '4 inches long was next to the 

 source of heat, and that of 8 inches farthest from that 

 source, or behind the long one, the absorption of the 8 

 inches was sensibly zero. In like manner eight inches 

 of carbonic acid gas when in front of a column of 41*4 

 inches of the same gas absorbed 6J per cent, of the 

 whole radiatibn, while placed behind that column the 

 effect was nearly zero. The reason is that when the 

 8-inch stratum is in front, it stops the main portion of 

 the rays which give it its thermal colours, 7 while placed 

 behind these same rays have been almost wholly with- 

 drawn, and to the remaining 94 per cent, of the radia- 

 tion the gases are sensibly permeable. 



It is inferred from an extension of this reasoning that 

 the sum of the absorptions of the two chambers taken 

 separately must always be greater than the absorption 

 effected by a single column of the gas of a length equal 

 to the sum of the two chambers ; this conclusion is 

 illustrated in a striking manner by the experiments, 

 It is also found that when the mean of the sums of the 

 absorptions is divided by the absorption of the sum, the 

 quotient is sensibly the same for all gases. It may 

 farther be inferred that the sum of the absorptions 



7 Analogous to transparent media which receive their colour by stopping 

 or absorbing some of the colours of white light and transmitting others. 



