SECT. ii. EFFECT OF THE THICKNESS OF A GAS. 47 



ferent thicknesses of the same gas or vapour on radiant 

 heat. The experiments extend from a thickness of 0*01 

 of an inch to that of 49*4 inches. The instrument em- 

 ployed for ascertaining the action of the smaller thick- 

 ness was a horizontal hollow cylinder closed at one end 

 by a plate of rock-salt. A second cylinder was fitted 

 into this with its end also closed by a plate of rock-salt. 

 This cylinder moved within the other like a piston, so 

 that the two plates of rock-salt could be brought into 

 flat contact with one another, or could be separated to 

 any required distance, and the distance between the 

 plates was measured by a vernier. At one end of 

 the cylinder there was a source of constant heat, and 

 the differential goniometer already described at the 

 other. With this apparatus Mr. Tyndall found that 

 olefiant gas maintains its great superiority over the 

 other gases in absorptive power at all thicknesses. A 

 layer of that gas not more than O'Ol of an inch thick 

 intercepted about one per cent, of the total radiation. 

 This great absorption corresponded to a deflection of 

 11 of the needle of the goniometer, and such was the 

 delicacy of the apparatus that it would be possible to 

 measure the action of a layer of this gas of less thickness 

 than a sheet of writing paper. A layer of olefiant gas 

 two inches thick intercepts nearly 30 per cent, of the 

 entire radiation. A shell of olefiant gas two inches thick 

 surrounding our globe would offer no appreciable hin- 

 drance to the solar rays in coming to the earth, but it 

 would intercept, and in great part return, 30 per cent, of 

 the terrestrial radiation ; under such a canopy the sur- 

 face of the earth would probably be raised to a stifling 

 temperature. 



The apparatus for measuring the action of the greater 

 thicknesses of gas was a hollow brass cylinder 49 '4 

 inches long, closed at both ends by plates of rock-salt, 

 and divided internally into two compartments or cham- 



