1*32 Prof. Tyndall on the Absorption and 



incident on the interior surface being absorbed. Thus the plate 

 of glass adjacent to the pile must be much more intensely heated 

 with the unblackened tube than with the blackened one. The 

 difference in the amount of heat received by the pile-end plate 

 in the respective cases is rendered very manifest by the experi- 

 ments of Professor Magnus himself; for he finds that, with the 

 same source, twenty-six times the amount of heat transmitted 

 by the coated tube is transmitted by the uncoated one. What, 

 therefore, Professor Magnus ascribes to a change of quality by 

 reflexion, would, if I am correct, be due to the higher heating 

 in the case of the naked tube, and consequent greater chilling by 

 the cold air, of the plate of glass close to the pile. To this must 

 be added the effect produced by cooling the distant end of the 

 tube itself, to which heat has been communicated from the first 

 glass plate by the process of conduction, and the cooling of which 

 comes most into play when the tube is uncovered. 



The difference between Professor Magnus and myself as regards 

 the action of aqueous vapour admits now of easy explanation. 

 His effect being one of convection, and not of absorption, the 

 quantity of vapour present in his experiments — probably not 

 more than 1 per cent, of the volume of the gas, certainly not 2 

 per cent. — vanished as a convecting agent, in comparison with 

 the air. 



It is hardly necessary to repeat these reflections with reference 

 to the experiments of Dr. Franz. The taking of the chilling of 

 his plates for absorption has caused him to find no difference of 

 effect when he doubled the length of his tube. With a tube 450 

 millimetres long, he finds precisely the same absorption as with 

 a tube of 900. He finds the action of carbonic acid to be the 

 same as that of air, although at atmospheric tensions the action 

 of the former is 90 times that of the latter*. He finds the 

 vapour of bromin^ more destructive to radiant heat than nitrous 

 acid gas, whereas the latter is beyond comparison the most 

 destructive. The heat rendered latent by the evaporation of the 

 bromine of course augmented the chill, and thus magnified the 

 effect which in reality he was measuring. In reference to heat- 

 ing the glass plates by the flame made use of in his experiments, 

 I will cite a single passage from the memoir of Dr. Franz. It 

 refers to the vapour of iodine produced by throwing the sub- 

 stance on a heated surface in a vessel closed with glass plates. 

 "The mirror," he writes, "showed a deflection of only 178. 



* The sensible equality of all the transparent gases and air was regarded 

 as evident by Dr. Franz. " It might be seen," he writes, "from the outset 

 that no decided difference would be observed between them" (p. 342). 

 Similarly, Professor Magnus, speaking of aqueous vapour, writes, "Although 

 it might be foreseen with certainty that the small amount of aqueous vapour 

 in the air could have no influence on the radiation," &c. (p. 43). 



