30 Prof. Tyndall on the Relation of 



moist air. There seems to be no doubt that this cause is to be 

 sought in the employment of the rock-salt plates. 



When the absorption by dry air was compared with that by 

 air saturated with moisture without using rock-salt plates, a 

 small difference was found between them, but always less than 

 1 per cent. When rock-salt plates were employed, the difference 

 was considerably greater, and when moist air was passed through 

 the tube for a longer time, it attained a value similar to what I 

 had obtained in previous experiments*. 



Besides the defect arising from the hygroscopic character of the 

 rock-salt plates, Prof. TyndalPs method labours under another 

 difficulty. The absorptive powers of the various gases were mea- 

 sured by him by first adjusting the compensating source of heat 

 so that the radiation through the exhausted experimental tube 

 produced no deflection of the galvanometer, and then determin- 

 ing the deflection caused by dry air. The value thus obtained 

 for dry air formed the unit for the determination of the other 

 gases, all of which were compared in the same way with the 

 vacuum. Therefore the smaller the observed difference between 

 dry atmospheric air and the vacuum, the greater the apparent 

 absorptive power of the other gases. Hence, if this difference 

 were to be equal to nothing, the absorption by the other gases 

 would come out infinitely great. 



In the method which I have employed, the determinations are 

 altogether independent of any comparison between an exhausted 

 space and a space filled with air. For the deflection caused by 

 atmospheric air is observed first each time, and then that caused 

 by the gas under examination or by the exhausted tube ; so that 

 each time the values obtained for both under precisely similar 

 circumstances are compared together. 



V. On the Relation of Radiant Heat to Aqueous Vapour. 

 By John Tyndall, F.R.S. fyc.-\ 



I HAVE already placed before the Royal Society an account 

 of some experiments which brought to light the remarkable 

 fact that the body of our atmosphere, that is to say the mixture 

 of oxygen and nitrogen of which it is composed, is a comparative 

 vacuum to the calorific rays, its main absorbent constituent being 

 the aqueous vapour which it contains. It is very important that 

 the minds of meteorologists should be set at rest on this subject 

 — that they should be able to apply, without misgiving, this 

 newly revealed physical property of aqueous vapour ; for it is 



'* Poggendorff's Annalen, vol. cxiv. p. 635. \VhW., Mag. vol.xxiii. p. 249.] 

 t From the Philosophical Transactions, Part I. for 1863, having been 

 read at the Koyal Society December 18, 1862. 



