250 Prof. Magnus on the Passage of Radiant 



atmosphere ; and mainly to the aqueous vapour ." He has, he 

 continues, led by the experiments which I have made, again 

 investigated this subject, and the experiments have shown that 

 the action of aqueous vapour was enormous. On the 10th of 

 October he found that the absorption by the air of the laboratory 

 consisted of three components. If the first, which is due to 

 pure air, be designated by the number 1, the second, produced by 

 the transparent aqueous vapour is 40, and the third, caused by 

 the effluvia of the laboratory and the carbonic acid, is 27. The 

 total action of foreign substances on that day was certainly 67 

 times as great as that of the atmospheric air alone, and that of 

 the aqueous vapour was certainly 40 times as great. 



This statement has caused me to repeat the experiments in 

 this direction which I published in PoggendorfFs Annalen, 

 vol. cxii. p. 539 (Phil. Mag. vol. xxii. p. 85). But neither by 

 using the apparatus described in page 87 and depicted on Plate J. 

 fig 2, in which the heated bottom of a glass vessel sent its heat 

 to the pile through the air without the intervention of any plates, 

 nor even when the heat of a lamp passed through a tube closed 

 with glass plates, could any difference be perceived between air 

 saturated with aqueous vapour at 15° C, and perfectly dried air. 

 It follows again from this, that aqueous vapour, so long as it is 

 not separated as fog, exercises at 15° C. no appreciable influence 

 on the transmission of thermal rays, and that the rays of the sun, 

 so long as the air is clear, reach the earth in the same manner, 

 whether the atmosphere is saturated with vapour or not. 



Besides the above experiments I have made similar ones with 

 plates of rock salt, but I soon found that the use of the latter 

 presents considerable difficulties ; for rock salt in saturated air 

 readily attracts moisture, and becomes covered with a solution of 

 salt which may become so considerable as to drip off. If a plate 

 of rock salt be placed in an inclined position under a bell- 

 glass, under which is a vessel containing water, the solution 

 gradually collects towards the lower parts, and falls in drops 

 into a vessel placed underneath. The attraction of the water 

 was observed in this manner between 10° and 25° C. ; the 

 water underneath the bell-glass had no higher temperature. For 

 the sake of comparison, a glass plate was placed each time near 

 the rock-salt plate under the same bell-glass, but it never showed 

 a trace of moisture. The plates of rock salt used were all quite 

 white and transparent. Plates from Northwich in Cheshire were 

 principally used ; but plates from Wielicza, from Stassfurth, Ischl 

 and Hall in Tyrol, which I happened to possess, showed the same 

 deportment ; and salt from Barcelona, prepared from sea-water, 

 behaved in a similar manner. 



If the plate of salt, after it has become covered in a moist atmo- 



