26 MM. Lecher and Pernter on the Absorption 



cent., will appear altogether unintelligible (Hoorweg finds 

 1'5 per cent., and Garibaldi even 92 per cent ; this last, it is 

 true, at a lower temperature); and still less intelligible the 

 constant reference to the absorption of the sun's rays by the 

 action of aqueous vapour — a fact which Secchi*, for example, 

 amongst others, supports. It is true that Tyndall employed 

 aqueous vapour of much higher tension than could be present 

 on Mont Blanc. 



The weight of vapour present in the layer of air between 

 the Glacier des Bossons and the summit of Mont Blanc may 

 be calculated by means of a formula given by Hannf . The 

 data required are the difference in height A = 3610 metres, 

 and the tension of the aqueous vapour p , in millimetres, on 

 the Glacier des Bossons, which, according to Violle's measure- 

 ments, was 5 # 3 millims. ; also the temperatures of the upper 

 and lower stations t x and t 2 , which, according to Violle's data, 

 were 1° C. and 9°'5 C. 



~ 0-0010582 AnQOnl1 t ». 1101 



Q= — —r^oX 0*2832 U — 10 6si 7 l =11'3 kgrm. 



1-r*^ 2 



Then 11 kgrm. would saturate a layer of air of 1040 cubic 

 metres at a temperature of 12°, which was the temperature in 

 Tyndall's laboratory. One metre of this layer would absorb 

 0*017 per cent, if the whole of the absorption were due to the 

 aqueous vapour: this number must be multiplied by 6 to make 

 it applicable to Tyndall's experiments; and then, from the 

 equations 



99-90 = 100e- 1 - a; ? 

 95 snlOOe-*-*, 

 we find that, talcing Violle's measurements as correct, a 5-per- 

 cent, absorption of heat-radiation from a body heated to 270° 

 would require a length of at least 50 metres of saturated aqueous 

 vapour at 12° C, whilst Tyndall {and, in a smaller degree, 

 Hoorweg) finds the same absorption for a length of 1'22 metre. 



From what has been said it is clear that a definitive solution 

 of the question of the actual absorption in our atmosphere can- 

 not be given at present — the more so, since the carbonic acid 

 and the vegetable and animal organisms in the air must be 

 taken into account in deciding this question. 



A decisive solution can only be obtained when the absorp- 

 tion is known for the different parts of the spectrum. 



For the absorptions, so far as they have been measured, are 



* Fogg. Ann. exxvi. 



t Zeittchrift der bsterr. Ges. fiir Meteor. Bd. ix. 



