520 Prof. Tyndall on the Action of Free Molecules on 



from a single fact, M. Soret, in 1868, made some further 

 experiments on solar radiation. The intensity was measured 

 by first allowing the rays to fall directly on the thermometer 

 of the actinometer, and then by allowing them, prior to meeting 

 the thermometer, to pass through 5 centimetres of water. 



Calling the first temperature T and the second t, the ratio ^ 



will be obviously greatest when the absorption by the water 

 is least. And as Ave know that water exerts its chief absorbent 

 power on the ultra-red rays of the spectrum, the variations in 

 this ratio observed at different atmospheric thicknesses will 

 enable us to infer the nature of the heat "arrested" by the 

 atmosphere. 



M. Soret found the ratio to be greater in the middle of the 

 day than when the sun is near the horizon. At 12.30, for 

 example, on the 9th of March the ratio was 0*594, while at 

 5.10, on the same day, it was only 0-409. A smaller fraction 

 of the total heat was absorbed by the water near midda}' 

 than near sunset. At midday, therefore, the solar heat was 

 more thoroughly sifted of its calorific rays, and more trans- 

 missible by water, than it was when the atmospheric thickness 

 was much greater. It would seem difficult to reconcile this 

 result with the notion that aqueous vapour is the absorbing 

 constituent of the atmosphere. 



A year subsequently MM. Desains and Branley found, both 

 at Paris and at Lucerne, that the sun's heat was always more 

 transmissible through water and alum in the morning than at 

 midday. I have too much confidence in the able experimenters 

 here named to think any of them wrong. How, then, is the 

 discrepancy between them to be accounted for ? I think in 

 the following way. What is called the glow of the Alps varies 

 greatly with the quantity of suspended matter in the air. 

 When pronounced, it shows that the more refrangible consti- 

 tuents have been in great part removed from the sun's rays. 

 The proportion of the less-refrangible rays in the total radia- 

 tion is augmented in this way, the relative transmissibility of 

 the heat being diminished. It was, I would suggest, heat 

 that had its character impressed upon it in this way by scat- 

 tering, and not by absorption, that yielded the result obtained 

 by M. Soret. 



Whatever may be the value of this explanation, one result 

 of great interest to me was established by the two French 

 experimenters. Simultaneous observations were made by 

 them on the summit of the Bigi and at Lucerne, the vertical 

 distance between both stations being 4756 feet. Within this 

 stratum 17 "1 per cent, of the solar rays was absorbed. 



