Radiant Heat, and its Conversion thereby into Sound. 525 



But the most impressive illustration of the action of aqueous 

 vapour is now to be referred to. In 1865 I subjected to ex- 

 amination the radiation from the electric light produced by a 

 battery of 50 of Grove's cells, and found, by prismatic analysis, 

 the invisible calorific radiation to be 7*7 times the visible. The 

 determination was afterwards made by the method of filtration, 

 whereby the one class of rays was detached with great sharp- 

 ness from the other, and both of them rendered measurable. 

 By this method the invisible radiation was found to be 8 

 times the visible. A close agreement was therefore established 

 between the results of the two methods. Computed from the 

 diagram of Miiller, the invisible radiation of the sun is twice 

 the visible. This smaller ratio might, of course, be referred to 

 the original quality of the solar emission, the ratio holding- 

 good up to the surface of the sun. But having placed, as I 

 thought beyond doubt, the action of aqueous vapour on radiant 

 heat, and believing the action of the vapour to be substan- 

 tially the same as that of water, I reasoned and experimented 

 as follows in 1865 : — " The sun's rays, before reaching our 

 earth, have to pass through the atmosphere, where they en- 

 counter the atmospheric vapour, which exercises a powerful 

 absorption on the invisible calorific rays. From this, apart 

 from other considerations, it would follow that the ratio of the 

 invisible to the visible radiation in the case of the sun must 

 be less than in the case of the electric light. Experiment, 

 we see, justifies this conclusion. If we cause the beam from 

 the electric lamp to pass through a layer of water of suitable 

 thickness, we place its radiation in approximately the same 

 condition as that of the sun; and on decomposing the beam 

 after it has been thus sifted, we obtain a distribution of heat 

 closely resembling that observed in the solar spectrum." 



If, therefore, we could get above the vapour-screen which 

 swathes the earth, the " powerful absorption " referred to in 

 the paragraph just quoted would disappear, the ratio of the 

 invisible to the visible solar rays being augmented corre- 

 spondingly. That such would be the case I have long taken 

 for granted ; but I hardly hoped for a corroboration so im- 

 pressive as that furnished by the recent observations of Pro- 

 of vapour-tension. From this and from a similar result obtained by a 

 second method of calculation, be draws tbe conclusion " tbat there can be 

 very little error in agreeing with Dr. Tyndall that the absorptive power 

 of dry air is sensibly nothing, and that the total absorptive power of the 

 atmosphere is due to the water-vapour it contains." A most interesting 

 discourse on Solar Heat, by M. Violle, of Grenoble, will be found in the 

 Revue Scientifique for 1878, p. 944. I guard myself against saying that 

 the diathermancy of dry air is perfect. 



