112 Prof. Magnus on the Influence of the 



drawback in the case of aqueous vapour, that it condenses on the 

 rock-salt ; and as even the thinnest layer of water only transmits 

 an exceedingly small quantity of heat, the result is interfered with 

 by that thin layer of water. To remove this drawback 1 quite 

 avoided rock-salt plates, by bringing the thermopile inside a tube 

 in a vertical position ; the top part of the tube was closed by very 

 thin glass, which was directly heated externally, and thus served 

 as source of heat. Yet as there is complete discordance between 

 the results which I obtained for aqueous vapour by this method and 

 those which Professor Tyndall has obtained with the use of rock- 

 salt plates ; and as this physicist, although the influence of rock- 

 salt plates is easily ascertained, always reverts to the statement 

 that heat is absorbed by aqueous vapour with several thousand 

 times greater energy than by air; and as this extraordinary ab- 

 sorptive capacity of vapours has, both by himself and others, 

 already been made the basis of an explanation of several im- 

 portant phenomena in meteorology and in terrestrial physics, 

 I have considered it a duty incumbent on me to compare, if pos- 

 sible, in another manner, the absorption of heat by aqueous vapour 

 with that by air. For this purpose the comparison of its radia- 

 tion seemed especially fitted, as all solid walls were thus avoided. 



Such a comparison has been made by Professor Frankland in 

 London*, and described in a paper " On the Physical Cause of 

 the Glacial Epoch." He says there that he has devised a simple 

 method of experimentally proving the radiation of aqueous vapour 

 so that several persons could see the effect at once. " A charcoal 

 chauffer, 14 inches high and 6 inches in diameter, is placed 

 about 2 feet from, and in front of, a thermo-electric pile, the 

 radiation from the chauffer and fuel being carefully cut off from 

 the pile by a double metallic screen. The deflection of the gal- 

 vanometer due to the radiation from the ascending and heated 

 carbonic acid being now carefully neutralized by a constant 

 source of heat radiating upon the opposite face of the pile, a 

 current of steam is made to ascend through an iron tube pass- 

 ing vertically through the chauffer. Instantly the galvanometer 

 deflects for heat much more powerfully than it did previously to 

 its compensation, when it was exposed to the full radiation from 

 heated air and carbonic acid. When the current of stt am is in- 

 terrupted, the needle immediately returns to zero. If now a 

 current of air be forced up the central tube instead of steam, 

 either no deflection at all, or a slight one for cold occurs. The 

 heat of the chauffer effectually prevents any condensation of the 

 steam." 



It is possible that the condensation of steam was avoided by 

 the heat of the chauffer ; yet this would presuppose that the 

 * Phil. Mag. S. 4. vol. xxvii. p. 326, note. 



