Prof. Tyndall on Recent Researches on Radiant Heat. 259 



rays of heat. Such a solution on the surfaces of my plates 

 might account in part for the extraordinary absorption which 1 

 have observed. In a series of experiments made with the express 

 intention of wetting the plates of salt by precipitation, Prof. 

 Magnus exalts the absorption to 4 times that of air; but 

 though the plates were visibly wet, no nearer approach than 

 this could be made to my result, which makes the absorption of 

 aqueous vapour 40, 50, and even 60 times that of air. It was 

 only on the inner surface of the salt, which came into contact 

 with the saturated air, that the moisture was precipitated in the 

 experiments of Prof. Magnus; the outer surface, which was in 

 contact with the common air of his laboratory, remained dry; 

 and even the wetted surface, when exposed for a time to the 

 same air, became dry also. I would here, at the commencement, 

 remark that it is with this common outer air, and not with air 

 artificially saturated with moisture, that I find the absorption of 

 aqueous vapour to be 50 or 60 times that of the air in which it is 

 diffused. In fact, if I am correct, the action of aqueous vapour 

 upon radiant heat might be applied in the construction of a 

 hygrometer surpassing in delicacy any hitherto devised. 



I think it would be hardly possible for a person of any experi- 

 mental aptitude whatever, to work, as I have done, for three years 

 with plates of rock-salt, which must be kept polished and bright, 

 without becoming aware of all the circumstances referred to 

 by Prof. Magnus. But the truth is that I was well acquainted 

 with the peculiarities of rock-salt many years before this investi- 

 gation commenced*. A slight consideration of the conditions 

 of the case will, I think, show how improbable it is that a 

 precipitation, such as that surmised, could take place in my ex- 

 periments. First, then, the common air of the laboratory, 

 according to Prof. Magnus, does not produce the effect which 

 he considers may be active in my case; this, as already stated, 

 is the air which I have employed in all kinds of weather, dry as 

 well as moist. Secondly, this air is introduced into a tube 

 through which is passing a flux of heat from the radiating 

 source. Thirdly, the air on entering the tube is heated by the 

 stoppage of its own motion, and thereby rendered more ca- 

 pable of maintaining its vapour in a transparent state. The ex- 

 terior surface of my terminal plate of salt was, moreover, always 

 open to inspection, and it was never found wet; much less 

 could the inner surface be wetted, because the temperature 

 within the tube was higher than that without. 



* The action of moisture upon rock-salt was unhappily made strikingly 

 evident to me some months ago ; for through a chink in the roof of the 

 laboratory some water entered, which destroyed two of my plates, and left 

 me more or less a cripple ever since. 



