Prof. Tyndall on Recent Researches on Radiant Heat, 263 



his experiments did not actually prove it, it must be evident that 

 the small amount of aqueous vapour in the air cannot sensibly 

 affect the absorption ; and I apply the same consideration of 

 smallness of quantity to account for the neutrality of the aqueous 

 vapour, when mixed with air, as a chilling agent by contact. With 

 regard to absorption, however, the quantity of vapour usually 

 afloat in the atmosphere is quite enormous in comparison with 

 some of the quantities with which I work. Indeed it is com- 

 mon with me to operate with quantities of various vapours which, 

 multiplied thousands of times, would not equal in volume the 

 vapour of the atmosphere. 



Further, an inspection of my experiments showed me long ago 

 that those substances which, in the liquid condition, are highly 

 absorbent of radiant heat, are also highly absorbent in the 

 vaporous condition. Indeed, prompted by this fact, I have already 

 commenced experiments for the purpose of examining whether 

 the same amount of matter does not exert the same absorption, 

 whether it be in the liquid or the vaporous state. Now, water 

 is proved by Melloni to be the most opake liquid that he had 

 examined ; and it would be perfectly anomalous to me, on a priori 

 grounds, if the vapour of this liquid proved so utterly neutral 

 as the experiments of Prof. Magnus would make it. 



But I have also spoken of the exposure of the naked face of 

 the pile to the gas experimented with. My experience of this 

 arrangement is not without instruction. 



I had a square aperture cut into a tin tube, and the face of a 

 pile introduced into the aperture, and cemented air-tight all 

 round. The tube was closed at the ends and put in connexion 

 with an air-pump. The tube being exhausted and the needle 

 of the galvanometer connected with the pile at zero, on allowing 

 air to enter, its motion was soon arrested, and an equivalent 

 amount of heat was generated. This heat, communicated to the 

 face of the pile, was sufficient to dash my needles against the 

 stops at 90°. I do not entertain a doubt of being able to cause 

 my needles to swing through an arc of 500° by the heat thus 

 generated. When, on the contrary, the tube was full at the 

 commencement, and the needle at zero, two or three strokes of 

 the pump sufficed to send the needle up against the stops, the 

 deflection now being due to the chilling of the inner face of the 

 pile. In fact this very deportment of a gaseous body on enter- 

 ing an exhausted receiver, and on being pumped out of a full 

 one, has enabled me to solve the paradoxical problem of deter- 

 mining the radiation and absorption of a gas or vapour without 

 any source of heat external to the gaseous body itself. The pile 

 of Prof. Magnus was exposed to a similar action to that here 

 described, though he never, to my knowledge, refers to it. It 



