426 Prof. Tyndall on the Absorption and 



reference to the influence of aqueous vapour. With both the 

 gas-flame and the boiling water as sources of heat, he finds the 

 effect of dry air to be precisely the same as that of air which he 

 has allowed to pass in minute bubbles through water, and thus 

 saturated with aqueous vapour. 



I was engaged in experiments on this substance when my other 

 duties compelled me to close this inquiry for a time. I believe, 

 however, I may safely say that not only is the action of aqueous 

 vapour on radiant heat measurable, but this action may be made 

 use of as a measure of atmospheric moisture, the tube used in my 

 experiments being thus converted into a hygrometer of surpassing 

 delicacy. Unhappily, as in other cases touched upon in this 

 memoir, I have been unable to give this subject the development 

 I could wish ; but the results which I am in a position to record 

 are nevertheless interesting, 



On a great number of occasions I compared the air sent in 

 directly from the laboratory into the experimental tube with the 

 same air after it had been passed through the drying-apparatus. 

 Calling the action of the dry air unity, or supposing it rather to 

 oscillate about unity (for the temperature of my source varied a 

 little from day to day), on the following days the annexed ab- 

 sorptions were observed with the undried air of the laboratory : — 



Absorptions by undried air. 

 October 23rd . . 63 I November 1st . . 50 



October 24th . . 62 

 October 29th . . 65 

 October 31st . . 56 



November 4th . . 58 

 November 8th . . 49 

 November 12th . . 62 



Nearly ^ths °f the above effects are due to aqueous vapour ; 

 which, therefore, in some instances exerted nearly sixty times the 

 action of the air in which it was diffused. 



The experiments which I have made on aqueous vapour have 

 been very numerous and varied. Differing as I did from so 

 cautious and able an experimenter, I deemed it due to Professor 

 Magnus and myself to spare no pains in securing myself against 

 error. I have experimented with air moistened in various ways, 

 sometimes by allowing small bubbles of it to ascend through 

 water, sometimes dividing it by sending it through the pores 

 of common cane immersed in water. Between the drying- 

 apparatus and the experimental tube I have introduced tubes 

 containing fragments of glass moistened with water, and allowed 

 the air to pass over them ; large effects were in all such cases 

 obtained, the absorption being usually more than eighty times 

 that of dried air. Fragments of un wetted glass, which had been 

 merely exposed to the air of the laboratory, had dry air led over 



