EADIATION OF HEAT BT GASEOUS MATTEE. 
89 
might be enabled to investigate the interesting question raised by this surmise of Pro- 
fessor Magnus. I have failed, however, to obtain his result. My naked glass tube, which 
is nearly of the same length as his, gives me a result which is more than 140 times less 
than his in the case of air and oxygen, and more than 160 times less than what he has 
obtained with hydrogen. Our sources of heat are, it is true, different, but the disad- 
vantage is on my side ; for assuredly the rays from a gas-jet are, if anything, less affected 
by the transparent elementary gases than those from my source. Had 1 time, I would 
repeat the experiments with a flame ; but this, I regret to say, is out of my power at 
present. 
Another difference between Professor Magnus and myself has 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 he 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. 1 believe, however, I may safely say, that not only is the 
action of aqueous vapour on radiant heat measurable, but this action may he made use 
of as a measure of atmospheric moisture^ the tube used in my experiments being thus con- 
verted 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 1 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 ah’ sent in directly from the labo- 
ratory 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 absorptions were observed with the undried air of the 
laboratory : — 
Absorptions by undried air. 
October 23rd .... 
. 63 
October 24th .... 
. 62 
October 29th .... 
. 65 
October 31st ...... 
. 56 
November 1st . . . . 
. 50 
November 4th .... 
. 58 
November 8th .... 
. 49 
November 12th .... 
. 62 
Nearly i^ths of 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 
MDCCCLXII. N 
