EADIATION or HEAT BY GASES AND VAPOUES. 
27 
The proportion here holds good up to a tension of 2 ‘5 inches, when the deviation 
from it commences and gradually augments. Though these measurements were made 
with all possible care, I should like to repeat them. Dense fumes issued from the 
cylinders of the air-pump on exhausting the tube of this gas, and I am not at present 
able to state wdth confidence that a trace of such in a very diffiise form within the tube, 
did not interfere with the purity of the results. 
Table XXII. - — Nitrous Oxide. 
Absorption. 
Tension in inches, 
r 
Observed. 
Calculated. 
0-5 
14-5 
14-5 
1-0 
23-5 
29-0 
1-5 
30-0 
43-5 
2-0 
35-5 
58-0 
2-5 
41-0 
71-5 
3-0 
45-0 
87-0 
3-5 
47*7 
101-5 
4-0 
49-0 
116-0 
' 4-5 
51*5 
130-5 
5-0 
54-0 
145-0 
Here the divergence from proportionality makes itself manifest from the commence- 
ment. 
I promised at the first page of this memoir to allude to the results of Dr. Feaxz, 
and I will now do so. With a tube 3 feet long and blackened within, an absorption of 
3-54 per cent, by atmospheric air was observed in his experiments. In my experiments, 
however, with a tube 4 feet long and polished within, which makes the distance traversed 
by the reflected rays more than 4 feet, the absorption is only one-tenth of the above 
amount. In the experiments of Dr. Feanz, carbonic acid appears as a feebler absorber 
than oxygen. According to my experiments, for small quantities the absorptive 
power of the former is about 150 times that of the latter; and for atmospheric tensions, 
carbonic acid probably absorbs nearly 100 times as much as oxygen. 
The differences between Dr. Feanz and myself admit, perhaps, of the following expla- 
nation. His source of heat was an argand lamp, and the ends of his experimental tube 
were stopped with plates of glass. Now Mellon: has shown that fully 61 per cent, of 
the heat-rays emanating from a Locatelli lamp are absorbed by a plate of glass one-tenth 
of an inch in thickness. Hence in all probability the greater portion of the rays issuing 
from the lamp of Dr. Feanz was expended in heating the two glass ends of his experi- 
mental tube. These ends thus became secondary sources of heat which radiated against 
his pile. On admitting air into the tube, the partial withdrawal by conduction and con- 
vection of the heat of the glass plates would produce an effect exactly the same as that 
of true absorption. By allowing the air in my tube to come into contact with the 
radiating plate, I have often obtained a deflection of twenty or thirty degrees; the effect 
E 2 
