RADIATION OF HEAT BY GASES AND VAPOURS. 
9 
preparing the gases, I have resorted to the methods which I found recommended in 
chemical treatises, but as yet only to discover the defects incidental to these methods. 
Augmented experience and the assistance of my friends will, I trust, enable me to solve 
this point by and by. An examination of the whole of the experiments induces me 
to regard hydrogen as the gas which exercises the lowest absorptive power. 
We have here the cases of minimum gaseous absorption. It will be interesting to 
place in juxtaposition with the above results some of those obtained with olefiant gas, 
— the most highly absorbent permanent gas that I have hitherto examined, I select for 
this purpose an experiment made on the 21st of November, 
The needle being steady at zero, in consequence of the equality of the actions on the 
opposite faces of the pile, the admission of olefiant gas gave a permanent deflection of 
70°-3. 
The gas being completely removed, and the equilibrium re-established, a plate of 
polished metal was interposed between one of the faces of the pile and the source of 
heat adjacent. The total amount of heat passing through the exhausted tube was thus 
found to produce a deflection of 
75°. 
Now a deflection of 70°'3 is equivalent to 290 units, and a deflection of 75° is equiva- 
lent to 360 units ; hence more than seven-ninths of the total heat was cut off by the 
olefiant gas, or about 81 per cent. 
The extraordinary energy with which the needle was deflected when the olefiant gas 
was admitted into the tube, was such as might occur had the plates of rock-salt become 
suddenly covered with an opake layer. To test whether any such action occurred, I 
polished a plate carefully, and projected against it for a considerable time a stream of 
the gas ; there was no dimness produced. The plates of rock-salt, moreover, which were 
removed daily from the tube, usually appeared as bright when taken out as when they 
were put in. 
The gas in these experiments issued from its holder, and had there been in contact 
with cold water. To test whether it had chilled the plates of rock-salt, and thus pro- 
duced the effect, 1 filled a similar liolder with atmospheric air, and allowed it to attain 
the temperature of the water ; but its action was not thereby sensibly augmented. 
In order to subject the gas to ocular examination, I had a glass tube constructed 
and connected with the air-pump. On permitting olefiant gas to enter it, not the 
slightest dimness or opacity was observed. To remove the last trace of doubt as to the 
possible action of the gas on the plates of rock-salt, the tin tube referred to at the com- 
mencement was perforated at its centre and a cock inserted into it ; the source of heat 
was at one end of the tube, and the thermo-electric pile at some distance from the other. 
The plates of salt were entirely abandoned, the tube being open at its ends and con- 
sequently full of air. On allowing the olefiant gas to stream for a second or two into the 
tube through the central cock, the needle flew off and struck against its stops. It vras 
held steadily for a considerable time between 80° and 90°. 
MDCCCLXI. c 
