EADIATION OF HEAT BY GASEOUS MATTEE. 
71 
27 per cent. ; while a layer so opake that it cut off the beams of the sun itself trans- 
mitted 23 per cent, of the rays emitted by a source heated to 100° C. 
At page 93 of ‘ La Thermochrose,’ Melloni examines the absorption of this substance 
for all sorts of rays, and by a series of ingenious experiments, and reasonings remark- 
able for their clearness and precision, he arrives at the conclusion that lampblack 
absorbs with the same intensity all descriptions of radiant heat^. At page 284, how- 
ever, he cites and discusses vdth the same precision a series of experiments made with 
smoked rock-salt, in which he shows that the same layer of lampblack transmits 8 per 
cent, of the rays from a lamp of Locatelli, 10 per cent, of those of incandescent pla- 
tinum, 18 per cent, of those from copper heated to 400° C., and fully 23 per cent, of 
those emitted by a source of 100° C. Now a transmission of 8 per cent, implies an 
absorption of 92 ; while transmissions of 10, 18, and 23 per cent, imply absorptions of 
90, 82, and 77. But that the self-same layer of lampblack absorbs 77 per cent, of the 
rays from one source and 92 per cent, of the rays from another, is at variance with the 
statement that lampblack absorbs heat from all sources with the same intensity. Sup- 
pose the surface of a thermo-electric pile to be coated by a layer of lampblack of the 
same thickness as that which coated Melloxi’s plate of salt; 23 per cent, of the rays 
from a source of 100° C. would go right through such a layer and impinge upon the 
metal face of the pile ; and if the latter were a good reflector, the heat incident upon it 
would be in gi*eat part retransmitted through the lampblack and lost to the instrument. 
For a source of 100° C., this loss would be many times greater than for a Locatelli lamp. 
Possibly, however, Melloni meant simply to assert that for practical purposes the 
absorption by the face of his pile might be considered to be the same for all kinds of 
heatf. 
§ 5 . 
I have now to record some new experiments on the action of vapours upon radiant 
heat. A number of glass flasks were prepared, of the shape and size of common test- 
tubes, each of which was furnished with a brass cap carefully cemented on to it. By 
means of this it could be attached to a stopcock, and thus connected with the experi- 
mental tube. The mode of operation was this. The liquid was introduced into the 
flask by means of a small glass funnel ; the stopcock (S) was then attached to the flask 
and connected with a second air-pump, which was always kept at hand. The air above 
the liquid was removed, and the air dissolved in it was allowed to bubble away, until 
nothing remained but the pure liquid below and the pure vapour above it. The stop- 
cock S was now shut off, and the flask united to the experimental tube. The exhaustion 
of the tube and stopcocks being complete, and the needle of the galvanometer at zero, 
the cock S was turned on and the mercury-gauge carefully observed at the same time. 
* “Done, le noir de fumee absorbe avec la meme intensite toute sorte de rayonnements calorifiques ” (p. 101). 
t The snn, through the floating carbon of the London atmosphere, sometimes presents a most instructive 
appearance. Entirely shorn of his rays and of perfectly uniform brightness, his colour at times is as red as 
blood. This is doubtless mainly due to the comparative transparency of the floating carbon for the longer 
undulations. 
