EADIATION OF HEAT BY GASES AND VAPOURS. 
5 
of the second wire were now so attached to a second pile that when the latter was 
caused to approach the copper ball, the current thus excited passed through the coil 
in a direction opposed to the first one. Gradually, as the second pile was brought nearer 
to the source of heat, the needle descended from the stops, and when the two currents 
were nearly equal the position of the needle was close to zero. 
Here then w’e had a powerful flux of heat through the tube ; and if a column of gas 
four feet long exercised any sensible absorption, the needle was in the position best 
calculated to reveal it. In the first experiment made in this way, the neutralization of 
one current by the other occurred when the tube was filled with air ; and after the 
exhaustion of the tube had commenced, the needle started suddenly oif in a direction 
which indicated that a less amount of heat passed through the partially exhausted tube, 
than through the tube filled with air. The needle, however, soon stopped, turned, 
descended quickly to zero, and passed on to the other side, where its deflection became 
permanent. The air made use of in this experiment came direct from the laboratory, 
and the first impulsion of the needle was probably due to the aqueous vapour precipi- 
tated as a cloud by the sudden exhaustion of the tube. When, previous to its admis- 
sion, the air was passed over chloride of calcium, or pumice-stone moistened with sul- 
phuric acid, no such effect was observed. The needle moved steadily in one direction 
until its maximum deflection was attained, and this deflection showed that in all cases 
radiant heat was absorbed by the air within the tube. 
These experiments were commenced in the spring of 1859, and continued without 
intermission for seven weeks. The course of the inquiry during this whole period w'as 
an incessant struggle with experimental difficulties. Approximate results were easily 
obtainable, but I aimed at exact measurements, which could not be made with a varying 
source of heat like the copper ball. I resorted to copper cubes containing fusible metal, 
or oil, raised to a high temperature ; but was not satisfied with their action. I finally 
had a lamp constructed which poured a sheet of gas-flame along a plate of copper ; and 
to keep the flame constant, a gas regulator specially constructed for me by Mr. ITulet 
was made use of. It was also arranged that the radiating plate should form one of 
the walls of a chamber which could be connected with the air-pump and exhausted ; 
so that the heat emitted by the copper plate might cross a vacuum before entering the 
experimental tube. With this apparatus I determined approximately the absorption 
of nine gases and twenty vapours during the summer of 1859. The results would 
furnish materials for a long memoir; but increased experience and impro\ed methods 
have enabled me to substitute for them others of greater value ; I shall therefore pass 
over the work of these seven weeks Avithout further allusion to it. 
On the 9th of September of the present year (1860) I resumed the inquiry. For three 
weeks I worked with the plate of copper as my source of heat, but finally rejected it on 
the score of insufficient constancy. I again resorted to the cube of hot oil, and continued 
to work with it up to Monday the 29th of October. During the seven weeks just 
referred to, I experimented from eight to ten hours daily ; but these experiments, though 
