On the Transmission of Radiant Heat through Gaseous Bodies. 61 
diminution of the current would be necessary ; far larger, indeed, than 
the absorption of the air, if any, could produce: while if I used a 
feeble source, and permitted the needle to point to the lower degrees 
of the galvanometer, the total quantity of heat in action was so 
small, that the fraction of it absorbed, if any, might well be insensible. 
My object then was to use powerful currents, and still keep the 
needle in a sensitive position; this was effected in the following 
manner :—The galvanometer made use of possessed two wires coiled 
side by side round the needle ; and the two extremities of each wire 
were connected with a separate thermo-electric pile, in such a manner 
that the currents excited by heat falling upon the faces of the two piles 
passed in opposite directions round the galvanometer. A source of 
heat of considerable intensity was permitted to send its rays through 
the tube to the pile at its opposite extremity; the deflection of the 
needle was very energetic. The second pile was now caused to ap- 
proach the source of heat until its current exactly neutralized that of 
the other pile, and the needle descended to zero. 
Here then we had two powerful forces in perfect equilibrium ; and 
inasmuch as the quantity of heat in action was very considerable, the 
absorption of a small fraction of it might be expected to produce a 
sensible effect upon the galvanometer-needle in its present position. 
When the tube was exhausted, the balance between the equal forces 
was destroyed, and the current from the pile placed at the end of the 
tube predominated. Hence the removal of the air had permitted a 
greater amount of heat to pass. On readmitting the air, the needle 
again descended to zero, indicating that a portion of the radiant heat 
was intercepted. Very large effects were thus obtained. 
I have applied the same mode of experiment to several gases and 
vapours, and have, in all cases, obtained abundant proof of calorific 
absorption. Gases vary considerably in their absorptive power—pro- 
bably as much as liquids and solids. Some of them allow the heat 
to pass through them with comparative facility, while other gases 
bear the same relation to the latter that alum does to other diather- 
manous bodies. 
Different gases are thus shown to intercept radiant heat in different 
degrees. I have made other experiments, which prove that the self- 
same gas exercises a different action upon different qualities of radiant 
heat. The investigation of the subject referred to in this Note is 
now in progress, and I hope at some future day to lay a full descrip- 
tion of it before the Royal Society. 
** Photochemical Researches.” —Part IV. By Robert W. Bunsen, 
For. Memb. R.S., and Henry Enfield Roscoe, Ph.D., Professor of 
Chemistry in Owens College, Manchester. 
In the three communications * which they have already made to the 
Royal Society upon the subject of photochemistry, the authors showed 
that they have constructed a most delicate and trustworthy instrument 
by which to measure the chemical action of light, and by help of which 
they have been able to investigate the laws regulating this action. 
* Phil. Trans. 1857, pp. 355, 381 and 601. 
