39 



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. 



VIII. " Photochemical Researches." Part IV. By ROBERT W. 

 BUNSEN, For. Memb. R.S., and HENRY ENFIELD ROSCOE, 

 Ph.D., Professor of Chemistry in Owens College, Man- 

 chester. Received May 26, 1859. 



(Abstract.) 



In the three communications * which they have already made to the 

 Royal Society upon the subject of photochemistry, the authors showed 

 * Phil. Trans. 1857, pp. 355, 381 and 601. 



