8 4* Prof. Tyndall on the Absorption and Radiation 



front chamber F, with which the cylinder A B is connected by 

 suitable screws and washers. Within the cylinder A B moves a 

 second one, 1 1, as an air-tight piston, and the bottom of the 

 second cylinder is stopped by the plate of rock-salt S'. This 

 plate projects a little beyond the end of the cylinder, and thus 

 can be brought into flat contact with the plate S. Fixed firmly 

 to A B is a graduated strip of brass, while fixed to the piston is 

 a second strip, the two strips forming a vernier, vv. By means 

 of the pinion K, which works in a rack, the two plates of salt may 

 be separated, their exact distance apart being given by the ver- 

 nier. P is the thermo -electric pile with its two conical reflectors ; 

 C is the compensating cube, employed to neutralize the radiation 

 from the source C. H is an adjusting screen, by the motion of 

 which the neutralization may be rendered perfect, and the needle 

 brought to zero under the influence of the two opposing radia- 

 tions. The graduation of the vernier was so arranged as to 

 permit of the employment of plates of gas varying from O01 to 

 2*8 inches in thickness. They were afterwards continued with 

 the pieces of the experimental tube, already referred to, and in 

 this way layers of gas were examined which varied in thickness 

 in the ratio of 1 : 4900. 



In my former experiments the chamber F was always kept 

 exhausted, so that the rays of heat passed immediately from the 

 source through a vacuum ; but in the present instance I feared 

 the strain upon the plate S, and I also feared the possible intru- 

 sion of a small quantity of the gas under examination into the 

 front chamber F, if the latter were kept exhausted. Having 

 established the fact that a length of 8 inches of dry air exerts no 

 sensible action on the rays of heat, I had no scruple in filling 

 the chamber F with dry air. Its absorption was nil, and it merely 

 had the effect of lowering in an infinitesimal degree the tempe- 

 rature of the source. The two stopcocks c and c' stand exactly 

 opposite the junction of the two plates of salt S, S' when they 

 are in contact, and when they are drawn apart these cocks are in 

 communication with the space between the plates. 



After many trials, the following mode of experiment was 

 adopted: — The gas-holder containing the gas to be examined 

 was connected by an india-rubber tube with the cock c 1 , the other 

 cock c being at the same time left open. The piston was then 

 moved by the screw II until the requisite distance between the 

 plates was obtained. This space being filled with dry air, the 

 radiations on the two faces of the pile were equalized, and the 

 needle brought to zero. The gas-holder was now opened, and 

 by gentle pressure the gas from the holder was forced first 

 through a drying apparatus, and then into the space between 

 the plates of salt. The air was quickly displaced, and a plate of 



