110 Mr. W. F. Barrett on a Physical 



and produced a certain deflection in a delicate galvanometer ; 

 this deflection was neutralized by a compensating cube contain- 

 ing water heated by steam and placed before the opposite face of 

 the pile. The needle of the galvanometer was brought precisely 

 to zero by the adjustment of a double metal screen placed between 

 the cube and the pile. Another double screen of plated metal 

 w r as now introduced between the tube and the source ; a deflec- 

 tion of the needle took place indicating the amount of heat fall- 

 ing on the posterior face of the pile; and as this amount exactly 

 neutralized the radiation from the flame, it corresponded to that 

 radiation, and consequently gave the total heat emitted by the 

 source. This deflection was carefully noted; the double silvered 

 screen was now removed ; the needle then descended to zero, 

 and everything was ready for the first experiment. 



After taking the total heat in the above manner, the brass 

 adjusting screen before the compensating cube was left untouched, 

 the slight variations which occurred throughout the day being 

 remedied by altering, within small limits, the size or distance of 

 the source. 



The experimental tube having been exhausted, and the needle 

 of the galvanometer standing at zero, air from the laboratory was 

 drawn first over a U-tube filled with marble moistened with a 

 solution of potash, and then over sulphuric acid into the experi- 

 mental tube. Whilst the air was passing into the tube the needle 

 was closely watched through a telescope, and it was found that 

 the only effect of 30 inches of this pure air w r as slightly to aug- 

 ment the amount of heat passing through the tube, this action 

 being due to the feeble dynamic radiation of the air. After the 

 tube had been exhausted, air direct from the laboratory, and 

 retaining therefore its carbonic acid and aqueous vapour, was 

 allowed to enter ; the needle instantly moved in the direction of 

 absorption, and when the tube was filled, a deflection of 9 0, 3 

 was observed; this deflection corresponds to an absorption of 

 15 per cent, of the total radiation. The tube was again ex- 

 hausted, and air was drawn into it after passing over sulphuric 

 acid; this dry air gave a deflection of 8°-7, or 13*8 per cent, 

 absorption. The small amount of carbonic acid present in the 

 air could in this case have been the only agent which intercepted 

 nearly 14 per cent, of the whole radiation. 



To be assured of this remarkable result, fragments of solid 

 potash were placed in a glass tube about 4 inches long, and com- 

 mon air passed over the potash alone into the tube, when a de- 

 flection of only 4°, or an absorption of 6*4 was obtained. This 

 seems to establish the fact that, with a source such as a carbonic 

 oxide flame, where the chief radiating body is heated carbonic 

 acid, the absorption by the minute amount of carbonic acid in 



