248 H. Wild on the Absorption of 



When coal-gas was now introduced on the left-hand side and the 

 air- pump allowed to work there only, a deflection of about 800 

 millims. occurred, which corresponds to a deflection of 8°. The 

 substitution of ordinary coal-gas, therefore, on one side of the 

 thermopile in place of a stratum 1 foot in thickness of the air 

 of the room, caused a deflection of the magnet amounting to 8°, 

 and corresponding to a diminution of thermal action. According 

 to our experiments, therefore, the absorbing effect of a stratum 

 of moist air 1 foot in thickness, and saturated with aqueous va- 

 pour at 18°, is only sixteen times smaller than that of coal-gas. 

 It is self-evident that this number is only a rough approximation, 

 since in such tubes, open at both ends, we cannot speak of a per- 

 fectly definite length of an interposed stratum of gas. Accord- 

 ing to the statements of Professor Tyndall, a stratum of coal-gas 

 4 feet in thickness absorbs 81 per cent., and one 1 foot in thick- 

 ness absorbs 61 per cent, of the total radiation. Taking into 

 account, therefore, the above given absorptions by moist air under 

 similar circumstances, the absorption by coal-gas is, according to 

 him, only eight times as great as that by moist air when the 

 stratum is 4 feet thick, and twenty-four times as great when the 

 thickness is only 1 foot. According to our experiments, there- 

 fore, the absorption by aqueous vapour is relatively somewhat 

 greater even than Professor Tyndall has estimated it to be. 



Moreover, with the above disposition of the apparatus no de- 

 flection whatever of the magnet-mirror could be observed when 

 the two Leslie cubes were removed and the pumps again allowed 

 to play. In fact the effect of the currents of air on the thermo- 

 pile was rendered inappreciable by allowing these currents to 

 enter at the most distant ends of the tubes. 



With a second similar experiment we obtained 98 millims. as 

 a mean of the difference of the readings of the scale, instead of 

 110. The temperature of the air at the time was 16° C, and in 

 order to dry and to moisten it, two sets of four U-shaped tubes 

 were employed ; the tubes of each set were connected by means 

 of caoutchouc tubes and filled with pieces of glass, pure concen- 

 trated sulphuric acid being poured into the four tubes of the one 

 set, and distilled water into those of the other. 



In one of the following experiments, the above-described dis- 

 position of the apparatus was so modified as to obtain a single 

 tube, 120 centims. long, on the left side of the thermopile, by 

 placing both tubes end to end; in this manner the lateral aper- 

 tures near the ends of the compound tube were at a distance of 

 90 centims. from one another. The lateral apertures near the 

 middle of the long tube were connected with each other by a short 

 caoutchouc tube. Finally, in order to produce perfect neutraliza- 

 tion a double screen of metal was more or less interposed between 



