Radiation of Heat by Gases and Vapours. 175 



which, with the other sources, would be masked by the errors of 

 observation, became with it true quantitative measures of absorp- 

 tion. 



§ 4. The entire apparatus made use of in the experiments on 

 absorption is figured on Plate III. S S' is the experimental tube, 

 composed of brass, polished within, and connected, as shown in 

 the figure, with the air-pump, A A. At S and S' are the plates 

 of rock-salt which close the tube air-tight. The length from S 

 to S' is 4 feet. C is a cube containing boiling water, in which 

 is immersed the thermometer t. The cube is of cast copper, 

 and on one of its faces a projecting ring was cast to which a 

 brass tube of the same diameter as S S', and capable of being 

 connected ah*- tight with the latter, was carefully soldered. The 

 face of the cube within the ring is the radiating plate, which is 

 coated with lampblack. Thus between the cube C and the first 

 plate of rock-salt there is a front chamber F, connected with the 

 air-pump by the flexible tube D D, and capable of being exhausted 

 independently of S S'. To prevent the heat of conduction 

 from reaching the plate of rock-salt S, the tube F is caused to 

 pass through a vessel V, being soldered to the latter where it 

 enters it and issues from it. This vessel is supplied with a con- 

 tinuous flow of cold water through the influx tube i i, which dips 

 to the bottom of the vessel; the water escapes through the efflux 

 tube ee, and the continued circulation of the cold liquid com- 

 pletely intercepts the heat that would otherwise reach the plate S. 

 The cube C is heated by the gas-lamp L. P is the thermo- 

 electric pile placed on its stand at the end of the experimental 

 tube, and furnished with two conical reflectors, as shown in the 

 figure. C is the compensating cube, used to neutralize by its 

 radiation* the effect of the rays passing through S S\ The 

 regulation of this neutralization was an operation of some de- 

 licacy; to effect it the double screen H was connected with 

 a winch and screw arrangement, by which it could be ad- 

 vanced or withdrawn through extremely minute spaces. For 

 this most useful adjunct I am indebted to the kindness of my 

 friend Mr. Gassiot. N N is the galvanometer, with perfectly 

 astatic needles and perfectly non-magnetic coil ; it is connected 

 with the pile P by the wires w w; Y Y is a system of six chloride- 

 of-calcium tubes, each 32 inches long; ft is a U-tube containing 

 fragments of pumice-stone, moistened with strong caustic pot- 

 ash ; and Z is a second similar tube, containing fragments of 

 pumice-stone wetted with strong sulphuric acid. When drying 

 only was aimed at, the potash tube was suppressed. When, on 



* It will be seen that in this arrangement I have abandoned the use of 

 the differential galvanometer, and made the thermo-electric pile the differ- 

 ential instrument. 



