6 
PEOFESSOE TTKDALL ON THE ABSOEPTION AND 
more accurate, must unhappily share the fate of the former ones. In fact the period 
was one of discipline, — a continued struggle against the difficulties of the subject and 
the defects of the locality in which the inquiry was conducted. 
My reason for making use of the high sources of heat above referred to w’as, that the 
absorptive power of some of the gases which I had examined was so small that to make 
it clearly evident, a high temperature was essential. For other gases, and for all the 
vapours that had come under my notice, a source of lower temperature would have 
been not only sufficient, but far preferable. I was finally induced to resort to boiling 
water, which, though it gave greatly diminished effects, w'as capable of being preserved 
at so constant a temperature, that deflections which, with the other sources, would be 
masked by the errors of observation, became with it true quantitative measures of 
absorption. 
H- 
The entire apparatus made use of in the experiments on absorption is figured on 
Plate I. S S' is the experimental tube composed of brass, polished mthin, and connected, 
as shown in the figure, Avith 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 contain- 
ing boiling Avater, in Avhich 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 air-tight Avith the latter, was carefully 
soldered. The face of the cube within the ring is the radiating plate, which is coated 
Avith lampblack. Thus betAAeen the cube C and the first plate of rock-salt there is a 
front chamber F, connected Avitji 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 reach- 
ing the plate of rock-salt S, the tube F is caused to pass through a A^essel V, being 
soldered to the latter Avhere it enters it and issues from it. This A^essel is supplied with 
a continuous floAV of cold Avater through the influx tube i ^, Avhich dips to the bottom 
of the A'essel; the Avater escapes through the efflux tube ee, and the continued 
circulation of the cold liquid completely 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 shoAvn 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 neutrali- 
zation was an operation of some delicacy ; to effect it the double screen H was connected 
Avith a Avinch and screAV arrangement, by which it could be advanced 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 ; ifiis connected with the pile P by the Avires 
low, YY is a system of six chloride-of-calcium tubes, each 32 inches long; E. is a 
* It will be seen that in this arrangement I have abandoned the use of the differential galvanometer, and 
made the thermo-electric pile the differential instrument. 
