12 MM. Lecher and Pernter on the Absorption 



condensation of the vapour. The whole flexible tube ended 

 in the stopper to, by moans of which it could bo adjusted in 

 the brass tube /, as shown in the plate at id. The stopper 

 has incisions in it at the sides, through which the steam 

 escaped after impinging upon the little plate a. The whole 

 of the apparatus for the production of vapour was separated 

 from the apparatus A B C by means of screws not shown in 

 the plate. 



The pressure in A could be read off on the mercurial mano- 

 meter R. The air-pump D, a good Ekling's pump, served for 

 the exhaustion of A. In the experiments with vapours the 

 tube N, arranged after the fashion of a wash-bottle, was 

 employed, which contained the liquid the vapour of which was 

 to be examined, and which was placed in a bath connected 

 with the water-tap, so that its temperature might be kept con- 

 stantly the same as in the vessel A. In this way the precipitation 

 of the vapour in the vessel A was avoided. Dry and pure air 

 was allowed to pass slowly through the fluid in small bubbles, 

 so as to ensure its saturation with vapour at the temperature 

 of the experiment. 



Whilst A could have been filled with air in about a minute 

 and a half, we opened the tap b so little (g was closed) that 

 it required two hours to fill A completely. The apparatus A 

 will then be full of vapour at the maximum tension corre- 

 sponding to the temperature. A small quantity may have 

 condensed on the walls, and so reduced the tension slightly. 

 This is, we believe, the only source of error in our method 

 which has not been completely removed ; it can, however, 

 hardly have much influence on the results. That a little 

 vapour may have condensed on the thermopile and on the 

 radiating surface a is of no consequence, since though possibly 

 a small part of the radiation may be absorbed by it, yet the 

 very fine, thin layer of fluid will act like a slight increase in 

 the layer of lamp-black. 



The air destined to serve as the carrier of the vapour came 

 from the gas-holder G, into which it was brought direct from 

 the court of the Institute. From G it goes through several 

 chloride-of-calcium tubes, and then through the three special 

 pieces of apparatus, 0, P, Q. In P and Q are pieces of glass 

 the size of hazel-nuts, which have been allowed to soak for a 

 day in boiling sulphuric acid, and have then been placed in 

 the vessels P and Q. After repeated washing with distilled 

 water, the corks were adjusted and made air-tight, and then 

 dry pure air was drawn through them for three weeks by 

 means of a Bunsen's filter-pump. Next, the whole apparatus 

 was filled with chemically pure sulphuric acid by means of 



