999 



apparatus I'or determining vapour-pressures. In llie experimental space 

 inside tlie vacuum-glass B:^ of tlie lij'drogen-vaponr crjostat beside 

 the helium-thermometer (^ 2) and the auxiliary capillary ofUHAPPUis 

 is placed the small vessel S, which is meant to receive the condensed 

 hydrogen and whose volume is known ^). The high-pressure i-eservoir 

 R contains the pure hydrogen which has been obtained by distillation. 

 A sutHicient quantity of it is collected in the calibrated arm (in the 

 drawing on the right hand side) of a pressure tube P, which is half 

 full of mercury by means of a pipette P^ (between the stop-cocks 

 /iTif'j and KwJ, using a differential manometer J/ placed in parallel 

 with it. The pressure-equilibrium which is changed by the admission 

 of hydrogen is re-established by the admission of compressed air, 

 which is contained under high pressure in the supplycylinder R^. 

 In this manner the pressure in the vapour-pressure apparatus is 

 gradually raised to the equilibrium pressure between liquid and vapour, 

 corresponding to the temperature in the cryostat. K\\\ is then closed 

 and a known quantity of gas present in the pressure-tube P can be 

 carried over into S as liquid. ') 



S is calibrated. When a known quantity of hydrogen has been 

 added, K]y^ is closed and the operations are stopped until the equi- 

 librium of temperature which is distui-bed by the adiabatic com- 

 pression of the gas has been restored, as shown by the indication 

 of the differential manometer M. When at the same time the tem- 

 perature in the cryostat, as checked by means of two platinum 

 thermometers, has become constant, the pressure-measurements can 

 be started. For this purpose the air-side of the differential mano- 

 meter is connected along Kw^ with the open standard gauge of the 

 laboratory (Comm. N°. 44). Concurrently with the determination of the 

 pressures in the different parts of the apparatus one or more readings 

 of the gas-thermometer are taken. In this manner determinations 

 were made for a series of temperatures ; in a few cases moreover 

 measurements were made at one and the same temperature with 

 different quantities of liquid, in order to make sure that the equili- 

 brium pressures with little and with much liquid in the vapour- 

 pressure bulb S were the same at the same temperature, as must be 

 the case, if the temperature is uniformly distributed and the gas is pure. 



(7'(9 he continued). 



') For the cryostat and the arrangement of the various apparatus inside it, as 

 also for the method of regulating the temperature, we refer to fig. 1 of Comm. 

 N^. 151a. For simplicity the auxiliary capillary of Ghappuis has there been left out. 



-) For this pui*pose the right-hand arm of the pressure-tube P is calibrated. The 

 density of liydrogen is obtained by estimation by means of the law of the straight 

 diameter from the data of Comms, N". 137a and N'\ 151c. 



