( 334 ) 



§ 2 The measuring apparatus. (See PI. I). 



In the vapour pressure apparatus of' the small pattern A about 

 120 cc. gas can be condensed, in t he large pattern />' about 1 Liter. 

 Pattern .4 consists of a bulb a of 0.5 cc. with a glass stem h, con- 

 nected to a manometer with a steel capillary c. The manometer 

 nudists of' two tube- w^wi, "' - rm - ' M,, ' e filled with mercury; they 

 are connected by an india rubber tube. An air-trap </ prevents im- 

 purities from coming into the gas. Tin.' ulass stem is moreover 

 surrounded by a copper cylinder e and a glass jacket / by means 

 of which heal is conducted from above, thus preventing the tempe- 

 rature at any pari of the apparatus from falling below that of the bulb 

 which is placed in the cryostal at the place where the temperature 

 is measured. By pressing the mercury higher up or lower down 

 one can condense different quantities of gas successively. 



The construction of' pattern B differs a little from that of' A; this 

 i- especially in order to avoid the apparatus becoming difficult 

 to handle because a too large quantity of mercury would be 

 required. The manometer m^rt, consists of' a /'-tube of glass of 

 which the two limits are separated by a glass cock /,. This /'-tube 

 is blown on to another '///., which contains the gas. Tin.' two limbs 

 of the latter are cylinders of' 70 cm. length and 0.5 liter contents 

 separated by a cock /-, . The limb <l x connected to the manometer 

 is fixed at its upper end to the steel capillary c which is connected 

 with the bulb a. The limb <l t carries a ulass cock k, , through which 

 the apparatus is filled. The reservoir is filled to 1 atm. excess of 

 pressure witli gas. By first shutting k, and then opening it we can 

 condense first the gas of c/ 1 which is under an excess of pressure, 

 then also that of '/., . 



A enables us to judge of' the purity of the gas when we investi- 

 gate in how far the vapour pressure measured depends on tfie 

 fraction of the quantity of gas already condensed. By means of B, 

 where the quantity of condensed gas always amounts to the same 

 portion of the total quantity, we can determine in how far the 

 vapour pressure i> independent of the increase or decrease of the 

 quantity of the condensed gas itself. 



The oxygen is prepared from potassium permanganate through 

 heating. After it has been carefully purified and dried over a KOH- 

 solution and IV K the gas is condensed in a bulb immersed in liquid 

 air. Then the liquid air is removed ami the apparatus are filled with 

 the evaporated gas. 



The pressure was read with a cathetometer ; this does not require 

 a very high degree of accuracy because of the great variability of 



