1882.] 



On the Critical Point of Mixed Gases. 



115 



meter having thus been proved, an ordinary Cailletet tube was chosen, 

 having a capillary part about 2 millims. in diameter, and a total 

 capacity of about 50 cub. centims. This was accurately calibrated, 

 and then filled with the hydrochloric acid gas, by passing it through 

 in a regular stream for about four or five hours ; after sealing off, the 

 bent end was placed under pure dry mercury, under the receiver of 

 an air-pump, and a sufficient quantity of the gas withdrawn to make 

 room for any amount of carbonic acid gas required to be introduced ; 

 the introduction was effected by passing a very fine capillary tube, bent 

 at a particular angle and through which pure carbonic acid was stream- 

 ing, round the bend of the tube while it remained under mercury, great 

 care being taken to prevent the slightest trace of air from getting in. 

 When sufficient carbonic acid had been introduced the tube was 

 transferred to one of the iron bottles containing pure dry mercury, 

 which was connected with another iron bottle containing the air 

 manometer, and with the pump in the usual way. 



The critical point of the mixture was first determined, and then the 

 tensions of the saturated vapour at different temperatures, together 

 with the fractional volume to which the gas was reduced at the point 

 of liquefaction, and also the relation between the liquid and gaseous 

 volumes at different heights in the tube. 



At the end of the experiments the tube was carefully lifted out of 

 the bottle, the outside of it well cleaned and dried with bibulous 

 paper, and the end of it placed under distilled water. The small 

 quantity of mercury in the bend of the tube was shaken out, and the 

 water allowed to rush up the tube and absorb the hydrochloric acid 

 gas ; the solution was afterwards made up to 500 cub. centims. with 

 distilled water, and 50 cub. centims. titrated with standard nitrate of 

 silver, which gave the quantity of chlorine equal to the amount of 

 hydrochloric acid in the tube. The small residue of mercury was 

 dried and weighed, and the space it occupied subtracted from the 

 total capacity of the tube, the remainder, after correction for tem- 

 perature and pressure, being of course the volume of the mixed gases ; 

 from this was subtracted the volume of the hydrochloric acid gas 

 calculated from the amount of chlorine obtained by titration, the 

 remainder being carbonic acid, with, of course, any slight impurity 

 of air or other inert gas that might be present. 



The following tables give the tensions of the saturated vapour, at 

 different temperatures, of the different percentage mixtures of pure 

 hydrochloric acid and carbonic acid gases. They are also plotted in 

 the form of curves on Plate I. 



I 2 



