Properties of Liquid Mixtures. 289 



D, D'. The whole is shown flat in the diagram ; but as a 

 matter of fact the side tubes C D and C D' were bent round 

 till the bulbs nearly touched, to ensure their being of the same 

 temperature. The apparatus was cleaned out with chromic 

 acid, washed, and dried ; the capillaries were then drawn out 

 and two of them sealed up, the third being left with the bit 

 of wider tubing beyond the capillary untouched. By means 

 of this it was attached to a mercury-pump, exhausted, and 

 the capillary fused. The point B was then opened under 

 mercury and fused off when the gauge contained sufficient : 

 in the same way one of the bulbs was half filled with the 

 mixture through D, arid then the other with water (which 

 must, of course, be freed from air) through D' '. The apparatus, 

 all of glass and hermetically sealed, is then ready for use : a 

 glass millimetre scale is fastened with rubber bands to the 

 gauge-tube, and it was immersed in a large glass jar of water. 

 The scale was usually read by the telescope of a cathetometer 

 and sometimes the screw micrometer of the telescope used 

 to subdivide the graduation. The differential method avoids 

 the necessity for any very great care in maintaining or mea- 

 suring the temperature of the apparatus. It was found quite 

 sufficient to heat the water-bath by leading a current of 

 steam into it, and when the required temperature was 

 reached, stop the steam for a moment and read the differ- 

 ence of level. When the highest temperature (90°) was 

 reached, some of the water was siphoned off, replaced by 

 cold, the whole mass well stirred, and a reading taken. There 

 was no noticeable lag in the indications of the gauge, the 

 readings at the same temperature, rising and falling, being 

 in good agreement. 



The fourth apparatus used was the Beckmann boiling-point 

 apparatus, in its usual (second) form : with that observations 

 at 100° were obtained of a kind to confirm the measurements 

 made at somewhat lower temperatures with the vapour-? 

 pressure apparatus. 



Observations of Vapour-pressure. 



The following observations were obtained with the differ-? 

 ential pressure-gauge : t is the temper,: turo centigrade, p 

 the vapour-pressure of the mixture, tt that of water, tt— p 

 is therefore the difference observed with the gauge, and 

 {it— p)/7r represents the relative lowering of the vapour- 

 pressure of water by the addition of the quantity of phenol 

 mentioned. 



