Young — Vapour- Pressures, Sfc, of Thirty Pure Substances. 401 



sealed end, and reading the volume of vapour, care being taken that the 

 portion of the tube above the liquid was at a slightly higher temperature 

 than the lower part, so that no liquid should be left adhering to the walls of 

 the tube. The sum of the two readings gives the total volume ; but as the 

 tube was calibrated with mercury, which lias a convex meniscus, whilst 

 that of the liquid is concave, the first reading must be corrected for this 

 reversal of meniscus. When the tube is inverted, the concave meniscus of 

 the liquid corresponds in position with the convex meniscus of mercury during 

 calibration, and no correction is required. 



Weight of substance in tlie sealed tube. — The total weight of substance in 

 the tube is the sum of the weights of liquid and saturated vapour. The 

 weight of liquid was ascertained in exactly the same manner as with the 

 pressure-apparatus. In calculating the weight of saturated vapour from its 

 volume, temperature, and pressure (the vapour-pressures had previously been 

 determined), it was assumed in most cases that the density of the saturated 

 vapour was normal at 0°, and at the temperature of running tap-water. The 

 total weight of vapour was always small ; and the error involved by this 

 assumption was negligible when the boiling-point was higher than, say, 60°. 

 In the case of the more volatile liquids, such as isopentane and normal 

 pentane, it was assumed that the ratio of the actual to the theoretical density 

 had the same value as the mean ratio for the substances previously examined 

 at the same " reduced " pressure. The correction even with isopeutane is 

 exceedingly small, and is almost negligible so far as the determination of the 

 specific volume of liquid is concerned; but it should not be omitted when the 

 specific volumes of saturated vapour are to be determined. 



The arrangement for heating the tube is shown in fig. 8 (p. 402). CC is 

 a jacketing-tube, the bulb of which contains a pure liquid which is to be 

 boiled under reduced and known pressures. The lower end of the jacketing- 

 tube is narrow, and is provided with a piece of rather thick-walled 

 india-rubber tubiug, E, through which the volume-tube c passes. This 

 india-rubber tube must be kept under water at least twenty-four hours 

 before being used ; the volume-tube can then be pushed up through it 

 without difficulty. It is necessary to keep the india-rubber tube from slipping 

 from the jacketing-tube by twisting a piece of copper wire tightly round it. 



The little glass tube, F, which is provided with a very short piece of 

 india-rubber tubing, O, contains water, which moistens the lower extremity 

 of the india-rubber tube, E, and the portion of the volume-tube immediately 

 below it. The jacketing-tube is supported by a retort-stand with two 

 clamps. 



The pure liquid is poured into the jacketiug-tube through a long, narrow 



