1904.] Properties of Solutions of Chloroform in Water, etc. 399 



the chloroform had been pumped off from the solution. This constant 

 product then gave the necessary datum for calculating the concentration 

 of the original solution introduced into the densimeter, the product of 

 vapour pressure and volume at each stage gave the datum for calcu- 

 lating the quantity of chloroform pumped off from the solution, and 

 therefore for deducing the corresponding concentration of solution. 

 Or, also, by plotting vapour pressures as abscissae, and the product 

 of vapour pressure and volume of vapour space as ordinates, the ratio 

 of vapour pressure and amount of chloroform absorbed at each stage 

 could be shown. 



The method of " variable vapour space " has, however, two working 

 disadvantages, which caused us in the end to abandon it and replace it 

 by the method of " constant vapour space." The first objection is that 

 the amount of solution taken is small, hence it is difficult to measure it 

 with accuracy, and to make it equal on the two sides. 



The second objection is that at the low concentrations the increase of 

 volume for a small fall in pressure is very large, and hence the deter- 

 minations become inaccurate, a small error in pressure reading making 

 a large deviation. The values for high pressures are also inaccurate, 

 but for a different reason ; these readings are taken with small volumes 

 of vapour space, and unless the vapour spaces are accurately equal on 

 the two sides, there is a large disturbance due to inequality in pressure 

 of the previously dissolved gases pumped off on the two sides. 



The results, however in the intermediate pressures are accurate and 

 are given below, as they confirm those given by the other method. 



Method of " Constant Vapour Space." — In using this method we have 

 always introduced a volume of 5 c.c. of the solvent on one side, and 

 5 c.c. of a solution of known strength on the other, and have invariably 

 adjusted the levels so that at the temperatures of observation there 

 was a vapour space of exactly 5 c.c. on each side. 



In some cases we have started with a saturated solution of chloroform 

 and have then made dilutions of different percentages of that solution, 

 in the manner described below. In the later experiments we found it, 

 however, more expedient, on account of knowing the exact concentra- 

 tion directly, to prepare a solution of known strength, say 1 per cent., 

 and for the more dilute solutions to use various percentage dilutions 

 of this stock solution. The more concentrated solutions were obtained 

 by making up solutions varying by 1 per cent, in strength, and 

 | per cent, differences were got by mixing these with each other in 

 equal proportions. For percentages less than 0*1 per cent., a 0' 1-per- 

 cent, solution was first prepared by making a ten-fold dilution of the 

 1-per-cent. solution, and this (H-per-cent. solution was then diluted 

 similarly to the 1-per-cent. solution. 



