398 



Prof. B. Moore and Mr. H. E. Koaf . 



[Apr. 12, 



datum for calculating the concentration in chloroform of the solution 

 corresponding to the observed vapour pressure in the vapour space. 



The ratio of the vapour concentration in the fluid to the concentra- 

 tion in the vapour space gives the coefficient of distribution {coefficient 

 de partage, Theilungscoefficient) : this should remain constant if the 

 absorption of the chloroform vapour by the liquid were normal and 

 strictly proportional to the vapour pressure, and if it varies it points 

 to a physical or chemical aggregation or compound between the chloro- 

 form and the fluid or its constituents (vide infra). 



Method of Beading. — The readings were taken with a cathetorneter,* 

 placed about 4 feet from the tubes, both for greater accuracy in 

 reading than direct measurement would give and to avoid changes in 

 temperature. 



Two Methods of Experimentation. — "We have employed two different 

 methods of experimentation in investigating the variation in vapour 

 pressure with varying concentration. It is obvious that the concen- 

 tration of a measured volume of a strong solution introduced into the 

 densimeter may be diminished by pumping off more and more chloro- 

 form, by increasing the volume of the vapour space above the solution. 



A series of readings of differences in pressure may thus be obtained 

 in which the vapour space on the two sides is kept equal and of known 

 and increasing value throughout the series. This method we have 

 called the method of " variable vapour space." 



On the other hand, a series of solutions of known and steadily 

 diminishing or increasing concentration may be introduced into the 

 densimeter and measured one after another as to their vapour pressure, 

 in each case with a known fixed volume of vapour space. As stated 

 above, the concentration at which each vapour pressure in the series is 

 measured is then accurately known. This method we have called the 

 method of " constant vapour space." 



Method of " Variable Vapour Space." — In this method, unless the 

 volume of the tubes of the densimeter is very large, the volume of 

 solution and solvent respectively introduced must be very small. We 

 have usually taken J c.c. on each side, either of a saturated solution 

 or of a very strong solution of known strength, and by altering the 

 levels of the tubes and mercury receiver, have taken a long series 

 of readings, in each case with equal volume of vapour space on each 

 side, at every increasing volume of vapour space, until the difference in 

 pressure became of small value, and the product of volume and vapour 

 pressure became approximately constant, showing that practically all 



# The instrument used was made by Pye and Co., of Cambridge, and, by means 

 of a vernier and divided screw-beacl, read to -^ T) of a millimetre. We have 

 frequently observed that we were able to take readings within five divisions, that 

 is mm > which is far within the accuracy of other portions of our determi- 

 nations. 



