1910-11.] Molecular Weights of Dissolved Substances. 
259 
XIII. — A Method for Determining the Molecular Weights of Dis- 
solved Substances by Measurement of Lowering of Vapour 
Pressure. By Alan W. 0. Menzies. 
(MS. received November 26, 1910. Head January 9, 1911.) 
[Abstract.] 
Although various methods have from time to time been described for 
determining the molecular weights of dissolved substances by measurement, 
not of boiling-point elevation, but of the reduction of vapour pressure of a 
solvent due to the presence of a dissolved substance, none of these methods 
has come into general use. The method here proposed, operating on this 
principle, offers the possibility of simply determining the molecular weights 
of non-volatile solutes in any of the ordinary solvents with an accuracy at 
least equal to that of the ebullioscopic methods, and in an apparatus at 
least as easily manageable. 
As will be seen from the figure, the apparatus * consists of an inner 
test-tube AB, furnished with a pressure gauge tube PP and a glass 
stopper D, surrounded by a jacket C, in which the pure solvent is kept 
boiling. The jacket is connected to a small reflux condenser by a short 
length of wide rubber tubing furnished with a screw-clip. When the glass 
stopper is removed and the clip closed, the vapour of the liquid boiling in 
the jacket is obliged to escape by blowing through the gauge tube, which is 
open at both ends, into the test-tube. The latter fits into its jacket by a 
ground joint G, and is graduated in cubic centimetres. The gauge tube is 
graduated in millimetres of length. The purpose of the jacket is to 
maintain the test-tube and its contents at a constant temperature — the 
boiling-point of the solvent ; while the purpose of the test-tube is to contain 
the solution whose vapour pressure is being measured. The pressure 
measurement is made in terms of the difference of level of the solution in 
the gauge tube and the test-tube — that is, in terms of millimetres of a liquid 
of low density compared to mercury. Differences of pressure that are small 
in terms of millimetres of mercury may therefore be measured with 
accuracy, and this permits of the use of very dilute solutions. 
In making a determination, the bulb of the jacket is charged two-thirds 
full with the pure solvent, which is boiled ten minutes to expel dissolved 
* This is made by Greiner & Friedrichs, Stiitzerbach, Thuringia. 
