( 101 ) 
of the freezing point was determined of solutions quite free from aur. 
I regret that Mr. Dirrericit, who was acquainted with my inves- 
tigations, has not mentioned, that in 1896 I published results which 
agreed perfectly with those he found with his aneroid and which 
he published in 1897. 
I intended to investigate other substances than NaCl in the way 
Prof. JAHN recommended, but my manometer got defect while I 
was engaged with experiments on KCl, so that I had to put off 
this investigation. 
At the end of this treatise I feel obliged to express my thanks 
to Prof. H. C. DrBerrs for the great kindness with which he placed 
at my disposal the apparatus required for my researches. 
Physics. — Prof. van DER WAALS presents on behalf of Mr. 
K. H. J. Cunmus a paper on: “The determination of the 
refractivity as a method for the investigation of the compost- 
tion of co-existing phases in mixtures of acetone and ether”. 
Introduction. 
The aim of this investigation was to examine the relation between 
the compositions of the co-existing vapour- and gas phasis and to 
find out the relation between the composition of the vapour and 
the pressure. 
When I began my experiments only the investigation of LiNr- 
BARGER!) had been published; since then those of LeareLpr?) and 
HARTMAN *) have also appeared. 
The great difficulty of the investigation of this relation lies in 
the determination of the composition of the vapour; I have tried to 
do so without first condensing the vapour, and without chemical 
analysis, by means of the determination of the refractivity. I was 
induced to use this method by the experiments of Ramsay and 
TRAVERS on the refractivity of gases and some gaseous mixtures #). 
1) Journ. of the American Chem. Soc. Vol. XVII N°. 8; Aug. 1895. Chem. News, 
Vol. 72, N°. 1871, v.v. Oct. 1895. 
2) Phil. Mag. (5) Vol. 40, 46. 
3) Dissertation for a doctor's degree, Leyden 1899. 
*) Proc. Roy. Soc. Vol. LXII, p. 225, 1897. 
