568 DR. J. W. CAPSTICK ON THE RATIO OF THE 
result is only very slightly affected by the presence of a little impurity. Impurities 
make themselves felt mainly by their modifying the density of the gas, and error 
from this source is avoided by the vapour density determination having been made in 
nearly every case on the same samples of the material as were used in the velocity of 
sound experiments. Hence impurity only has an effect in so far as the y of the foreign 
substance is not the same as that of the gas under investigation. An estimate of the 
amount of this error can be got by an application of equation (5) of my former paper. 
If, for instance, the methylene chloride contained 1 per cent. of chloroform, the error 
in y would be less than one part in a thousand. 
Except where the contrary is stated, the materials were obtained from KAHLBAUM 
and were dried and fractionated before being used. The following table of the 
observed range of boiling-points of definite quantities of the liquids as actually 
employed in the experiments will give some indication of the degree of purity 
obtained. 




TABLE I. 
Name. | Quantity. B. P. (uncorrected). 
Methylene chloride .. =: ... . 50 ¢.c. 43°5 to 443 
Ojnilorgowornin 5 go Sn 300 60°8 ,, 61:2 
Carbon tetrachloride. . . . . . 50 76°8 ,, 76°9 
Hithylene chloride. . .... . 60 83°9 ,, 84-1 
Kithylidene chloride. . . . «.-. 40 59 ,, 6071 
Nill Oilevals 55 a Ss a 110 45 ,, 46°5 
NIGAM org 5 5 6 a 0 2 oe 70 69 ,, 71 
[relat imo 6 A) gb So | 200 56 
Methyliacetate.. =. «9. «2. . 400 56 to 57-5 
Carbonvbisulphides 9s) - a ome 250) 46°8 ,, 47 




As regards the degree of accuracy attainable in the measurements Tinust again 
refer to my former paper. On p. 11 is given a typical set of measurements of the 
dust figures, and on p. 27 a series of experiments on ethyl chloride that afford a test 
of the accuracy of the vapour density determinations. 
The experiments on carbon dioxide (Table XXII., below) show further, that whilst the 
work was in progress no change took place in the apparatus that affected the results. 
The first three experiments on carbon dioxide were made when the apparatus was 
first set up; the last three were made two years later, after it had been repeatedly 
dismantled for cleaning, and repairs and most of the original tubes had been replaced 
by others. The two sets of three give almost identical results. 
Many cf the gases used in the experiments have low saturation pressures at the 
atmospheric temperature—a fact which tends to increase the experimental error, for 
small errors in reading the height of the mercury column in the vapour density 
apparatus have a more serious effect when the total pressure to be measured is small, 
and in addition to this, the small range of pressure available makes it difficult to 
