1892-93.] Dr J. Gibson on Composition of Sea- Water, 319 
The values for were determined by direct weighings of 
portions of the different sea-waters measured at 0° C. in a modified 
form of Sprengel tube, and then dividing the weights thus found by 
the weights of equal volumes of distilled water measured also at 
0° C. in the same Sprengel tube. 
All the weighings were fully corrected for the displaced air, the 
density of which was calculated from observations of the tempera- 
tures at the time of weighing, of the barometric height and of the 
wet and dry bulb thermometer. 
The determinations of ^ were made by Dittmar’s modification 
of Yolhard’s method, exactly in the manner described by Dittmar 
in his “ Challenger ” Eeport. 
The degree of accuracy of which this method is capable may be 
gathered from the following summary of my corrected results : — 
In 122 samples of sea-water obtained from the Moray Firth, the 
Pentland Firth, the North Sea, and including seven surface-samples 
from high northern latitudes (62° Klat. to 79° N.lat.), the following 
values for the ratio 
oSq-IOOO 
X 
= D were obtained : — 
Mean of 122 determinations, . . . 1’4563 
Maximum ,, ... 1*4585 
Minimum ,, ... 1*4535 
This corresponds to a range of differences of from -f 0*024 to 
-0*042, according as we compare the effect of the mean value for 
D with the effect of the maximum or minimum values respectively 
in calculating from the density. 
The first and most obvious bearing which this result has is, that 
whatever explanation may be given of the magnitude of the differ- 
ences X - y in the “ Challenger ” Report, they cannot possibly be due 
to the method used by Dittmar in his determinations of x- This 
becomes the more clear when we remember that the values 
which Dittmar gives for x are, generally speaking, the mean of 
two determinations, whereas mine are derived from single deter- 
minations. 
If then they are due to experimental errors, we must attribute 
them almost entirely to Buchanan’s specific gravity determinations. 
There is, however, another possible explanation. These differences 
