316 Proceedings of Boyal Society of Edinhurgh. [sess 
described and applied a method of this nature, which consists simply 
in the determination of a certain ratio which is necessarily a constant, 
if the assumption be made that there is no appreciable difference 
in the chemical composition of sea-water in different localities. 
This ratio, following the example of Dittmar, I have denoted 
D. Dittmar uses this letter to denote a ratio which varies with the 
temperature, though it is constant for any given temperature. 
Thus he puts 
jy 4S.-4W, 
Where 
48 ^ = The specific gravity of sea-water at the temperature t referred 
to pure water at 4° C. as standard. 
W^ = The specific gravity of water at the temperature t referred to 
water at 4° C. as standard. 
\ = The total halogen, calculated as chlorine, per kilo, of sea-water. 
Dittmar determined the value of D for a mixture of “ Challenger ” 
sea- water samples at a number of different temperatures, and showed 
that with these mixtures the values of D for different temperatures 
when calculated by an equation D = a + ht + ct^ were in very close 
approximation to those found experimentally. 
He came to the conclusion that for a given temperature the ex- 
cess of the specific gravity of sea-water over that of pure water is 
proportional to is obvious that if the assumption thus made 
be correct for all sea-waters, we may in every case calculate x from 
the known density or the density from a known x- ^’or 
and 
A-,W, = Dx. 
Dittmar determined x experimentally, by a special modification 
of Yolhard’s method, in 315 of the “Challenger” samples, and 
compared these with the theoretical values calculated from 
Buchanan’s observed specific gravities by means of the formula 
“ D 
The differences x ~X i^etween the theoretical values and those 
actually found are given in Table I. column 9 of his Eeport. It 
