of Edinburgh, Session 1884-85. 29 
different strengths of the same salt are expressed by the empirical 
formula, 
MY = 1800. + na + n 2 p - n 3 y . 
Where a, p, y are three constants and n the number of salt 
molecules per 100 H 2 0, it is found that — is equal to twice the 
solubility, — saturation is therefore reached when the further ad- 
dition of a molecule of salt would produce a diminution of the 
mean molecular volume of the molecules already present. The last 
molecule added enters into solution with a volume sensibly equal to 
the above mean volume as is shown by the following equation, 
(na + n 2 P - n 3 y) - (fn - i)a + (n - i) 2 P - (n - i) 3 y) =* a + np - n 2 y . 
when „ „ P±J 
* 
3. On the Salinity of the Water in the Firth of Forth. By 
Hugh Robert Mill, B.Sc., F.C.S., Scottish Marine 
Station, G-ranton, Edinburgh. (Plates III., IY., Y.) 
Contents and Summary. 
I. 1. — Method of collecting Water Samples from the surface and from 
various depths, with a description of the manner of making the 
neccessary observations, e.g., temperature, colour and transparency 
of water, and note-taking. Illustration of Buchanan’s slip water- 
bottle, pp. 30-34. 
2. — Determination of the Density.— D escription of the hydrometer 
and the manner of using it, and of calculating the results. Detail 
of experiments to determine the error of the method; each determin- 
ation of density being in duplicate supplied data for this purpose. 
Probable uncertainty of density shown to be +0 '00005, and that 
the method is capable of improvement, pp. 35-41. 
3. — Determination of Alkalinity. — M ethod employed, Dittmar’s modi- 
fication of Torn^e’s. Accuracy attained shows a deviation of 1 '8 
per cent, of carbonic acid found (total carbonic acid found being 
about 0'005 grammes per 1000 '000), pp. 41-44. 
II. 1. — Notes of previous Work in Estuary Analysis . — Murray in 1816, 
and Davy in 1843-44 on the Firth of Forth ; Macadam in 1855 on 
the Clyde and in the Beauly and Cromarty Firths in 1866 ; Kyle on 
the Plate in 1874 ; and the German and Danish Commission for 
the Investigation of the German Seas, on the Baltic, Skager Rack, 
and Cattegat, 1877-1881, pp. 44-46. 
