154 THE FRESH-WATER LOCHS OF SCOTLAND 
not in their atmospheric ratio (1 : 4) but in the ratio of approximately 
1 : 2. When we come to natural waters, we find that high salinity 
lowers the absolute solubility of gases somewhat, and that carbon 
dioxide is greatly affected by the chemical affinity for it of dissolved 
carbonates. With regard to the latter gas, all hitherto published 
data are untrustworthy as to the amount in solution in the gaseous, as 
distinct from the ionised, state, and we are not likely to become better 
informed until a sound experimental method of measuring the tension 
of the gas in solution becomes universal. The saturation-solubilities 
of oxygen and nitrogen, at partial pressures of ^ atm. and ^ atm. 
respectively, as in air, are set down in the following table, expressed 
in c.c. at 0° and 760 mm. per litre of liquid : — 
Oxygen. 
Nitrogen. 
Pure water at 0° C. 
at 15° 
at 30° 
Sea-water (salinity = i 
per cent. ) at 0° 
at 15° 
at 30° 
10-29 
7- 22 
5-57 
8- 36 
5-84 
4-50 
18-56 
13- 63 
10- 94 
14- 40 
11- 12 
9-26 
In the best-explored lake, that of Geneva, a series of experiments 
showed the content of oxygen to be 6'8-7"6 c.c. per litre, and nitro- 
gen 14*6-15'9 c.c. per litre, at various depths and at temperatures 
ranging from 4° to 9° C. There was very little variation from the 
surface down to 300 metres (984 feet), which is doubtless due to the 
even vertical temperature of the lake and the complete circulation 
which it consequently enjoys. The amounts of gas dissolved are seen 
to be rather below saturation in the case of oxygen, and very near 
saturation in the case of nitrogen. Very different are the waters of 
the Caspian: in the South Basin 5*6 c.c. of oxygen at 100 metres 
(328 feet), tailing down to 0-73 c.c. at 715 metres (2345 feet), are 
recorded ; in the North Basin 2'3 c.c. at 150 metres (492 feet) down 
to 0*13 c.c. at 575 metres (1886 feet). Here the bottom waters are 
altogether destitute of oxygen, and there is no animal life below 400 
metres (1312 feet). Wherever the bottom waters are inadequately 
ventilated, reduced sulphur compounds are apt to be generated in the 
deposits, and this sometimes leads to the presence of an abnormal gas in 
solution, namely sulphuretted hydrogen. 
The quantity of gas held in solution in any part of a lake is 
governed by a multiplicity of factors. It depends first of all on 
the circulation of the lake : thus lakes of uniform temperature, 
especially the shallower ones, are well aerated from top to bottom ; 
lakes with a discontinuity layer (Sprimgschicht) receive an ample 
