384 THE FRESH-WATER LOCHS OF SCOTLAND 
depth of the lakes, the summer temperature at the bottom is probably 
everywhere >4° C. The surface temperature varies in the course of 
the year at any rate from 0° to 24-26° C. The high summer tempera- 
ture is to a great extent due to the broad littoral zone, the water of 
which in early summer is heated above the temperature of the air in 
direct sunlight. The heat collected is distributed by currents through 
the M'hole water of the lake. Owing to the shallowness and small 
size of the lakes, they follow the variations in the temperature of the 
air on the whole fairly exactly. The summer period of stagnation is 
probably longer than the winter period throughout the greater part 
of the zone. In all the deeper lakes there is a very distinct main 
" Sprungschicht,"' which in late summer probably occurs at a depth 
of 20-25 m. On the temperature in Baltic lakes see further 
especially Halbfass (1901, p. 60), Ule (1898, p. 32), and Seligo (1905b, 
p. 201). 
The transparency^ which, as is well known, depends almost ex- 
clusively on the amount of material dissolved in the water, is always 
slight; greatest in winter (7-8 m.), less in summer (rarely over 
4-5 m.) : see Halbfass (1901, p. 78) and Ule (1892, p. 63). Seiches 
have been studied by Halbfass (1903b, p. 16 ; 1904, p. 65). 
The colour ^ of the Baltic lakes is hardly ever blue, as it may be 
exceptionally in alpine lakes, the water of which is purer ; it is rarely 
^ There is a long series of researches dealing with the colour of fresh water. 
The older literature is cited in Forel (vol. ii. p. 462). In recent times there are 
investigations by Spring (1883, p. 55 ; 1886, p. 814 ; 1896, p. 94 ; 1897, p. 578 ; 
1899b, p. 99 ; 1905, p. 101) ; Klunzinger (1901, p. 321 ; 1902, p. 338) ; Ule (1892, 
p. 70; 1894, p. 1 ; 1901, p. 16a); Aufsess (1903, p. 1 ; 1904a, p. 186; 1904b, 
p. 678 ; 1905, p. 1). 
There are two theories to explain the variations in the colour of the water— 
the one physical (ditfraction theory), which maintains that the colour can be con- 
sidered "rtis Farhe triiber Medien''^ ; the other chemical, which considers the colour 
as a special property. The investigations of v. Aufsess, made for a great part 
in lakes under different conditions, specially show that the latter view is the 
right one. It is simply and solely the solution of different substances which are 
carried down to the lakes in various ways which gives the water a colour 
differing from the pure blue. The substances which cause variations in the 
colour are chalk and organic humous materials. Large amounts of chalk give a 
green colour ; large cjuantities of dissolved organic substances vary the colour 
through green over to yellow. The green lakes occur chiefly in chalk areas ; the 
yellow and brown waters are found especially in the regions where large masses 
of decomposing plant materials occur. It is in the first instance the geological 
nature of the lake-basin and of the lake's drainage area which determines the colour 
of the lake-water. So far as one can judge simply from observations of the colour 
of lake- water and from some knowledge of the geological nature of the drainage area, 
I may say that all I have seen on my numerous journeys most distinctly indicates 
that V. Aufsess's view is right. According to Bourcart (1906, p. 108), inorganic 
salts, especially calcareous salts, have no colouring influence at all. Ferric salts 
also may produce a change of colour, especially in bog- water (Spring, 1897, p. 578). 
