LAKES. 
253 
these interesting questions can be fully and definitely 
answered. 
The Colour of the Swiss Lakes. 
Switzerland owes much of its charm to the lakes, 
and the lakes owe their beauty in great measure to 
their exquisite colouring. In this respect they differ 
considerably: the Lake of Geneva is blue, but most 
of the Swiss lakes are more or less green, and 
some brownish. What is the reason of these differ- 
ences? 
The blueness is not due to, though it may be 
enhanced by, the reflection of the sky. Pure water 
is of an exquisite blue. Of all the Swiss lakes the 
Lake of Lucel in the Val d’Herins is perhaps the 
clearest, and it is of a lovely blue. Various sugges- 
tions have been made to account for the green colour 
of some lakes. The most probable explanation appears 
to be that suggested by Wettstein, and ably sup- 
ported by Forel ,* namely, that the blue is turned 
into green by minute quantities of organic matter in 
solution. Forel took water from several lakes, and 
thoroughly filtered them, but they retained their colour, 
showing that it was not due to particles in suspen- 
sion. Fie then took a block of peat, and infused it 
* Le Leman , vol. II. 
