BY HARDOLPH WASTENEYS. 107 
In a lake of this type one of the most important factors 
m connection with its biological and chemical composition 
which we have occasion to enquire into is the period or 
periods during which the water in the lake is in a state of 
stagnation or circulation, these periods depend almost 
entirely on the temperature of the water throughout its depth 
at different times of the year. In colder climates there may 
be two periods of circulation of the water in a lake, but in a 
climate hke ours there can be ouly one unless indeed, the 
lake be over 200 feet deep when stagnation would probably 
be permaneni 
During the months between July and May, there is a 
difference of from 5 to 20 degrees Fahrenheit between the 
temperatures of the surface and bottom water in this lagoon, 
it is to be expected therefore that during that period the bottom 
layers of water are certain to remain at the bottom by virtue 
of their greater density. This period is termed the * period 
of stagnation.” It causes several changes to take place in 
the character of the water at the bottom of the lagoon. The 
most conspicuous change is that of colour. Whilst the 
colouring matter in the water remaining at the surface is 
bleached by the action of the sun’s rays, that at the bottom 
grows gradually darker until, near the close of the stagnation 
period it has a decided opalescent turbidity and a rich brown 
colour, which deepens after being drawn to the surface. 
This colour is probably due to the presence of iron in the water, 
as well as to the sedimentation of the organic matter, which is 
much increased in the bottom layers during this period. 
The oxygen dissolved in the bottom water disappears during 
stagnation, owing to its absorption by the organic matter, 
putrefaction takes place, and the water acquires an offensive 
odour, in which the familiar sulphuretted hydrogen plays a large 
part. The effect of stagnation on the microscopic organisms 
is no Jess remarkable, owing to the absence of oxygen and 
light little life exists at greater depths than 20 feet. but the 
water at the bottom acquires an abundant supply of food 
material, both organic and inorganic, suitable fer microscopic 
life. 
Towards the end of May, however, on the approach of 
colder weather, the decrease in temperature at the surface 
increases the density of the water there, gradually leading 
to a complete vertical circulation of the water in the lagoon, 
