eS ee ee a ee 
RELATION OF ALGAE TO DISSOLVED GASES. 187 
PRELIMINARY TESTS. 
As a preliminary step I made a series of determinations 
of the dissolved gases, oxygen and free COz, in the waters 
of the lagoon and stream of the Garden and in some of 
the ponds in the city parks, extending from May until the 
freezing of the ponds in January. The autumn and winter 
of 1911, up to January, were mild and most favorable for 
such studies. There was no severe freeze to kill or check 
the autumn growth, which was especially rich in diatoms. 
The summer was unusually warm; much of the time rang- 
ing from 90° to 100° F., from May to October. But this 
made only a slight difference in the supply of water in 
the ponds, which are fed from the City System, and an 
almost constant level is maintained, except when heavy 
rains cause a slight temporary rise and overflow at the out- 
let dam of the lagoon. The hot summer must have had 
some considerable influence on the temperature of the water, 
as it nearly always showed the same temperature as the 
air during the summer and early autumn. The chemical 
composition of the water fluctuates somewhat with the 
heavy rains, but aside from this it must be the same as 
that of the Mississippi River, from which it comes. 
While the chemical value of the water as a culture 
medium for alge is not materially changed by the process 
of purification at the city settling plant, yet there are 
slight changes that would have to be taken into account 
where chemical indicators are involved. Miss Hayden (710), 
in her recent study of the alge in these ponds, makes a 
general statement on authority, to this effect. The annual 
report of the Water Commission for 1906, the last avail- 
able, shows that the process removes nearly all of the 
bicarbonate of calcium and delivers the water with quite a 
strong degree of alkalinity. This was a point with which 
I was later to reckon. 
INVESTIGATIONS—-FIELD. 
~The accompanying table of oxygen and carbon-dioxide 
content of the water in the lagoon shows an increase of oxygen 
