232 Wisconsin Academy of Sciences, Arts, and Letters . 
synthetic activities. This makes the half bound carbon diox¬ 
ide a very important factor from the biological standpoint. 
Whenever this gas is present in excess of the amount required 
for the conversion of the normal carbonates into bicarbonates, 
it appears in the form of free carbon dioxide and such a water 
will give an acid reaction with phenolphthalein. On the other 
hand, when no free carbon dioxide is present and some of the 
half bound carbon dioxide is lost, leaving an excess of the nor¬ 
mal carbonate, the water gives an alkaline reaction with this in¬ 
dicator. In table II (p. 244) the degree of alkalinity or acid¬ 
ity of the water is shown in terms of carbon dioxide. The minus 
sign shows that the water gave an alkaline reaction with phe¬ 
nolphthalein and the numbers indicate the amount of free carbon 
dioxide necessary to produce a neutral reaction. The plus sign 
indicates that the water gave an acid reaction and the degree of 
acidity is stated in terms of free carbon dioxide. 
In lake Amatitlan the water was distinctly alkaline down to 
a depth of 15 m. so that it would have required 3.34 cc. of free 
carbon dioxide per liter to make it neutral. It was neutral at 
20 m. and 25 m. and acid thence to the bottom where the acidity 
was equivalent to 2.78 cc. of free carbon dioxide per liter of 
water. Similar results for free carbon dioxide are found in tem¬ 
perate lakes soon after the summer stratification has been well 
established. 
In lake Atitlan the water gave an alkaline reaction at all 
depths. In the upper 70 m. the alkalinity was equivalent to 2.23 
cc. of free carbon dioxide per liter of water and it amounted to 
only 1.11 cc. at and below 200m. The water of a thermal spring 
located at the village of Atitlan had an acidity equivalent to 
23.26 cc. of free carbon dioxide per liter, while the fixed carbon 
dioxide amounted to 124.65 cc. 
The water of lake Coatepeque had the highest degree of alkal¬ 
inity. In the upper 20 m. it amounted to 5.57 cc. while below 
75 m. it was equivalent to 3.23 cc. (See table II, p. 244.) 
During the period of complete circulation the water in each of 
these three lakes doubtless had a substantially uniform degree 
of alkalinity, but when the lower water ceased to take part in the 
circulation its alkalinity began to decrease. This result was due 
to the liberation of carbon dioxide in the respiration of the or¬ 
ganisms which occupied this stratum and in the decomposition 
of organic material which also took place in this region. 
