234 Wisconsin Academy of Sciences, Arts y and Letters. 
Rainwater possesses a small amount of carbon dioxide when it 
reaches the earth and in passing through the ground it obtains 
still more. The. normal carbonates of calcium and magnesium 
are only slightly soluble in pure water, but this carbonated water 
readily changes the comparatively insoluble normal carbonates 
to the much more soluble acid carbonates or bicarbonates. Thus, 
when this ground water reaches a lake through springs, it gen¬ 
erally contains a liberal amount of both bicarbonates and free 
carbon dioxide. But, unless the volume of spring water enter¬ 
ing a lake is relatively large in proportion to the volume of the 
lake, the free carbon dioxide content of the whole body of water 
is not greatly affected from this source. 
The unusual acidity of the water in lake Ilopango was not due 
to the absorption of carbon dioxide from the air because the 
quantity was larger than would naturally be obtained from this 
source owing to the low partial pressure of this gas in the atmos¬ 
phere. Neither was there any evidence that it was due to car¬ 
bon dioxide derived from decomposition and respiration. These 
processes would have to be taking place very vigorously in order 
to maintain such a high degree of acidity in the upper water 
which |was kept in circulation by the wind and freely exposed 
to the air where the tendency would be to dispose of any excess 
of this gas. But there was no indication that these processes 
were proceeding with such vigor. The fact that there were 
about 3 cc. of dissolved oxygen per liter of water at the bottom 
shows also that the 7.23 cc. of carbon dioxide in this water did 
not come entirely from respiration and decomposition because, if 
it had been produced by these processes, the free oxygen would 
all have been consumed in its production. 
Of the sources mentioned above, there remains, then, only 
the ground water to be considered. Ordinary spring waters 
generally contain an abundance of free carbon dioxide and 
when the volume of water derived from springs is relatively 
large ini proportion to the volume of the lake, the acidity of the 
lake water is affected appreciably. But in a lake having as 
large a volume of water as lake Ilopango, it would require an 
unusually large inflow of spring water to produce and main¬ 
tain such a high degree of acidity as was shown by the waters 
of this lake. No definite data were obtained relative to the 
volume of spring water entering lake Ilopango, but about a 
quarter of the shore was examined and no springs were noted 
