Judmj—Lakes in Central America. 
227 
The results obtained on these lakes are not in agreement with 
those obtained by Downes 1 on water reservoirs situated in the 
Panama Canal Zone. He found that these artificial bodies of 
water were permanently stratified and that the transition zone 
lay at a depth of only 2 m. to 3 m. 
Dissolved Gases. 
Oxygen. Samples for the estimation of the dissolved gases 
were obtained by means of a closing water bottle which had a 
capacity of about one liter. The sample bottles had a capacity 
of about 250 cc. so that three samples of water were obtained 
from each depth at a single haul. Two of these samples were 
used for the determination of the dissolved oxygen and the third 
for carbon dioxide. The water was transferred from the water 
bottle to the sample bottles by means of a rubber tube and pre¬ 
cautions were taken to prevent contact with the air. The sur¬ 
plus water was used to flush out the bottles containing the oxy¬ 
gen samples. The Winkler method was used for the determina¬ 
tion of the quantity of dissolved oxygen and in general the re¬ 
sults given in the table are the mean of duplicate samples. 
In this method of determining the dissolved oxygen, 1 cc. 
of a solution of maganous chloride is added to the sample and 
then 1 cc. of a solution containing sodium hydroxide and potas¬ 
sium iodide. The sample is thoroughly shaken and the precipi¬ 
tate is allowed to settle. It is now treated with 2 cc. of con¬ 
centrated, chemically pure hydrochloric acid, which dissolves 
the percipitate. In the chemical reactions which take place, 
iodine is liberated in proportion to the amount of dissolved 
oxygen present. The amount of the free iodine is then deter¬ 
mined by titration with a standard solution of sodium thiosul¬ 
phate. One cubic centimeter of a N/10 sodium thiosulphate 
solution is equivalent to 0.0008 gm., or 0.5598 cc. of oxygen at 
0° and 760 mm. since 1 1. of oxygen weighs 1.429 gms. at nor¬ 
mal temperature and pressure. 
During the winter circulation the general mixing of the water 
at all depths is sufficient to produce a fairly uniform distribu¬ 
tion of the dissolved gases and other substances held in solution. 
The water at various depths is exposed to the air from time to 
1 A Study of the Water Supplies of the Isthmus of Panama. Proc. Med. 
Asso. of Isthmus of Panama, vol. Ill, 1911, p. 133-150, 7 pis. 
