370 REPORT OF THE COMMISSIONER OF FISHERIES. 
from this cause occur in nature, for supersaturation does not arise 
suddenly and aquatic animals would avoid the regions of excess in the 
rare cases where access to them is possible. The possibility of inju- 
rious or fatal excesses of dissolved air, especially in natural waters, 
seems not to have occurred either to fish culturists or biologists. 
The two faults, excess and deficiency of air, are so correlated that 
the same process of correction applies to each. The same exposure 
to the air which aerates water with a deficiency of air deaerates water 
with an excess of air. In superaerated water, such as that of the Woods 
Hole aquaria, there may be a deaeration in the more complete sense; 
both nitrogen and oxygen are to be removed. But in hardly any case 
does the term: aeration apply in its complete signification. Oxygena- 
tion alone is usually the strict meaning. In natural waters the term 
deaeration likewise does not in most cases completely apply. Denitro- 
genation alone is the stricter meaning. Oxygenation, however, may 
accompany denitrogenation, and thus water is in the broad and looser 
sense aerated and deaerated at the same time and by the same process. 
When an actual case of air-supersaturated water confronts the fish 
culturist or the management of aquaria, the practical measures to be 
taken will suggest themselves according to the source of the excess of 
air. If a gravity plant supplied by pumps is in operation the whole 
suction system is open to suspicion of leaks. Such leaks, of course, 
give out no water but suck air, and are therefore not always easily 
recognized. By stopping the pumps and removing the proper valve 
the hydrostatic pressure may be allowed to rest back on the suction 
pipe and will speedily develop the leaks if the pipe is exposed. If it 
is underground they may not show readily, or at all. Repair of all 
the leaks will completely remedy the difficulty. The suction pipes, 
especially if wooden, may be beyond repair, in which case nothing but 
a complete renewal will entirely prevent trouble. Pending this, local 
deaeration may be practiced at each aquarium, pond, or trough sup- 
plied with the water. For an aquarium a large pan with many per- 
forations may be suspended above, the higher the better, and the 
water delivered into this. If the exposure of the slender streams and 
the splashing at the surface are not sufficient correction, the scale of 
the device has but to be increased, most conveniently by adding more 
perforated pans. The great desideratum is sufficient fall in which to 
expose the water. 
When the supersaturated supply is from springs or wells the condi- 
tion is more serious. A radical correction is impossible, for the air, 
or modified air, which causes the excess is deep in the earth and can 
not be controlled. If, as is usually the case, there is no great differ- 
ence of level between the rising water and the ponds, troughs, or 
tanks in which it is used upon fishes, it is the more difficult or impos- 
sible to completely deaerate. The natural remedy is to use the water 
