358 REPORT OF THE COMMISSIONER OF FISHERIES. 
given quantity of dissolved air per unit of water, at a given tempera- 
ture and pressure, occasions a fatal process among fishes, a sufficient 
increase in the pressure or decrease in the temperature may render the 
water perfectly harmless to fishes; but it does so by abolishing the 
excess of air, though no change occurs in the absolute quantity of air 
concerned. The temperature factor alone is not so easily varied, and 
no direct experiments have been made involving it, but the statement 
above can hardly fail to be corroborated by such tests. For the pres- 
sure factor some interesting experimental facts have been obtained. 
Seup placed in live boxes at or near the top of a reservoir storage 
tank of the Woods Hole water which was causing gas symptoms in 
aquaria were usually killed within twenty-four hours, the characteris- 
tic embolism and external symptoms always present. At the bottom 
of this tank, the depth of water being 8 or 9 feet, several days were 
required to produce the symptoms, and death occurred only after a 
still longer time. At half the depth the results were intermediate. 
There was a constant flow of water through the tank and it was evi- 
dently the hydrostatic pressure which inhibited the usual process. 
Carrying these observations further, a large glass jar was arranged to 
hold aquarium water with a constant flow and under a pressure vary- 
ing between 6 and 7 pounds per square inch in addition to atmos- 
pheric pressure. Five adult scup were placed in this jar and remained 
alive under the pressure, without food, for twenty-nine days without. 
developing any gas symptoms. The same water which flowed through 
the jar would at the beginning of the experiment at atmospheric pres- 
sure produce external lesions within twenty-four hours and was fatal 
within two or three days, the time varying considerably. After 
removal of pressure at the end of the experiment, all the five scup 
died within five days with free gas in the vessels of each. They were 
fed for the first time on the fourth day after the removal of pressure. 
During various experiments at Woods Hole some evidence was inci- 
dentally brought out indicating that starvation retarded the gas-disease 
process. This it may be conceived to do by a general lowering of 
metabolism. 
Except under experimental conditions, no cases of as disease caused 
by reduction of pressure alone have been observed by the writers, and 
it is doubtful whether any occur. In a former paper by one of us 
(Gorham, 1899) it was thought that the reduction of pressure was the 
only cause. The factor of the supersaturation of the water was not 
recognized at that time. From experiments performed in connection 
with that former work and new ones in connection with the present 
study we are sure that mere reduction of hydrostatic pressure—that is, 
the reduction incident on bringing fishes to the surface of the water— 
is not sufficient to produce the disease in those fishes which have been 
studied. A number of scup were kept ina live car at the surface of the 
