GAS DISEASE IN FISHES. 361 
the quantity of air dissolved is great enough. Supposing the temper- 
ature constant, it is the interrelation of the dissolved air factor and 
the pressure factor which determines the fact of the excess, and since 
the condition of excess of air is to be defined only as a preponderance 
of the dissolved air factor over the pressure factor, the cause of the 
gas disease may be defined broadly as due to an excess of air; and 
more narrowly, since there is much evidence that nitrogen alone is 
essentially concerned, as due to an excess of nitrogen. 
SUPERSATURATION OF NATURAL WATERS. 
The symptoms and fatality at Woods Hole were the result of 
artificial conditions. A modification by the hand of man of the condi- 
tions under which air is usually taken up by the water resulted in an 
excess of the air so takenup. The pertinent question will immediately 
suggest itself whether natural waters ever acquire a similar excess, or 
any excess at all, of air or of the constituents of air. Such excesses 
are found to occur. Natural springs of water and flowing wells are 
known to emit a gas, sometimes in considerable quantities, which has 
approximately the composition of air. These are not very common. 
Conditions at Erwin, Tenn.—Such a spring occurs on the reserya- 
tion at the Fisheries Station at Erwin, Tenn., in a limestone region 
near the foot of a considerable mountain ridge. This spring has a 
superficial area of about 600 square feet and its maximum depth is 
about 4 feet. The bottom is partly of mud, partly of gravel and the 
outcropping of the limestone strata. The water wells up chiefly from 
the gravel, and from each wellspring a quantity of gas in large bub- 
bles is evolved at intervals of a few moments. The gravelly bottom 
about the sources of water holds mechanically large amounts of gas, 
for, upon tapping it gently with a stick, an unusually large quantity is 
liberated-and comes bubbling up through the water. The evolution 
of gas then ceases for a longer period than usual, but begins again 
spontaneously within a few minutes. This periodical delivery of gas 
continues day and night at all seasons. Evidently there is a constant 
flow of gas accompanying the flow of water and at all times in the 
earth or gravel beneath the spring and through which the water rises 
are entangled large quantities of gas, a small fraction of which is 
evolved every few minutes as the pressure beneath determines. 
This gas is air with the nitrogen and carbon dioxid considerably 
increased. (Table I, p. 372, sample 1.) As springs do not usually 
discharge both water and free air, the original access of air is of more 
than passing interest. It is evident that it must be mainly derived 
from the atmosphere. 
The region about this spring is mountainous and largely of a lime- 
stone formation, in which caverns have been formed by the usual 
process of solution of the limestone by water containing carbon dioxid. 
