GAS DISEASE IN FISHES. woe 
ature, for in its course through the gills it must be cooled to or nearly 
to the temperature of the water. Its stream is too thin and it is too 
intimately exposed to the water to maintain an appreciably higher 
temperature. The blood, then, before it can return a second time to 
the gills, undergoes a rise in temperature, and as the solyent power of 
liquids decreases with increase of temperature, this rise tends toward 
the release in gaseous form of some of the dissolved air. No doubt 
the amount released is small per unit of time, but the free gas can not 
be reabsorbed and the process of release is continuous, so that a fatal 
embolism is only a question of time. 
This seems a fairly satisfactory explanation of the means by which 
the gas arrives free within the blood vessels. It requires the assump- 
tion that in water of normal condition with respect to dissolved air 
the blood of fishes does not become completely saturated in the gills— 
else gas would be thrown out constantly by the higher temperature of 
the systemic circulation, which is of course contrary to fact. There 
is experimental proof that in mammals ordinary respiration does not 
saturate the blood—that is, that all the oxygen which it is capable of 
holding under the conditions does not enter it (L. Fredericq, 1896; 
O. Hammarsten, 1901, 531). That the observation holds good for fishes 
is extremely probable. It must further be assumed that under the 
conditions of supersaturation existing in the Woods Hole water the 
blood does take up all the air it will hold at its temperature in the 
gills; or, if it falls short of this, that it takes up more than it can hold 
at the maximum temperature to be encountered in its circulation 
through the body. This latter supposition is the more probable and, 
while no determinations support it, it is thoroughly in accord with 
the facts and may be provisionally accepted. | 
Temperature is not the sole cause which may play a part in the pre- 
cipitation of the gas. For the separation of the solute, or dissolved 
substance, from a supersaturated solvent, there must be a nucleus 
about which the precipitating dissolved particles may gather—an 
excitant to start the process of precipitation. This is strikingly 
illustrated by supersaturated solutions of certain salts. A crystal of 
the same salt as that dissolved when introduced into such a solution 
will cause the immediate separation of this salt, which gathers about 
the crystal as a nucleus. Likewise water may be heated, in a per- 
fectly clean and smooth flask, above the boiling point without ebulli- 
tion. If a solid foreign particle, such as a fragment of pumice stone, 
be dropped into the flask, boiling instantly begins. ‘To apply this 
principle to the present case, the minute floating corpuscles may be 
considered as the nuclei for the separation of gas from blood, which 
is supersaturated with it. The difference in temperature is the more 
important and fundamental cause of the release of gas, while doubtless 
the corpuscles at least provide Joc? for the change of state. 
F. C. 1904—23 
