368 FISHES. 



preserves always a certain proportion of azote, which it employs 

 perhaps, partly to fill the natatory bladder. 



There are also fishes which swallow atmospheric air, and convert 



Edwards, considering the bearings of this phenomenon, actually took a large mouthed 

 vessel into which he put five ounces and half of aerated water ; he brought the water 

 to the temperature of 68 deg. Fahr. Into this vessel he put a little fish called the 

 bleak (cyprinus alburnus) and in a few minutes the animal died. He then tried 

 another fish in the same vessel, and with the same quantity of water, but with a 

 decrease of its temperature, amounting to ten degrees only, when the animal conti- 

 nued to live until its secretions became so abundant that they actually corrupted the 

 water. Here, then, we have the important principle determined that the more the 

 temperature is raised beyond certain limits, the greater is the degree of influence of 

 the air required for the support of life. 



But of all the experiments to which fishes have been subjected, that is by far the 

 most intensely interesting which is carried on by exposing them merely to the action 

 of the atmosphere. 



The fishes themselves, indeed, have suggested this modification of man's experi- 

 ments, because, when, in a given quantity of water which hasbeenaerated, a fish has 

 reduced the combined portion of air contained in it until its respiration becomes very 

 difficult, then the creature rises to the surface and actually takes it from the air itself, 

 and so essential is this operation to them, that, as Sylvestre has proved by experi- 

 ment, the fish cannot live in a small quantity of water if we exclude the atmospheric 

 air from its surface. This naturalist placed a diaphragm on a vessel in which a fish 

 had been previously put, and he found that the result was a speedier termination to 

 the life of this animal than befell another animal of the same kind in another vessel 

 which was freely exposed to the atmosphere. Hence, then, we have a most unde- 

 niable proof that the structure of the gills of fishes was endowed with a double 

 faculty of extracting oxygen from water, and also from air, two fluids so differing in 

 their chemical constituents. 



The existence of fishes in the atmosphere is one of the most mysterious facts in 

 comparative anatomy. If we take a fish out of water it dies, some in a few minutes, 

 and others in a few hours, and hence the conclusion was universally received that 

 fishes died in the atmosphere because the state of the air in it was too thick as 

 compared with its condition when dissolved in water. Now it is likewise true that 

 nearly all the vertebrated classes of animals perish from the very contrary cause, 

 namely, the transition from the atmosphere into aerated water, so that whilst the 

 latter die in water because they have too little air, fishes die in the atmosphere 

 because they have too much. 



The most marked effect derived from the exposure of fishes to the atmosphere, is 

 the loss by perspiration. Dr. Edwards took out of water a chub and a gudgeon, 

 and wiping them and then weighing them, he exposed them to the air. Their gills 

 continued to move till death, but their bodies were observed to get gradually dry, and 

 when they died they were stiff and dry as chips. He then weighed them, and found 

 that the first had lost nearly one-fourteenth of its weight, the other one-fifteenth. 

 This naturalist having, in his researches on the reptiles, found a great many inter- 

 esting results produced by the loss of perspiration from exposure to air, and resolved 

 upon ascertaining if the same consequences could be obtained in the fishes. We 

 extract his account of the effects, from the very excellent translation of Dr. Edwards' 

 work by Doctors Hodgkin and Fisher. 



" To simplify the examination of this subject, let us here consider, as we have 

 done in our researches on the batrachians, the losses by perspiration, as solely at 

 the expense of the water contained in the animal. Capacity of saturation with 

 water implies the quantity of this liquid which an animal is able to contain, between 

 the point of greatest repletion, or saturation, and that of the greatest inanition, 

 compared with the weight of its body. The means of carrying the body to the 

 point of saturation when it is capable of absorbing water, is to place it in that fluid, 

 until the increase in weight has arrived at its maximum. This is exactly the con- 



