SECRETION AND ABSORPTION OF GAS, ETC. 123 



Moreau's further experiments showed that the air- 

 bladder may in reality so act as to assist the animal in 

 balancing itself at any required depth, but that the action is 

 an exceedingly slow one, and not dependent in any way on 

 the muscles. He took a fish which had been living at the 

 surface and determined its specific gravity by means of the 

 arrangement show in F"ig. 2. He then sank the fish in 

 a cage to a considerably greater depth, and after about two 

 days pulled it up again, and redetermined its specific 

 gravity at the same pressure as before. He always found 

 that the fish was now much lighter. It had gradually 

 secreted air into its air-bladder while at the deep level, 

 so that at last its specific gravity corresponded with that 

 of the water. On coming up again the extra air of course 

 caused it to swell, so that it was much lighter when brought 

 back to its old level. 



When the air-bladder was artificially emptied, either 

 by puncture with a trocar, or (in the case of fish with a 

 patent duct) by producing a vacuum in the vessel contain- 

 ing the animal, the immediate result was that the fish sank 

 helplessly to the bottom. After a few hours, however, it 

 was able to swim about again as usual, gas having gradually 

 been secreted into the air-bladder. This experiment may 

 very conveniently be performed on a goldfish. All that is 

 necessary is to place the animal in a large-sized bottle pro- 

 vided with a perforated rubber cork, through which passes 

 a tube connected with a filter-pump. As soon as sufficient 

 air has escaped from the swimming-bladder the tube is re- 

 moved. The fish at first remains helplessly at the bottom 

 of the bottle, but gradually recovers, and next morning will 

 be swimming about again as usual. Another experiment 

 described by Moreau is to take two fishes, and attach a float 

 to one and a sinker to the other. At first the fish with the 

 float is unable to leave the surface, while that with the 

 sinker is unable to leave the bottom. After some hours, 

 however, the two fishes will be swimming about together 

 as usual, the fish with the float having absorbed some of 

 the gas in its air-bladder, while the fish with the sinker has 

 secreted gas. This experiment seems to indicate that the 



