RESISTANCE OF FISHES. 325 



and placed in fresh tap water, as soon as all movement had 

 ceased. In none of these cases was there recovery or even 

 further movement. However when a fish was removed while 

 the movements were still faintly visible, it usually recovered 

 and became normal once more. The nearness to the death 

 point at which a fish might still be resuscitated varied with the 

 species and with the individual. This variation was not fully 

 determined. It is probable that if errors were made in deter- 

 mining the death point, they were upon the side of exceeding 

 the actual time, for in nearly all cases, a fish was not regarded 

 dead until no movement had been visible for from two to five 

 minutes. 



III. APPARATUS. 



The apparatus used in the experiments consisted of the gas 

 control apparatus, already mentioned, 1 and of three large wide- 

 mouthed bottles, with connections, etc. In brief the gas-control 

 apparatus consists of a series of perforated pans, of boilers, and 

 coolers. The water may be made to pass through the pans into 

 the boilers and through the coolers, in the order named, or it 

 may be turned directly into the boilers and on into the coolers. 

 If gas is to be introduced, it is injected through a gas introducer, 

 between the first and second coolers. In these experiments, the 

 only gas introduced was carbon dioxide. This gas was intro- 

 duced from the cylinders in which it is purchased. According to 

 Shelford and Alice's analyses, the gas in these cylinders contains 

 99.4 per cent, carbon dioxide and .6 per cent, nitrogen (different 

 cylinders may vary slightly). Variations in the oxygen con- 

 centration were obtained by treatment with the apparatus. 

 The very low concentrations (.1 c.c. per liter) were obtained by 

 boiling the water vigorously in the apparatus, and in order to 

 do this it was necessary to start with the hot tap water of the 

 laboratory. Because it was necessary to use the hot tap water 

 in some experiments it was thought best to use it for all, and this 

 plan was followed except in experiments with high oxygen (10 

 c.c. per liter). In these cases the cold tap water was used. In 

 either case the temperature at which the water was used was the 



1 For lull description of gas control apparatus, together with drawings, see 

 Shelford and Alice ('13), p. 214. 



