A simple apparatus is described with which the oxygen consumption of a small fish may be estimated, first in water and then at successive time intervals in a toxic solution, so that the progressive effect of the solution on the rate of oxygen intake may be graphed.

Chloroform solutions of concentration 1/3000 to 1/5000 gradually depress the respiration rate and are ultimately fatal. A 14/100,000 solution appears to produce a state of anaesthesia in which the oxygen intake is about 30 % normal.

Sodium cyanide solutions bring about a progressive decline in the oxygen intake which is closely accompanied by a decline in the rate of opercular movement. The critical concentration is about 0.00004N, which depresses the respiration rate to 32 % normal in 90 min. At greater dilution the survival time lengthens rapidly. Sodium sulphide solutions give very similar results but are somewhat less toxic than cyanide solutions, and the value for the critical concentrations is about 0.0002N.

Heavy metal salts (mercuric chloride, copper sulphate, lead nitrate) produce at first an increase in the respiration rate. Then the oxygen intake declines, but the rate of opercular movement continues to increase, reaches 180-240/min., continues at this increased rate for some time in the case of dilute solutions and then falls rapidly when the oxygen intake is reduced to 38 % normal.

It is concluded that in the case of cyanides and sulphides, where respiration is inhibited at the tissues, the close agreement between the oxygen intake and opercular movement rate may be attributed to the decline in carbon dioxide production. In the case of the heavy metal salts, on the other hand, where respiration is obstructed at the gill surfaces, it would appear that the carbon dioxide content of the blood is raised so that the respiratory centre is stimulated and the opercular movements increase in speed and amplitude. The continued fall in oxygen intake and accumulation of carbon dioxide maintains this effect for some time, but eventually the fish becomes exhausted, the respiratory movements fail and the fish dies.

The work concludes with some suggestions for further investigation.

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