REACTIONS OF FISHES TO ATMOSPHERIC GASES 229 



each experiment; one was placed in a box into which normal 

 water was allowed to flow and another in one into which treated 

 water was allowed to flow. The fishes were each observed in 

 detail and a full record of all their movements made for a period 

 of ten minutes for each fish, when they were interchanged and 

 the experiments repeated. A detailed series of such experiments 

 were performed with boiled water, a partial series with carbon 

 dioxide, and carbon dioxide in boiled water. The respiratory 

 mo\'ements were increased in vigor or in number, usually in both 

 when the fishes were in carbon dioxide or low oxygen water. 

 They rose to surface often and gasped and gulped. Although 

 the details of .these experiments formed an important part of 

 the method of work, they are not presented here because their 

 inclusion would burden the paper with much detail not of prime 

 importance, from our point of view. 



The chief sources of error in this gradient method lie in the 

 attempts at studying the fishes at temperatures higher than that 

 of tap water. It was found that a difference of 1° C. between 

 the ends of the experimental tank very seriously interfered with 

 the gradient. The movement of the fishes back and forth in the 

 tanks tended slightly to mix the water of the two ends but we 

 found no evidence that this interfered with the main gradient. 



2. REACTIONS TO SINGLE GASES 



It is usually difficult to vary a single factor in an experiment, 

 In the experiments in which we intended to vary a single gas. 

 the same kind of water was used in both ends of the experimental 

 boxes, gas being added to the water at one or both ends, as 

 necessity demanded. In adding a gas to the water, a very small 

 quantity of other gases were also added (table 1 A) and anj^ 

 effect of increased gas pressure upon salts in solution QlcCoy 

 and Test '11) of necessity took place. However these sources 

 of error probably in no way interfered with the essential char- 

 acter of the reaction of the fishes to the gas used. 



