36 American Fisheries Society 



more than 0.5 per cent. The balance employed in all weighings 

 was sensitive to a milligram with a load of one and one-half 

 kilogram. 



Feeding the trout was unquestionably the most difficult and 

 yet the most essential step in the process of accurately measuring 

 their metabolism. The effort, therefore, was made to teach the 

 fish to take their food directly from forceps. Wherever this was 

 feasible, the rest of the experimental procedure was quite simple. 

 In a few instances I was actually successful in so training the 

 trout that they would come to the edge of the aquaria and leaping 

 out of the water, snap the food held with pincers. In this way the 

 washing out of soluble constituents of the food by the water was 

 entirely prevented. 



The food was kept in small weighing bottles, and the amount 

 consumed was determined by the difference in the weight before 

 and after feeding. The method of feeding the trout ad libitum, 

 had the advantage also that at no time were there unconsumed 

 particles of food left in the aquaria which might favor contamina- 

 tion of the water, and thus greatly affect the significance of the 

 results. 



Unfortunately this method of direct feeding could not always 

 be utilized, as will be shown in a later section. The food was 

 generally prepared in large quantity and stored in a frozen condi- 

 tion. The contents of every jar was carefully analyzed, and its 

 composition was checked at least twice in the course of an exper- 

 iment. Portions of this stock food, enough for several feedings, 

 were put in weighing bottles, and kept in the ice chest in the 

 laboratory. 



Usually forty-eight hours after feeding, the trout was removed 

 to a jar with a fresh supply of water, and the old water containing 

 the solid and dissolved excreta of the preceding period was filtered 

 and prepared for analytical treatment. 



Large aluminum tumblers, the bottoms of which were perfo- 

 rated with a number of fine holes, were used for this purpose; 

 the tumblers serving as Gooch crucibles. The tumblers were pro- 

 vided with a thick pad of fine glass wool, dried in the oven and 

 weighed. The contents of the aquarium were siphoned into the 

 tumbler and filtered through the glass wool, with the aid of suction, 

 into a large bottle. The glass wool was found very efficient in 



