466 BULLETIN OF THE BUREAU OF FISHERIES. 



result. At i to 600,000 brook trout yearlings were not killed during three days, but 

 the effect of iodine was seen in coagulating the slime on the bodies of the fish. 



In the form of potassium iodide, i part of iodine in 25,000 parts of tap water killed 

 brook trout fry in two to six hours, but i to 50,000 was harmless during a trial of two 

 weeks. Lake trout fry were uninjured by dilutions of i to 200,000 and weaker. 



The administration of the chemical agents has been accomplished in several different 

 ways and in various dilutions. The constant uniform dilutions were maintained by 

 running a solution of known strength continuously into the fish troughs with the 

 measured water inflow at the head of the trough, an intimate mixture being insured. 

 A barrel was used as a receptacle for the solution and a constant head for the flow of 

 solution was maintained by the use of a floating siphon. (Fig. 95.) In this way it 

 is easy to keep the water supply of the experiment constantly impregnated to any 

 desired degree with any soluble agent. 



The intermittent treatments were applied by adding single doses of the chemical 

 to the water of the fish trough, obtaining fairly complete distribution by stirring. Two 

 daily doses were given, and the water supply was so adjusted as to change the contents 

 of the trough every four-hour period. The dilution of the chemical then proceeded with 

 the flushing out of the trough by the water flow. The comers of the trough may have 

 still held appreciable quantities of the agent after the four-hour period, but the flow 

 must have diluted it beyond any effective strength long before the succeeding portion 

 was added. 



Experiment i (table ix). — Iodine administered as potassium iodide and given con- 

 tinuously under constant head from a floating siphon into a trough receiving an ade- 

 quate and measured water supply direct from Craig Brook and having a definitely 

 known outflow. The water supply was 24 liters per minute. The siphon flow of KI 

 solution was so regulated that a constant concentration of iodine of i : 5,000,000 was 

 maintained in the flowing water. The temperature of the water during the course of 

 the experiment varied but little from 19.5° C. The fish were fed raw beef liver. The 

 following fish were subjected to this treatment: Three tumored hybrid salmon, 6 tumored 

 brook trout, and 15 clinically clean brook trout. An equal number of controls living 

 under identical conditions, with the exception of the presence of iodine, were carried 

 through for comparative study. The greatest period during which fish were subjected 

 to this treatment was 31 days. Treated fish and controls were preser\'ed for study, 

 however, at frequent intervals during the course of the experiment. A brief statement 

 of results is given in table ix. This is self-explanatory. (Fig. 96, 97, 98, 99, 100, loi.) 



Experiment 2 (table x). — -Similar in all respects to experiment i, except for the 

 concentration of iodine, which was much greater, being i : 300,000. The fish were fed 

 raw beef liver. The fish subjected to the treatment were as follows: Four clinically 

 clean brook trout, 5 brook trout with pharyngeal discoloration (red floor), and 6 brook 

 trout with various tumors. Parallel controls for all these fish were also studied. The 

 longest period of treatment in this experiment was 17 days. Macroscopic evidence of 

 regression was furnished in some of the red floors and in some of the tumor fish as 

 shown by the disappearance of pharyngeal reddening and reduction of tumor measure- 

 ment. The results of microscopic study of these specimens, together with their controls, 

 are given in table x. 



Experiment j (table xi). — In this experiment iodine was administered intermit- 

 tently as potassium iodide, the greatest concentration of iodine in the water at any one 



