466 
BULLETIN OE 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 troiit 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 corners 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 preserved 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 
