CARCINOMA OF THE THYROID IN SALMONOID FISHES. 
465 
by treating under exactly the same conditions comparable fish with mercury in the 
form of HgCK. This gave the further advantage that if mercury should prove to have 
a similar effect upon the thyroid to that determined by Marine and Lenhart for iodine, 
the relation of these growths of the thyroid to one of the heavy metals might be deter- 
mined, mercury already having been shown to have an inhibitory and regressive effect 
upon genuine neoplasms. The experiments were carried out in the summer of 1910 
with the result, as may be seen by the accompanying tables, that mercury was found 
to have an effect upon the growing thyroid of the Salmonids indistinguishable from 
that obtained with iodine, with the exception that mercury appeared to produce these 
results more certainly and more rapidly than did iodine. To further amplify the 
Fig. 95. — Floating siphon. A is the siphon, B the frame, and C the container. The form of the frame is of course not 
essential, and should be adapted to the container. The illustrations show the glass tubing of much larger size than is neces- 
sary or practicable in small siphons. Small tubing is preferable. 
comparison, during the summer of 1911 experiments with arsenic, an element long 
known to have a favorable influence upon genuine neoplasms, vrere carried out, using 
arsenic in the form of AS2O5. These results are also sufficiently set forth in the tables. 
Experiments were begun by determining the toxicity of the iodine to trout when 
added to the water in the form of the pure element already dissolved in distilled water 
and then added to the dissolved potassium salt. The uncombined iodine is much more 
toxic than the potassium iodide. Lake trout fry were killed in less than 20 hours by 
one part of free iodine added to 400,000 parts of tap water. Dilutions of i to i ,000,000 
are safe, perhaps because the iodine is combined before it has time to produce a fatal 
