CARCINOMA OP THE THYROID IN SALMONOID FISHES. 



465 



by treating under exactly the same conditions comparable fish with mercury in the 

 form of HgCl 2 . 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 Salmonidae 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, were carried out, using 

 arsenic in the form of As 3 O 5 . 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 



