in the English Channel near Brighton. S3 



also gave by the action of the electric current a slightly 

 pinkish deposit. 



It is always necessary, when we wish to detect by means of 

 chlorine minute quantities of an iodide, to employ the chlorine 

 in a very diluted state, as when in excess it forms a soluble 

 chloride of iodine which will not act on starch. 



The sulphates and chlorides present in salt waters do not 

 interfere with the delicacy c>f the starch test; on the contrary 

 a concentrated solution of the chlorides will show the presence 

 of one millionth part of iodide of potassium more distinctly 

 than an equal volume of distilled water. This appears to 

 arise from the iodide being a little soluble in pure water. 

 I thought at first that a trace of an iodide might be contained 

 in the common chloride of sodium, and thus cause a deeper 

 tinge of blue colour ; but by employing a chloride of sodium 

 prepared from pure hydrochloric acid and pure soda, I 

 found the same degree of increased reaction. The iodide of 

 starch will likewise keep unchanged much longer in a solu- 

 tion of chlorides exposed to light and air than in pure water. 



The bromides when present in large quantity interfere 

 with the delicate reaction upon traces of iodine, but when the 

 quantity of iodine is not too small the reaction is very distinct, 

 as a small proportion of free bromine will, like chlorine, 

 decompose the iodide, and produce the characteristic re- 

 action. 



After these experiments I tested fresh sea-water for iodine 

 in the manner before described, but did not obtain the slightest 

 indication of it. I now added one millionth part of the iodide 

 of potassium, and the colour produced by the test did not 

 differ in the slightest degree from a solution of chlorides of 

 the same specific gravity as sea-water, treated in the same 

 manner, and from this I immediately inferred, that iodine, if 

 present in sea-water, must be so in very minute quantity. 



I took 73 pounds troy, of sea-water and boiled with a 

 quantity of caustic potash, sufficient to precipitate the alkaline 

 earth, and after filtration evaporated the fluid to four ounces. 

 On testing a small quantity of this concentrated water no 

 iodine was to be detected, and it was found on adding a 

 minute quantity of an iodide that the presence of bromides 

 in comparatively large quantity interfered with the test. But 

 although these results appeared to negative the presence of 

 iodine, 1 felt convinced it must exist in sea-water, being 

 present in so many sea plants and animals. 



Sarphate, in his ^^ Commentatio dc lodio^^ 1835, Leiden (a 

 treatise which received the prize), states that he could detect 

 no iodine in the sea-water near the Dutch coast. Professor 



