General Distribution of Iodine. 317 



of air passed through it. At the conclusion, the papers, over which 

 300 cubic feet of air had been drawn, were carefully inspected, but 

 not the slightest indication of iodide of starch could be detected, even 

 when moistened with distilled water. The soda and potassa were 

 separately treated with starch and nitric acid, and both exhibited the 

 rose-colour characteristic of the presence of iodine in small quantity. 

 At this stage of the inquiry, I entertained great hopes of being able 

 to verify Chatin's observations ; but, on analysing portions of the 

 original alkaline solutions through which no air had been drawn, I 

 found iodine present in them in quantity, to all appearance, as great 

 as it was in those portions of the liquids used in my experiments. 



The caustic alkalies employed by me, were therefore contaminated 

 with the very substance I was searching for in the atmosphere, and 

 it remained to inquire into the original source of this impurity. With 

 this view, I tested samples of the carbonate of potassa, carbonate of 

 soda, and lime-shell, which had been employed in the preparation of 

 the caustic solutions, and in all three, iodine was present in percep- 

 tible quantity. Desirous of making certain that the agents used by 

 me were as pure as other commercial substances of the same kind, 

 various specimens of each were obtained and submitted to the process 

 to be afterwards detailed. The samples first tested were those 

 usually to be purchased in Edinburgh and other places, but subse- 

 quently genuine and authenticated specimens were procured from 

 trustworthy sources ; and from every sample of carbonate of potassa, 

 carbonate of soda, and lime-shell, which I have as yet subjected to 

 examination, I have obtained distinct indications of the presence of 

 iodine. It became, therefore, quite evident, that so far as the de- 

 termination of iodine in the atmosphere was concerned, the experi- 

 ments as yet referred to, were of no value, and that it was requisite, 

 in any future experiment upon this subject, to avoid the introduc- 

 tion of the alkalies, which so invariably contained this element as a 

 foreign ingredient. 



Accordingly, in the next experiment the alkalies were dispensed 

 with, and their place was occupied by nitrate of silver. The appa- 

 ratus also was somewhat modified — the air being drawn through — 



1. A tube with slips of starched paper, kept somewhat damp. 



2. A gas bottle immersed in a freezing mixture ; and, 



3. A gas bottle containing a solution of nitrate of silver. 



To enable the condenser (2) to do its work thoroughly, and to 

 guard against any of the liquid in the gas bottle (3) being carried 

 away by excessive evaporation, they were buried in soil which was 

 saturated with water. A continuous current of air was kept up for 

 fully five hours, commencing at mid-day. At the conclusion of this 

 experiment the papers were not altered in the slightest degree ; the 

 gas bottle (2) contained about a quarter of an ounce of liquid; and 

 the nitrate of silver (3) had not been perceptibly changed. The 

 condensed liquid was neutral to test-papers — a drop of starch was 



VOL. LIII. NO. CVI. — OCTOBER 1852. Y 



