666 



and in all three iodine was present in perceptible quantity. Desirous 

 of making certain that the agents used in the investigations were as 

 pure as other commercial substances of the same kind, various speci- 

 mens were procured, from different sources ; and in every sample 

 which was subjected to examination the presence of iodine was de- 

 tected. So far, then, as the determination of iodine in the atmosphere 

 is concerned, the experiments were of no value. The alkalies through 

 which the air had been drawn undoubtedly contained iodine origi- 

 nally; and therefore no certain conclusion could be drawn as to the 

 probability of their being more highly iodized by contact with the 

 atmosphere. To the presence of iodine in potashes, or, to use words 

 more strictly botanical, in the ashes of forest twiher, further reference 

 will be made in a subsequent part of this paper. 



In the next experiment the alkalies were dispensed with, the air 

 being drawn through — 



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



2ndly. A gas-bottle immersed in a freezing mixture. 



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



A continuous current was kept up for fully five hom*s, 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 added to it, and subsequently nitrate 

 of potassa and hydrochloric acid, which together form a most delicate 

 means of detecting iodine. The result was negative. The nitrate of 

 silver solution was cautiously evaporated to half an ounce, sulphu- 

 retted hydrogen added to precipitate the silver, and liberate, as hydri- 

 odic acid, any iodine which might be present; the liquid raised iu 

 temperature, carefully avoiding ebullition, and filtered. The filtrate, 

 on the addition of starch, nitrate of potassa, and hydrochloric acid, 

 did not exhibit the slightest trace of iodine. Mr. Macadam therefore 

 concluded, that in the large volume of air which he had drawn through 

 the arrangement there had not been an appreciable amount of iodine. 



The experiments as yet referred to were made, at different heights, 

 on Arthur's Seat ; and their negative results led to arrangements being 

 made for a trial on a scale much more extensive. Through the kind- 

 ness of the proprietor of Kinneil Iron-works, the author was enabled 

 to proceed to Borrowstowness, and attach his apparatus to the receiver 

 from which the air, under great pressure, is forced into the blast 



