234 Botanical Society of Edinburgh, 



contents of the jar had shown its presence. The jar (4) with the 

 lead solution was treated in the same manner as described in a former 

 part of this paper, when referring to the employment of silver, and 

 the result was also negative. Notwithstanding the large scale on 

 which this experiment was conducted, a volume of air of not less 

 than 4000 cubic feet having been forced through the arrangement, 

 Mr. Macadam has been unable to verify the results of Chatin, yet he 

 feels disinclined to pronounce those results unwarranted, and has 

 therefore resolved to make another trial on a still larger scale. It is 

 proposed to fit up an apparatus of a stronger and more durable nature, 

 and to allow a volume of air of not less than 1 00,000 cubic feet to 

 pass through. 



Whilst the experiments on the atmosphere were proceeding, Mr. 

 Macadam was also examining large quantities of the rain-water which 

 fell in Edinburgh for the last two months. For this purpose, he 

 added to 3 gallons of the water some ounces of a solution of acetate 

 of lead. On standing twenty-four hours, a precipitate had fallen to 

 the bottom, from which the liquid was drawn off. The precipitate 

 was treated as formerly described, and no iodine was detected. As 

 the iodide of lead is slightly soluble in water, and as it might be pre- 

 sent in the liquid which had been removed from the precipitate, the 

 whole was evaporated to 1 oz., and afterwards tested for iodine, but 

 none was present. A second experiment was tried with a similar 

 volume of rain-water, viz. 3 gallons, substituting nitrate of silver for 

 the acetate of lead ; a precipitate was observed after standing for 

 twenty-four hours, but neither it nor the liquid contained a trace of 

 iodine. Another experiment, made with 3 gallons of rain-water, 

 which had been collected at Unst in the Shetlands, and to which 

 acetate of lead was added, gave the same negative results. 



Mr. Macadam is well aware, that, consequent on the evaporation 

 of water from the surface of the ocean, portions of the salts contained 

 in it are carried up and disseminated through the atmosphere, ready 

 to be rained down upon inland places, and that in this way iodine, 

 most probably as iodide of sodium, will be present in the air. Accord- 

 ingly at first he was confident that he should succeed in verifying 

 Chatin's observations in a district so near the sea as that around 

 Edinburgh, and more especially in the water obtained from Unst, 

 which had fallen in the immediate vicinity of the ocean ; but when 

 we consider what a very small per-centage of iodine is present in the 

 water of the ocean, many gallons being required to give even a faint 

 indication, equal to that exhibited by 55^-5^ th of a grain of an alkaline 

 iodide, and if, further, we suppose that when the water rises in va- 

 pour from the sea, it carries up the salts in the same proportions as 

 they exist in sea-water, it is evident that it would be requisite to eva- 

 porate some hundred gallons of rain-water, before even a minute trace 

 of iodine could be obtained. 



At a former part of this paper reference was made to the presence 

 of iodine in the potashes of commerce. The samples first tested were 

 those usually to be purchased in Edinburgh, but subsequently genuine 

 and authenticated specimens of both crude and refined potashes were 



