668 



nature, and to allow a volume of air of not less than 100,000 cubic feet 

 to pass through. 



While 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 three 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 present 

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

 was evaporated to one ounce, and afterwards tested for iodine, but 

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

 lume of rain-water, viz., three gallons, substituting nitrate of silver for 

 the acetate of lead. A precipitate was obsen^ed after standing for 

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

 iodine. Another experiment, made with three 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. Accordingly, 

 at the first he was confident that he should succeed in verifying Cha- 

 tin's observations in a district so near the sea as that around Edin- 

 burgh is, and more especially in the water obtained from Unst, which 

 had fallen in the immediate vicinity of the ocean ; but when we con- 

 sider 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 indica- 

 tion, equal to that exhibited by 1.500,000th 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- 

 ]jorate 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 ge- 

 nuine and authenticated specimens of both crude and refined potashes 

 were procured from Glasgow. It is to Canada and the United States 



