310 Mr Stevenson Macadam on the 



the atmosphere, jn the rain water, and in the soil, is comparatively 

 great, and to this he ascribes the absence of goitre and cretinism ; 

 whereas in the zone corresponding to that of the Alpine valleys, the 

 amount of iodine lias diminished to one-tenth of that found in the 

 Paiis zone, and to this scarcity of the element, he attributes the pre- 

 valence of goitre and cretinism, which in that zone are endemic* 

 Considering that the subject was one of great importance, more espe- 

 cially if the conclusions arrived at by Chatin — in reference to the 

 functions fulfilled by iodine, in preventing the occurrence of the dis- 

 eases referred to — could be legitimately deduced from the experiments 

 which he performed, I have recently undertaken a series of analyses 

 in reference to the general distribution of the element in question. 



My attention was principally directed to the atmosphere, and to 

 rain water, both of which, apart from the observations of Chatin, 

 I had reason to believe would contain iodine. It is well known 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 from this source iodine — principally as iodide of sodium- — 

 will most probably reach the air. This constant supply will be fur- 

 ther augmented by the iodine vapour which is disengaged from many 

 mineral springs, and which, amongst other possible compounds, will 

 more especially exist in the atmosphere as iodide of ammonium. 

 Independently, therefore, of any experiment, I thought it in the 

 highest degree probable, that iodine would be found present in the 

 conditions referred to, viz., as iodide of sodium and iodide of ammo- 

 nium, and it only remained to determine whether or not the quantity 

 of iodine was so great as to come within the range of our most delicate 

 tests. 



I commenced with the atmosphere. The process ^followed was 

 identical in principle with that pursued by Chatin. From statements 

 made in different parts of one of his memoirs, it would appear that 

 the apparatus he employed was a series of Liebig 1 s bulbs containing 

 a solution of carbonate of potassa, and attached to an aspirator by 

 means of which air was drawn through the liquid.-}- In the arrange- 

 ment I employed, the air was made to traverse, — 



1. A wide tube, containing slips of paper moistened with solution 

 of starch ; and, 



2. A double-necked gas bottle, containing three ounces of a dilute 

 solution of caustic alkali. 



At the commencement of the experiment, caustic soda was placed 



in the bottle (2) and not less than 150 cubic feet of air drawn through. 



The soda was then replaced by caustic potassa, and a similar volume 







* (Jomptes ltendus, tome xxxiv., p. 51 ; and Edin. New Phil. Journal, No. 104. 

 + romptes Rendus, tome xxxii., p. 669. 



