Page:Popular Science Monthly Volume 36.djvu/393

Rh fifty pounds to one ton of the wet weed), so complete was the embargo against Spanish barilla and salt that kelp assumed a value as high as one hundred dollars per ton. During this halcyon period Scotland and her western isles reaped an annual income of nearly three million dollars, a sum which raised to affluence many a Scottish lord whose sole possession hitherto had been a long title and a few miles of barren sea-shore. But the end of the war, and the success of Le Blanc's process for making sodium carbonate out of common salt, brought down the value of kelp with a sudden and disastrous drop. In 1831 the price had fallen to ten dollars per ton, a figure no longer remunerative. At this period kelp-making would have died out entirely but for the presence in it of a small quantity of iodine. The discovery of that element in 1812, by Courtois, and the demands for its manufacture, which had arisen between that date and 1840, were the sole cause for the continuance of kelp-making; for kelp was then, and has since remained, the only practicable source of iodine in Europe. During the past fifty years kelp has furnished fully ninety per cent of all the iodine and iodides which commerce has handled. The remaining ten per cent comes from South America, being derived from the well-known caliche of Peru. This substance, in being worked for the large amount of nitrate of soda it contains, also gives a small quantity of iodine as a profitable by-product. The caliche yields from two to three pounds of iodine per ton, against an average yield of twelve or fifteen pounds on the part of kelp. Many bothering questions of manipulation increase the labor and cost of the product from the former source, thus leaving the commercial advantage with kelp. With the origin of this new demand for kelp the industry received a strong impulse, and the price was restored to fairly profitable figures, ranging from fifteen to twenty dollars per ton, at which price it is still in commerce.

It may be proper to note several interesting facts connected with the growth and composition of iodine-bearing weeds. All sea-weeds do not contain iodine, although that substance is universally present in sea-water in the ratio of one to two hundred and. fifty thousand, proportions which, though minute, are amply sufficient to tempt assimilation by all growing sea-plants at least to an appreciable extent. Yet very few of them outside the family of Algæ. contain even a trace of that element. So that to this family has been committed the chief work of withdrawing iodine from sea-water, and of concentrating it in plant-tissues in a form easy of extraction. The power of iodine absorption on the part of the Algæ is the more remarkable when it is remembered that their growth transpires in the presence of three hundred times as much of the very similar element bromine, which latter, however, is absorbed in only one tenth the quantity. The localities where they