Iodine is widely distributed in the animal, vegetable, and mineral kingdoms. It is found in sponge and cod-liver oil; in all algse and marine plants, mainly as potassium iodide; combined with silver, mercury, and lead, as sodium iodide in many kinds of rock-salt, as sodium iodate in the mother liquor from nitrate of soda works, as calcium iodide in the ocean, and in combination with potassium, sodium, magnesium, and caxcium in many springs. The commercial sources of iodine are (1) seaweeds, (2) caliche, or raw nitrate of soda, (3) Stassfurt salts. Its preparation from these will be described in the order stated,
(1) From Seaweeds
The seaweeds which serve as sources of iodine are met with in the seas washing the coasts of different countries. The shores of Normandy and Brittany, certain coasts of Scotland and Ireland, and many other countries which it is unnecessary to mention here, produce numerous sea-plants containing more or less iodine, bromine, and salts of potash. Our hemisphere does not possess the exclusive privilege of yielding iodiferous plants. Some years ago an examination of about a hundredweight of saline matter from the coast of Patagonia showed that these ashes were as rich in iodine and in potash as good Breton ashes. Among all the kinds of seaweed used in this manufacture, the richest are the two varieties of Fucus digitatus, a. .plant of the deep seas, and which is habitually gathered as drift. Treating plants of this species collected on the north-west coast of Spain, at the southern extremity of the Bay of Biscay, gives very advantageous results. The arrival of the weeds on the coast depends on the season, the height of the tides, and the disturbance of the atmosphere and the sea. In a general manner, it may be said that the chief gathering of the weeds is in winter; that it is best at the time of high water, and especially at the equinoxes.
There are various manners of collecting the weeds. When the tide, after high water, begins to ebb, the weeds which it has brought up are left on the shore. They are then collected and washed a little before the water retires too far. They are made up in heaps of 1/2 to 1 ton, and carried to the works. The process of incineration, named the "trench " process, consists in: - 1st, Spreading out the weeds on the sand, or preferably on grass, to dry them. 2nd, Burning in trenches, made in the soil and lined with masonry. This process, among the inconveniences which it presents, has especially the fault of burning the weeds in an irregular manner, calcining one part whilst the rest is imperfectly burnt, and yielding ashes mixed with much earth and sand. Hence the yield is very irregular, and the iodine very rarely exceeds 3 parts in 1000. Being temporarily entrusted with the management of works where the returns were far from answering to the analytical richness of the weeds, Dr. Thiercelin modified the method of incineration as follows: - Having observed that the species named Fucus digitatus is much the most abundant, and at the same time the richest, it was decided to collect, this kind only.
As he did not make it a prominent point to dry the plant, he resolved to collect it at all seasons, not forgetting that the proportion of iodine varies with the season, so that from May to September not merely much less weed is to be gathered than from September to May, but that collected in summer yields, weight for weight, only the third of that obtained in winter. The weeds being collected, are immediately taken to the furnace-house, weighed, and put in work. The furnace-house is a shed about 60 ft. long by 18 ft. in width. It contains the furnace, a press, pits or holders for the weed, and a space where the ashes produced may be accumulated. The furnace has the form of a long rever-beratory, with the fire in front, and behind it a kind of gallery 15 ft. long and 3 ft. broad, with a vault 1 ft. 9 in. in height. The sole of the furnace has a slope of about 12° to the horizon. Upon this furnace follows a drying-floor to utilize the waste heat; and, lastly, a chimney 30 ft. in height. A hole in the shape of a hopper opens at the extremity of the furnace to receive the weeds, and is closed with a plate of sheet-iron. Three side-doors serve for working the weeds gradually forwards to the hottest part of the furnace, above which is placed a boiler to receive liquids for evaporation.
If the works are still empty, and if weeds arrive in abundance, one portion of the weed is placed in the pits and allowed to ferment; another portion is spread out on the furnace-top and the drying-floor, and the rest is introduced in the green state into the furnace by the hopper. The charge of green weed is about half a ton, and a little water is put into the boiler against the juice of the weed is ready, to prevent the iron plates from burning. The fire is then kindled, and we obtain - (1) A partial drying of the weeds spread on the furnace-top and on the drying-floor annexed. (2) A more complete desiccation in the interior of the furnace. The weeds are then raked downwards, and as the heat becomes more intense, the dry leaves take fire. When this result is obtained, the hottest parts are drawn into an ash-pit which runs the whole length of the furnace. The combustion which began in the furnace is here completed; it goes on slowly, and is continued by reason of the fresh supplies which are continually thrown upon the first. Whilst room is thus made in the furnace, owing to the desiccation and ignition, fresh quantities of dried weed are introduced through the hopper, taken from the drying-floor and the furnace-top, and green weed is spread out to dry in their place.
After the work has once commenced, it never ceases except from lack of weeds. In the meantime the weeds placed in the pits undergo alcoholic fermentation, so that at one time Thiercelin hoped, to collect and utilize the alcohol. However, putrid fermentation sets in after 4 or 5 days, and he never found more than 1 per cent, of alcohol in the liquid. After fermentation for 4, 5, or 6 days, the weeds are put under the press, and the juice is thus separated from the cakes. The juice is evaporated to dryness in the boiler above the furnace. The flakes of dried extract are then placed in the furnace beyond the bridge. The press-cakes are placed successively on the drying-floor and in the furnace, and yield ash like the unpressed weed. Is the fermentation necessary? Thiercelin believes that it is at least very useful, since it is said to prevent the formation of a volatile cyanide of iodine, which would occasion a loss of this substance. In any case he had recourse to it only when the- stock of weed was too great for the furnace to take up without waiting.