This is not the whole of their service in ocean matters, however. I think it may be said that if it were not for seaweeds animals could not live in the ocean, as truthfully as that if it were not for herbage no animals would be able to exist on land. Seaweeds are fed upon directly by all sorts of salt-water life, from mollusks as big as your thumb to turtles the size of a dining-table, and they make a shelter for thousands of little fellows who never leave their shadow. But this is a small part of the story. The diatoms, and other minute plants like them, form the main portion, if not all, of the food of a large number of sponges, polyps, mollusks, and other stationary, sluggish creatures, that otherwise, so far as I see, would not be able to live at all. These, in turn, are fed upon by larger predaceous animals. Thus, though the fishes and cetaceans may never bite a seaweed themselves (those large marine herbivores, the manatee and dugongs, subsist almost wholly upon it, however), they depend for food upon creatures that do. We may say, therefore, that the algæ form the basis of all ocean life. Men have been able to make marine plants of service to them also—a resource more important formerly than now. In the last century, for example, the kelp trade was the one great industry of the islands at the west of Ireland and Scotland, employing thousands of persons, and paying vast revenues to the lordly owners of the shores. Kelp is the name of any large, leathery sort of seaweed, whose leaves float at or near the surface, supported by bladder-like expansions; but in this case the word meant the ashes of any seaweed dried in the sun and then slowly burned in kilns, clouding the air with huge volumes of strongly odorous smoke. The slow burning of the seaweed left the ashes fused into a solid mass, which was broken up like stone before being sold. In France this substance was called _varec_; and in Spain, where the algæ were mixed with beach-plants, cultivated for the purpose, and burned in shallow pits in the ground, it went to market as _barilla_. In those days, kelp ash was the only source of the valuable alkali soda needed in manufacturing glass and soap. Then a French chemist discovered how to make such soda out of common salt, and the kelp ovens were abandoned, except a few in Scotland, supplying the demand for iodine and several other chemicals contained in this residuum which is so rich in iodine, used in photography and in medicine, that a ton of kelp ash will sometimes yield twenty pounds; yet only about 100,000 pounds are now produced in this way, while five times as much is obtained by chemical treatment of Chile saltpeter. It is a curious fact that barbarous people have long chewed seaweeds as a remedy in diseases for which physicians now prescribe iodine. Iodine is a violet dye, and the bluish and purple tints of many algæ, shells, and sea-animals appear to be due to the large amount of this element in sea-water.