Now, salt contains about 60 per cent. of chlorine gas, and therefore the bleachers can never stand still for want of it; but iodine is not so plentiful, and was discovered by M. Courtois, of Paris, in _kelp_, a substance from which he prepared carbonate of soda, or washing soda; but as this is now more cheaply prepared from common salt, the kelp is at present required only for the iodine salts it contains, as also for the chloride of potassium. Kelp is obtained by burning dried sea-weeds in a [Page 137] shallow pit; the ashes accumulate and melt together, and this fused mass broken into lumps forms kelp. The ocean bed no doubt has its fertile and barren plains and mountains, and amongst the so-called "oceanic meadows" are to be mentioned the two immense groups and bands of sea-weed called the Sargasso Sea, which occupy altogether a space exceeding six or seven times the area of Germany. The iodine is contained in the largest proportion in the deep sea plants, such as the long elastic stems of the fucus palmatus, &c. The kelp is lixiviated with water, and after separating all the crystallizable salts, there remains behind a dense oily-looking fluid, called "iodine ley," to which sulphuric acid is added, and after standing a day or two the acid "ley" is placed in a large leaden retort, and heated gently with black oxide of manganese. The chlorine being produced very slowly, liberates the iodine, as already demonstrated in experiment seven, p. 133, and it is collected in glass receivers. Iodine, when quite pure and well crystallized, has a most beautiful metallic lustre, and presents a bluish-black colour, affording an odour which reminds one at once of the "sea smell." _First Experiment._ A few grains of iodine placed in a flask may be sublimed at a very gentle heat, and afford a magnificent violet vapour, which can be poured out of the flask into a warm bottle. If the bottle is cold the iodine condenses in minute and brilliant crystals. (Fig. 136.) [Illustration: Fig. 136. A. Flask containing iodine heated by spirit lamp. B. Cold flask above to receive the vapour. C C. Sheet of cardboard to cut off the heat from the spirit lamp.] _Second Experiment._ Upon a thin slice of phosphorus place a few small particles of iodine; the heat produced by the combination of the two elements soon causes the phosphorus to take fire. _Third Experiment._