so that the specific gravity of a mass of frozen water is less than that of water at any temperature below 40°. It must not be forgotten that ice always contains a large quantity of air, by which it is rendered buoyant. Water, at rest, may be cooled many degrees below the freezing point without becoming solid. This is easily effected in a thin glass flask; but the moment it is agitated, it becomes a firm mass. Here we have the indication of another cause aiding in producing crystals of ice on the surface of water, under the influence of the disturbance produced by the wind, which does not extend to any depth. As oxygen and hydrogen gases enter largely into other chemical compounds besides water, it is important to consider some of the forms of matter into the composition of which these elements enter. To examine this thoroughly, a complete essay on chemical philosophy would be necessary; we must, therefore, be content with referring to a few of the more remarkable instances. The waters of the ocean are salt: this arises from their holding, in solution chloride of sodium (_muriate of soda_--_common culinary salt_) and other saline bodies. Water being present, this becomes muriate of soda,--that is, a compound of muriatic acid and soda: muriatic acid is hydrogen, combined with a most remarkable gaseous body, called, from its yellow colour, _chlorine_; and soda, oxygen in union with the metal sodium,--therefore, when anhydrous, culinary salt is truly a chloride of sodium. Chlorine in some respects resembles oxygen; it attacks metallic bodies with great energy; and, in many cases, produces the most vivid incandescence, during the process of combination. It is a powerful bleaching agent, is destructive to animal life, and rapidly changes all organic tissues. There are two other bodies in many respects so similar to chlorine, although one is at the ordinary temperatures solid, and the other fluid, and which are also discovered in sea-water, or in the plants growing in it, that it is difficult to consider them otherwise than as different forms of the same principle. These are iodine and bromine, and they both unite with hydrogen to form acids. The part which chlorine performs in nature is a great and important one. Combined in muriate of soda, we may trace it in large quantities through the three kingdoms of nature, and the universal employment of salt as a condiment indicates the importance to the animal economy of the elements composing it. Iodine has been traced through the greater number of marine plants, existing, apparently as an essential element of their constitution; in some land plants it has also been found, particularly in the Armeria maritima, when this plant grows near the sea:[216] it has been detected in some mineral springs, and in small quantities in the mineral kingdom[217] combined as iodide of silver, and in the aluminous slate of Latorp in Sweden.[218] Bromine is found in sea-water, although in extremely minute quantities, in a