ORDER II.—SEA WATER.
279. SEA WATER is a very heterogeneous compound, not only containing a considerable portion of saline substances, but holding also suspended in it an infinite number of minute animal and vegetable particles, to the gradual putrefaction of which its peculiarly nauseous and bitter taste, at the surface, is in some measure to be attributed.
The average quantity of salt in sea water is estimated to amount to about one-thirtieth part of its weight. It likewise contains a certain portion of muriat of magnesia, sulphat of magnesia or Epsom salts ([199]), and a small quantity of sulphat of lime ([192]). Sea water, taken from a great depth, has not the bitterness which the water of the surface has: it is only saline.
No natural waters, if we except certain brine springs and salt lakes, are so saline as those of the ocean; and the latter differ, in this respect, in different parts of the world. At the tropic, the sea is in general more salt than it is at the poles, a wise ordination to preserve it, in those climates, from the great tendency to putrefaction: and, at a considerable depth, it is always found more salt than at the surface. The water of the Baltic is much less salt than that of the Atlantic; and it is a remarkable circumstance, that its saline contents are increased by a west wind, but still more so by a gale from the north west.
Some philosophers have endeavoured, but to little purpose, to account, from second causes, for the saltiness of the ocean. Dr. Halley persuaded himself that it might have been gradually acquired, in very minute portions, by a deposit of salt washed down from the land by rivers, and that, as it could not be carried off by evaporation, instead of being diminished, it must be constantly increasing. But this idea of salting the sea with fresh water, is, to say the least of it, somewhat absurd, more particularly as it presumes that the sea was originally unimpregnated with salt. Had this been the case, the putrefaction of the immense mass of animal and vegetable substances which it gradually contained, would, in a short time, have proved fatal to the whole inhabitants of the earth.
The temperature of the sea, although it must necessarily vary in the different seasons, is much more uniform than that of any inland water exposed to the atmosphere. This is, in a great measure, attributable to its vast body of water, and the perpetual agitation to which it is exposed.
Sea water, when congealed by frost, is found to reject all, or nearly all, its saline particles; and consequently, when thawed, its ice yields water so fresh that it may be drunk without unpleasantness. The freezing of sea water is not unfrequently practised in the northern parts of the world, with a view to lessen the trouble and expense of extracting salt from it, for domestic and other uses ([202]). Salt water may likewise be rendered fresh and palatable by distillation, a mode which is now very generally practised at sea.
The sea shore has of late become so much frequented by invalids, for the purpose of bathing, that there is scarcely a fishing village, on the whole extent of our coast, but which is provided with some accommodation for bathers. As a cold bath, sea water is employed, with advantage, in all those cases of debility for which cold bathing has, in general, been recommended. It is also used as an external application in tumours and some other complaints; and, taken internally, as a remedy in various disorders.
It is to sea water that we are chiefly indebted for the salt which we use at table, and for all the purposes of domestic economy ([202]). From this water is also obtained those salts used in medicine, called Glauber’s ([203]) and Epsom salts ([199]).