Filter the salt-water from which you intend to extract the salt: evaporate it by boiling till you see on its surface a dark pellicle: this consists wholly of little crystals of salt just beginning to shoot: now slacken the fire, that the brine may evaporate more slowly, and without any agitation. The crystals, which at first were very small, will become larger, and form hollow truncated pyramids, the apices whereof will point downwards, and their bases be even with the surface of the liquor.
These pyramidal crystals are only collections of small cubical crystals concreted into this form. When they have acquired a certain magnitude they fall to the bottom of the liquor. When they come to be in such heaps as almost to reach the surface of the liquor, decant it from them, and continue the evaporation till no more crystals of Sea-salt will shoot.
OBSERVATIONS.
The Acid of Sea-salt is scarce ever found, either in sea-water or in the earth, otherwise than united with a fixed alkali of a particular kind, which is its natural basis; and consequently it is in the form of a Neutral Salt. This salt is plentifully dissolved in the waters of the ocean, and when obtained therefrom bears the name of Sea-salt. It is also found in the earth in vast crystalline masses, and is then called Sal-gem: so that Sea-salt and Sal-gem are but one and the same sort of salt, differing very little from each other, except as to the places where they are found.
In the earth are also found springs and fountains, whose waters are strong brines, a great deal of Sea-salt being dissolved in them. These springs either rise directly from the sea, or run through some mines of Sal-gem, of which they take up a quantity in their passage.
As the same, or at least nearly the same, quantity of Sea-salt will continue dissolved in cold water as boiling water will take up, it cannot shoot, as Nitre does, by the mere cooling of the water in which it is dissolved: it crystallizes only by the means of evaporation, which continually lessens the proportion of the water to the salt; so that it is always capable of containing just so much the less Sea-salt the more there is crystallized.
The brine should not boil after you perceive the pellicle of little crystals beginning to form on its surface; for the calmness of the liquor allows them to form more regularly, and become larger. Nor after this should the evaporation be hurried on too fast; for a saline crust would form on the liquor, which, by preventing the vapours from being carried off, would obstruct the crystallization.
If the evaporation be continued after the liquor ceases to yield any crystals of Sea-salt, other crystals will be obtained of an oblong four-sided form, which have a bitter taste, and are almost always moist. This sort of salt is known by the name of Epsom Salt, which it owes to a salt spring in England, from the water of which it was first extracted. This salt, or rather saline compound, is a congeries of Glauber's salt and Sea-salt, in a manner confounded together, and mixed with some of the Mother of Sea-salt, in which is contained a kind of bituminous matter. These two Neutral Salts, which constitute the Epsom Salt, may be easily separated from each other, by means of crystallization only. Epsom Salt is purgative and bitter; and therefore named Sal Catharticum Amarum, or bitter purging Salts.
There are different methods used in great works for obtaining Sea-salt out of water in which it is dissolved. The simplest and easiest is that practised in France, and in all those countries which are not colder. On the sea-shore they lay out a sort of broad shallow pits, pans, or rather ponds, which the sea fills with the tide of flood. When the ponds are thus filled, they stop their communication with the sea, and leave the water to evaporate by the heat of the sun; by which means all the salt contained in it necessarily crystallizes. These pits are called Salt Ponds. Salt can be made in this way in the summer-time only; at least in France, and other countries of the same temperature: for during the winter, when the sun has less power and rains are frequent, this method is not practicable.