The Salt-petre workers in France call these saline particles the Grain, and take care to separate them from the liquor (which, as long as it continues hot, keeps the salt-petre dissolved) before they set it to crystallize. This fact seems a little singular, considering that sea-salt dissolves in water more easily than salt-petre, and crystallizes with more difficulty.
In order to discover the cause of this phenomenon, we must recollect some truths delivered in our theoretical Elements. The first is, that water can keep but a determinate quantity of any salt in solution, and that if water fully saturated with a salt be evaporated, a quantity of salt will crystallize in proportion to the quantity of water evaporated. The second is, that those salts which are the most soluble in water, particularly those which run in the air, will dissolve in cold and in boiling water equally; whereas much greater quantities of the other salts will dissolve in hot and boiling water than in cold water. These things being admitted, when we know that sea-salt is one of the first sort, and salt-petre of the second, the reason why sea-salt precipitates in the preparation of salt-petre appears at once. For,
When the solution of Salt-petre and Sea-salt comes to be evaporated to such a degree that it contains as much Sea-salt as it possibly can, this salt must begin to crystallize, and continue to do so gradually as the evaporation advances. But because at the same time it does not contain as much salt-petre as it can hold, seeing it is capable of dissolving a much greater quantity thereof when it is boiling hot than when it is cold, this last-named salt will not crystallize so soon. If the evaporation were continued till the case of the Salt-petre came to be the same with that of the Sea-salt, then the salt-petre also would begin to crystallize gradually in proportion to the water evaporated, and the two salts will continue crystallizing promiscuously together: but it is never carried so far; nor is it ever necessary: for, as the water cools, it becomes more and more incapable of holding in solution the same quantity of salt-petre as when it was boiling hot.
And then comes the very reverse, with regard to the crystallizing of the two salts; for then the Salt-petre shoots, and not the Sea-salt. The reason of this fact also is founded on what has just been said. The Sea-salt, of which cold water will dissolve as much as boiling water, and which owed its crystallizing before only to the evaporation, now ceases to crystallize as soon as the evaporation ceases; while the Salt-petre, which the water kept dissolved only because it was boiling hot, is forced to crystallize merely by the cooling of the water.
When the solution of Salt-petre has yielded as many crystals of that Salt as it can yield by cooling, it is again evaporated, and being then suffered to cool yields more crystals. And thus they continue evaporating and crystallizing, till the liquor will afford no more crystals. It is plain that as the Salt-petre crystallizes, the proportion of Sea-salt to the dissolving liquor increases; and as a certain quantity of water evaporates also during the time employed in crystallizing the Salt-petre, a quantity of Sea-salt, proportioned to the water so evaporating, must crystallize in that time: and this is the reason why Salt-petre is adulterated with a mixture of Sea-salt. It likewise follows that the last crystals of Nitre, obtained from a solution of Salt-petre and Sea-salt, contain much more Sea-salt than the first.
From all that has been said concerning the crystallization of Salt-petre and Sea-salt, it is easy to deduce the proper way of purifying the former of these two Salts from a mixture of the latter. For this purpose the Salt-petre to be refined need only be dissolved in fair water. The proportion between the two salts in this second solution is very different from what it was in the former; for it contains no more Sea-salt than what had crystallized along with the Salt-petre under favour of the evaporation, the rest having been left dissolved in the liquor that refused to yield any more nitrous crystals.
As there is therefore a much greater quantity of Salt-petre than of Sea-salt in this second solution, it is easy to evaporate it to such a degree that a great deal of Salt-petre shall crystallize, while much more of the water must necessarily be evaporated before any of the Sea-salt will crystallize.
However, the Salt-petre is not yet entirely freed from all mixture of Sea-salt by this first purification; for the crystals obtained from this liquor, in which Sea-salt is dissolved, are still encrusted, and, as it were, infected therewith: hence it comes, that, to refine the Salt-petre thoroughly, these crystallizations must be repeated four or five times.
The Salt-petre men commonly content themselves with crystallizing it thrice, and call the produce Salt-petre of the first, second, or third shoot, according to the number of crystallizations it has undergone. But their best refined Salt-petre, even that of the third shooting, is not yet sufficiently pure for Chymical experiments that require much accuracy: so that it must be further purified; but still by the same method.