Those that have the greatest attraction for water require least for their solution: some of them possess so great an attraction for water, as to absorb it from the air. These are called deliquescent.

The first phænomenon in the mixture of salts with water is the separation of air from the water by a seemingly elective attraction. This appears by a muddiness which proceeds through all the parts of the fluid, and is occasioned by a number of little bubbles, which rise to the top so as to form a scum; when all these are risen, the water becomes transparent. This deserves to be taken notice of, as many have been deceived by it, especially those who have written on mineral waters. They often mention effervescence in them, where there was none, and the appearance of it was nothing more than the air escaping.

Another phænomenon is the alteration in the heat of the mixture. In some cases it becomes colder, in others hotter. When heat has been produced, it has been thought owing to the violent attraction between the salt and water; for the production of cold there has been no theory offered but that of latent heat, as stated in a preceding lecture.

Another phænomenon is, that if we add more and more salt to the water, it will be more and more slowly dissolved, and after a certain quan

When the water

tity it will dissolve no more. will dissolve no more, the point at which the salts cease to dissolve is called, as was before mentioned, the point of saturation. The proportion of water is very different with respect to different salts: some require only a quantity of water equal to themselves in weight. A saturated cold solution dissolves more salt if a little heat is thrown into it. Common salt is, however, one exception to this general rule; it dissolves with equal ease in the same quantity of cold water as in warm, Mr. Macquer thought the deliquescent salts were also exceptions; but this remark is by no means just. We are not agreed as to what salts are to be called deliquescent, and what not: their deliquescence may depend sometimes on the particular dampness of the air, and sometimes on their own particular condition.

From the phænomena attending the dissolution of salt, Sir Isaac Newton thought that there was an equal distribution of it through a determined space of water, and hence the deposition is in a regular order; hence also, if a space of water is increased by expansion, it is capable of arranging a greater number of particles of salt than it otherwise would do. That something of this kind really takes place is evident from experiment, as that part of the water which contains the least salt will have the greatest attraction for it; and thus the distribution of the salt will pro

ceed till the particles are arranged at equal distances through the whole fluid. There is, however, a seeming objection to this, viz. that when the salt is merely thrown into the water, the particles do not rise and disperse themselves immediately, but it is a long time before the salt is dispersed through the whole. This, though a seeming, is not a real objection. It is true, it is a long time before the salt dissolves, but this is in consequence of the very cause on which distribution depends; and in time the salt will be equally distributed through the whole. Throw a heavy salt into a glass of water (sulphat of copper, or blue vitriol for instance), it at first sinks to the bottom, and after some days begins to impart some of its colour and qualities to the particles of water immediately surrounding it. As the water in contact only acts upon the salt, it is soon saturated, and, being thus made heavier, remains round the salt in the state of an atmosphere. The whole of the attraction of the water, then, can only act on this surrounding atmosphere, as chemical attraction reaches to so small a distance. In a little time another stratum will be formed, containing less salt than the former. Innumerable horizontal strata will at length be formed containing less salt: hence the slow diffusion if not assisted with agitation. The sulphuric acid is used in bleaching, by dilution with the water in which the linen is steeped. The bleachers at first thought it was sufficient

merely to throw the acid into the water: by this, however, some of the linen was always corroded; for the sulphuric acid sinks to the bottom, and there remains a long time before it is distributed. When mixed thoroughly by agitation, the salt will never separate again; for the solution of salts is as perfect as any in chemistry.

Salts are separated by evaporation in two ways: 1st, By a brisk heat till all the water is dissipated. This is called evaporation to dryness, and the salt appears a shapeless, pretty firm mass, but the particles are very small. If this operation is often repeated, it is observed that some of the salt is always lost. 2d, By a gentle heat, and partial evaporation. The salts then shoot into regular masses with polished surfaces, more or less transparent, called crystals, from their resemblance to that stone. Salts deposited in this manner have a tendency to a particular form, though the different manner of conducting this process produces some variety in the figure of the crystals. The crystals are larger and more regular if the liquor is not evaporated too much, and if it is cooled slowly in a vault or cellar. There is a degree of transparency in the concretion; and if the process is repeated ever so often, the salt will have the same form. Each particular species of salt has a distinct form of crystals peculiar to itself. Nitrat of potass (common nitre) appears in the form of oblong pillars with six sides: muriat of soda (common

salt), in a cubical form: nitrat of soda (cubic nitre), in a rhomboidal: sulphat of soda (Glauber's salt), in large four sided prisms: so that, from the appearance, a person accustomed to see crystals can distinguish to what salt they belong. The common rule of crystallizing salts is to evaporate the fluid very slowly till a pellicle appears at top, and then set it by to crystallize. If we proceed till the salt begins to congeal, when warm the crystallization will be incomplete and irregular. It will be proper to filtrate the liquor, to free it from impurities, before we set it aside to crystallize. If the whole of the salt is not thrown down in the first operation, it must be repeated: The crystals are always better when large, than when small quantities are made. A circumstance, however, often attends this which is very troublesome, viz. what is called the vegetation of salts. If we set a glass vessel containing a saline solution in a cool, still place, to crystallize by slow evaporation, as soon as it begins to shoot it will be attracted by the sides of the vessels, and be protruded upwards in the form of a hollow cylinder. As soon as it reaches the top, it will bend down and creep along the outside of the glass, till it becomes a sort of siphon, by which the solution, as the salt is spongy, is drawn over the vessel; so that, unless we carefully watch it, we shall lose all except what was crystallized.

This phænomenon depends on capillary at