eans of heat, is one of the capital operations in chemistry; and where very great quantities of water are to be dissipated, as in the making of com- common salt and green vitriol, it is a matter of no small consequence to contrive the evaporating vessels in such a manner as to distillate a large quantity of liquid in a short time, and with little fuel. In the warmer countries, salt is made by the heat of the sun alone; and even in the southern parts of England, the heat of the solar rays is advantageously used to concentrate the brine. This, however, can be but very seldom practised, and artificial evaporation is almost the only kind made use of in this country. The two great requisites for promoting this, are, a sufficient degree of heat to form a large quantity of steam, and a current of air to carry it off; for, though the vapour is always formed by the heat, yet if the air stagnates on the surface of the fluid, that part next the surface is heated to such a degree, and so loaded with smoke, that the succeeding quantities of vapour are very considerably retarded in their ascent. For this reason, evaporation is greatly promoted by having as large a surface of fluid exposed to the air as possible, that the ascent may be very quick; and where it is possible to procure a considerable blast of air along the surface of the vessel containing the fluid, it will very much accelerate the dissipation of it. On the same account, blowing air through the water while evaporating, increases the quantity of vapour to a surprising degree. It is obvious, however, that, by either of these methods, only time can be saved: for as the blast of cold air, whether directed along the surface of the fluid, or into its substance, will diminish the heat considerably; though the evaporation goes on very fast, yet a proportionable quantity of fuel will be required; and it is doubtful whether this method might in the end be attended with all the advantages promised by it at first view.

Broad shallow vessels have likewise been found greatly to contribute to the quickness of evaporation, and thought to lessen the expense of fuel. That they do accelerate the ascent of the vapour is certain, because the air has more free access to the fluid than in a deeper vessel; but it is by no means clear that the quantity of fuel is thus lessened: on the contrary, there is great reason to suspect, that a considerably larger proportion of fuel must be employed to evaporate an equal quantity of fluid in a wide shallow vessel, than in a deeper one. They have moreover this great inconvenience, that, if their size be very large, it is in a manner impossible to make the fire act equally on the bottom of them; and the great expense attending the making of these kind of vessels, together with the danger of the fluid being mixed with accidental impurities from their wide-extended surface, forbids their use, except on particular occasions.

One great objection, however, to quick evaporation in any kind of vessel is, that this process is carried on for the sake of the residuum; and if the liquor is made to boil with great violence, part of this residuum will be carried away and dissipated in the air: what remains also will often be considerably different from what it would have been had the process been conducted in a slow and gentle manner. Vitriolated tartar is remarkably affected by quick evaporation. Neuman relates, that by strong continued boiling, it may be almost entirely dissipated along with the steam; and yet this salt, the most ponderous of all others, is composed of a very fixed acid, viz. the vitriolic, and a fixed alkali.

To obviate this inconvenience, chemical evaporations are generally ordered to be performed with a very gentle heat; by which means the dissipation of the saline matter is in a great measure, though not totally, prevented; for this seems somehow to depend on the action of the air. The great length of time, however, which this takes up, and the difficulty of preserving the liquid from accidental impurities, render evaporations in this manner sufficiently disagreeable.

A very strange difficulty occurs in attempting evaporations in this manner. If a saline solution is put into any kind of vessel wider at top than at bottom, and set to evaporate with a heat below what is sufficient to make it boil, the salt seems to disengage itself from the water, and ascends along the sides of the evaporating vessel in the form of a ring, which grows gradually higher and higher, till it comes to the top: it will then descend on the other side of the vessel, till it has got low enough to form a kind of syphon, from whence a great part of the solution will be infallibly drained off and lost: neither is there a possibility of preventing this saline concretion; for though you put it down ever so often, it very soon forms again. To remedy this inconvenience, Dr Black recommends the use of evaporating vessels wider at bottom than at the top; where some quantity of vapour would always be condensing and running down their sides, so as to dissolve the saline ring as soon as it began to form. It is evident, however, that by this method the evaporation would be rendered still slower than before.

On many accounts, distillation seems to be the most advantageous method of evaporating fluids, particularly as it both prevents any of the salts they may contain from being dissipated, and seems to be the method by which the largest quantities of liquid can be evaporated in the shortest time, and with least fuel. It is well known, that water in a still, or any covered vessel, can be kept boiling with much less heat than an equal quantity in an uncovered vessel. As the quantity of steam is the same that arises from an equal surface of water where the heat is the same, whether the vessel is covered or uncovered; it follows, that if proper vent is given to the steam in a covered vessel, an equal quantity will be raised in a close vessel with less fuel than in an open one; and, with an equal quantity of fuel, much more might be evaporated, in the same time, in a close than in an open vessel. This seems confirmed by what Dr Hales has advanced in his experiments concerning the freshening of sea-water. He has found, that, by blowing air through the water contained in a still, it is made to yield double the quantity which it otherwise would do. By this method, he observes, a still which holds 20 gallons, will yield, in 20 hours, 240 gallons of distilled water; and one which holds only five gallons, may be made to distil 64, in the same time. These are prodigious quantities; and which, we believe, could not be made to arise in an equal time from open vessels of the same size, by any manoeuvre whatever. Even deducting one half for the action of the bellows, the quantity is exceedingly great, being upwards of a gallon and an half per hour from a vessel holding only five gallons; and how difficult it would be to make this quantity arise from an open vessel of such a size, those who are much concerned in evaporation will easily know. See Distillation.