firmly to cement the particles of fand to one

another.

And, as the granules of fand are perfectly hard of themselves, so as not to admit of being broke down like the particles of chalk, it neceffarily follows, that the cement made of these materials must be much more perfect, in every respect, than the former.

$17.

That the reader may fee the full force of the above reasoning, it is neceffary he should be informed, that, when calcareous matter is reduced to a cauftic calx, it becomes in every sense of the word a perfect faline fubftance, and is, in this ftate, as entirely foluble in water as common falt or sugar; although with this difference, that lime can be fufpended by water only in a much smaller proportion.-Water can diffolve one third of

its

its weight of common falt, and keep it fuspended in a fluid state; but it can hardly diffolve one thirtieth part of quick-lime, before it is faturated *.

But,

The term faturation is employed to denote that fate of a Auid when it has aiffolved as much of a folid body as it can poffibly fufpend in it at one

time.

When any faline fubftance is put into water, it is diffolved by the water, and tufpended in it till it attains what is called the point of faturation ;-after which, if ever fo much falt be added, not one particle more will be diffolved, but it will remain at the bottom in its original folid ftate.

Water diffolves very different proportions of different falts before it is faturated. It will diffolve

its own weight of Glauber's falt, one third of its weight of common falt, and not one thirtieth of its weight of lime.

Hence it may very readily happen that, altho any particular falt could be wholly diffolved in water, a part of the falt may remain untouched, if too much has been added. Thus, if one ounce of lime is put into ten ounces of water, that water will become

3

fatu

But, although lime be as entirely foluble in water, when in its cauftic ftate, as any other purely faline substance, it fo quickly absorbs its air, as to have fome part of it rendered mild before it can be wholly diffolved on any occafion, in which state water cannot act upon it;-fo that, to obtain a total folution, that proportion of it that becomes mild, requires to be again and again calcined, after fresh folutions have been drawn from it.

As fuch a large proportion of water is neceffary to diffolve any quantity of lime, it feldom happens, even in making lime-water, but that more lime is added than is fufficient to faturate the whole of the water.In which case, some of it still remains at the bottom,

Qq

faturated before it fhall have diffolved one third of the quick-lime, and the remainder will remain in a folid ftate untouched.

bottom, in a condition capable of being diffolved, if more water be added to it.

But lime, it has been already faid, differs from purely faline fubftances in this respect, that it cannot poffibly be long fufpended in water; for it foon abforbs its air even from that element, and is thus reduced to a mild ftate, when it immediately chrystallizes, and feparates from the water *.

In

* Although purely faline substances, in every Atate, continue to be foluble in water, yet many of them become more or lefs fo, in proportion to the quantity of air that is united with them at the time'; and in so far resemble lime in this particular, that they are more easily diffolved when deprived of their air than when united with it.

Alkaline ** falts, ftrictly fo called, like lime, may be either in a cauftic or mild ftate; which appellations

they

** The term alkali is employed to denote a certain clafs of faline bodies, whofe moft certain diftinguishing charisteristic is, that they may be united with acids, and with them form neutral falts, as nitre, common falt, &c.

In confequence of this peculiarity, it neceffarily happens, that, in proportion as

these

they in like manner obtain when they are deprived of their air, or united with it.

When these falts are in a cauftic ftate, they are foluble in water in any proportion. They have even fuch a tendency to unite with it when in this ftate, that it is extremely difficult, if not altogether impoffible, to free them from the water till they are reduced to a mild state.--No art has ever yet been difcovered by which a cauftic volatile alkali could be exhibited in a folid form; and, although dry concretions of the fixed alkali are fometimes ob

certain degree

tained while it is poffeffed of a of caufticity, yet thefe are only effected in confequence of fome part of it becoming mild in the operation,; nor can they be kept in that ftate without the utmost care.

Ordinary pot-afh is an alkaline falt obtained from the afhes of burnt vegetables. This is, in fome meafure, deprived of its air in burning the plant; but, during the procefs, before the watery folution is thoroughly evaporated to drynefs, the alkali has abforb ed fome part of its air, and is in fome measure rendered mild, fo as to admit of being reduced

to