less hard, according to the nature of the clay,

and the proportion of fand that may have been mixed with it. And,

8th, When Shells, by the lapfe of time, and by long macerating in water, have loft the animal gluten that cemented them, and are crumbled down to a fine whitish powder, they are denominated Shell-marle.

It would be a curious difquifition to enquire how these maffes of calcareous matter were originally formed :-How they were reduced to a ftate that rendered them foluble in water, which must have been the cafe before they could admit of being chrystallized-What were the circumftances that contributed to render fome of these chrystals so much more perfect than others, &c. &c.-But thefe difquifitions, however curious they might be, are here omitted, as not abfolutely neceffary for the elucidation of our fubject. The explana

tion

tion of the nature of the different calcareous matters above given was neceffary; as, without a knowledge of these, it would have been impoffible to have explained, in a fatisfactory manner, the way in which these substances are more or less fitted to be employ ed as a cement, or a manure.

To avoid unneceffary repetitions, the reader is defired to observe, that, for the future, I fhall mention all matters that can be converted into quick-lime under the name of lime-ftone, whether they be in the form of marble, chalk, or common lime-ftone; diftinguishing either of thefe when it may become neceffary.

$ 6.

Lime-ftone, in the ftate we find it is always a compound fubflance.-in its purest ftate, it consists of a calcareous earth, united

with

with a confiderable proportion of water. For, faline matters, when chryftallized, always contain water.

Limeftone likewife contains another fubftance, the nature of which will be afterwards explained.

When lime-ftone has been exposed for a fufficient length of time to the action of a fufficiently intenfe fire, the whole of the water it contained is evaporated: So that lime-fhells are always lighter than the ftone of which they are made, by the whole weight, at least,, of the water the chrystals contained,

And, as perfect chryftals always contain a much larger proportion of water than those that are lefs perfect, it follows that, of two kinds of limestone of equal purity, that which is hardest and most transparent, will lose a greater proportion of its weight in calcination, than that which is fofter and more o

paque.

paque.-Hence marble lofes more weight by burning than chalk *.

*Such readers as are totally unacquainted with the circumstances that are neceffary to the formation of chryftals in general, will probably be at a lofs eafily to comprehend the chain of argumentation followed here and in fome other parts of this Effay. The following explanation will make it more intelligible.

As faline matters, properly so called, are more eafily chryftallized by art than any others, it will be beft to take our illuftrations from that class of bodies.

It is a property of faline bodies, that they may all be diffolved in water,

They may also be feparated from that water, and obtained in a dry form; but they affume very different appearances, according to the nature of the process that is followed for feparating them from that water,

If a watery folution of any falt be fuddenly evaporated by means of fire, there remains behind a white fubftance, sometimes flightly coherent, as in pearlafhes, &c. and fometimes it falls into a powdery

çalx,

Again,-As fand lofes nothing of its weight by calcination, it likewife follows, that,

calx, as in evaporating a folution of Glauber's falts, alum, &c.

But, if the water be gently evaporated by a moderate heat-before it becomes a dry powder, the falt quits the water, and fhoots out into regular figures of a transparent glaffy like appearance, which have obtained the name of chryftals.

These chryftals vary in figure, hardness, &c. according to the nature of the falts of which they are compofed.

But they all agree in one refpect, that they contain a confiderable proportion of water united with the faline matter.-Some kinds of falt abforb a very large proportion of water in this way.-Chryftallized Glauber's falts contain two thirds of their weight of water.-Common falt does not contain near fuch a large proportion.

Hence it happens that no chryftals of any kind of falt can poffibly be formed, unless these falts have been perfectly diffolved in water.

And the perfection and transparency of these chryftals depends entirely on their being allowed to fhoot