And the practice of the fouthern provinces of Britain fufficiently confirms the juftness of thefe obfervations. For, to the fouth of the Humber, on the east coaft, almost all the lime they use is made of chalk; yet there, are many buildings in thefe counties, in which the cement is as firm as in any part

of

every fenfe of the word; having the fame degree of firmness, of foftnefs, and every other quality of chalk.

This is the most perfect resemblance of chalk that can be made; but, if any quantity of that pure lime be reduced to a very thin pafte at the time of slaking, and be haftily dried to a certain degree,-it acquires a fort of confiftency, fo as to be capable of retaining its form. And, if this be kept in a place not exposed to the viciffitudes of the weather till it has attained its whole air, it will refemble chalk almost as much as the former, and might be employed for every pur❤ pofe that the chalk is used for.

The reader will pleafe to obferve, that this can only be done with lime that is perfectly free of fand 1 for, if it has the leaft particle of fand among it, nọ art can give it the foftnefs of chalk.

of the island :-Nor does the ordinary mode of building in these places indicate any deficiency in the quality of their mortar ;—för their houses are almoft all coated on the outfide with a cruft of lime, ftuck full of fmall pebbles, which remain in it very firmly for many years. We know well that this is the most trying manner of employing

mortar.

There is, however, greater danger that lime made of chalk fhould form, on fome occafions, a weak cement, than that from lime-ftone.

For, as chalk never contains any fand, its lime will always form a very foft cement, unless care be taken to mix a large proportion of fand with it in beating up the mortar; which is not fo indifpenfibly neceffary in forming mortar from lime-ftone ;—as it fometimes contains fo much fand as to form

a pretty

a pretty firm cement, without any additional fand at all.

Even if the lime-stone should be equally pure calcareous matter as the chalk, the lime of the first has a chance of becoming a firmer cement than that of the last.

For, as it is impoffible to reduce the pure lime-stone to a powdery calx, without fubjecting it to the action of a very strong fire, which, by diffipating the water, and fully drying the chryftals, carries off the whole of its fixed air; fo that the calx is almost entirely caustic.

the

But chalk may be reduced to calx by fuch a moderate heat as is fcarce fufficient to diffipate any of its air; fo that what affumes of lime made from it may be appearance nothing else than a powdered effete calcareous earth, which never can become a cement of any fort. But, as there is no danger of vitrifying chalk by over-burning, this inconve

nience

1

nience may be entirely obviated, by a careful and perfect calcination.

In those countries, therefore, where chalklime is common, care ought to be taken to choose only that kind of it for mortar, that has been calcined by a very strong fire, and to reject that which has been burnt by furze or brakes, as unfit for that purpose.

But, it is obvious, that, as this defect arises entirely from the unskilfulness of the operator, which may be cafily avoided, it ought not to be confidered as any objection to the quality of the lime confidered in itfelf.

§ 30.

It is unnecessary to extend our obfervations to all the other kinds of lime-ftone that may be met with; as these general observations on the two extremes, marble and chalk, may be easily applied to all the intermediate

kinds.

kinds. It has been already said, that the different friability of different forts of limeftone arifes entirely from a fmaller or greater degree of perfection in the chryftallization, that must have been occafioned by accidental circumstances that have occurred at the time the concretion was effected, and can have no influence on the quality of the lime when it is once more reduced to the state of a cauftic calx.

And, as it does not yet appear that there is the smallest difference between the chemical qualities of any one kind of foffile calcareous earth and another, when perfectly pure, there is no reason to fufpect that there can be any difference between one kind of lime and another, as a cement, unless what may arife from the nature of the extraneous bodies that may be accidentally mixed with that calcareous matter in its native ftate,-or from its being more or lefs perfectly calcined.

But the only extraneous matter that is ever found in lime-ftone is fand, in greater