giving too little fand, than in giving too

much.

It deferves, however, to be remarked, that the fand, when naturally in the lime-ftone, is more intimately blended with the lime than can poffibly be ever effected by any mechanical operation ;-fo that it would be in vain to hope to make good mortar artificially from pure lime, with fuch a small proportion of cauftic calcareous matter, as may fometimes be effected when the lime naturally contains a very large proportion of fand. But there feems to be no doubt that, if a much larger proportion of fand were employed, and if that were more carefully blended, and expeditiously worked, than is common, the mortar would be much more perfect than ufual in modern times. This I have tried experimentally, with the defired success.

$ 27.

Another circumftance that tends greatly to vary the quality of the cement, and to make a greater or fmaller proportion of fand neceffary, is the mode of preparing the lime before it is beat up into mortar.

When lime is to be employed for making plafter, it is of great importance that every particle of the lime-ftone be flaked before it is worked up. For, as the fmoothness of the surface is the circumftance most wished for in plafter, if any particles of lime fhould be beat up in it, and employed in work before they had had fufficient time to fall, the water still continuing to act upon them after it was worked up, would infallibly flake these particles, which would then expand themfelves with great force, and produce those excrefcences upon the surface of the pla

fter

fter that are commonly known by the name

of blifters.

Hence, therefore, if we hope to obtain a perfect kind of plafter, that shall remain fmooth on the furface, and free of blifters, there is an abfolute neceffity to allow the lime to lie for a confiderable time macerating in water before it is worked up. This operation is called fouring.

If the lime-stone be pure, and has been very perfectly calcined, there will be little danger but that the whole of the lime will fall at firft:-But, if it has been lefs perfectly burnt, there will be many particles that will require to lie a long time before they will be reduced to a powder. This operation is therefore more neceffary with impure than pure lime. But it ought on no occafion to be omitted, as there is not the fmalleft, pro bability but fome blifters would appear on plaster made even of the pureft lime, if

worked

worked up and used immediately after it had been flaked.

It is also a common practice to four the lime when it is intended, to be used in mortar. And, although it is not fo indifpenfibly neceffary in this cafe as when it is intended for plafter; yet, if properly performed, it is evident, that it must even here be of use; as any dry knots that may fall after the mortar is used, must tend to difunite the parts of it that had been already united, and render the cement much lefs perfect than if the whole had been properly mixed. the materials before ufing.

up with

But more circumfpection is requifite in fouring lime for mortar than for plafter, For,

As it is not neceffary that plafter should be endowed with a ftonny degree of hard-. ness, there is no lofs fuftained by allowing a great proportion, of the lime that is intend

ed

ed for that purpose to absorb its air before it be used; for a very small quantity of quicklime will be fufficient to unite the whole into one flightly coherent mass.

Therefore the only circumftance neceffary to be attended to in fouring lime for plafter is, that it be allowed to macerate long enough; as there is no danger of ever erring on the oppofite extreme. It is indeed neceffary that it should lie a very long time, on fome occafions, before we can be certain whether all the particles are thoroughly flaked. I have known pieces of lime-hells lie upwards of fix months exposed to all the viciffitudes of winter-weather, and fall after that time. Such flightly burnt ftones are indeed usually separated in fifting the lime for plafter; but, as fome small chips may escape, it is always fafeft to allow it to lie in the four a very long time.

This practice is also attended with another advantage of fome confequence.—For

if,