these chrystals separate from lime-water, a part of it becomes pure water again, and is inftantly

to a dry state by the force of fire. But, as the falt is not in this state perfectly mild, the cauftic part of the alkali attracts the moisture from the air with fo much power, as foon to obtain enough to reduce the whole to a watery solution, if it is not preserved from acid damp with the utmost care.

This, and every other faline fubftance that attracts moisture from the air, and diffolves in it, is called a deliquefcent falt.

But, if this alkali be expofed to the air for a fufficient length of time, till it has flowly absorbed its whole proportion of fixed air, and with it has become one chemical mixt, forming a perfectly mild alkali; it is then capable of being diffolved in water only in one certain proportion, like other falts; and may be made to fhoot into regular chryftals, which may be kept in a solid dry state, when the atmosphere is in a due temperature of heat, in the fame manner as any other salt.

In this cafe, the alkali, it is plain, leaves the water as foon as it has united with its air, in the fame manner as lime feparates from water, and affumes a dry chry.

ftalline

instantly capable of diffolving as much cauftic lime as it had loft by the former chryftallization;

ftalline form. The alkali may indeed be again diffolved by adding a larger proportion of water, which the other cannot ;-but, in the first particular, the parallel is alike.

*

Common falt is, in like manner, in part decompofed by the violent heat that takes place in our ordinary way of boiling it.-A part of its acid is diffipated; the alkali that remains is left in its cauftic ftate. Hence it has a perpetual tendency to absorb water;-in confequence of which the whole becomes a deliquefcent falt. If the evaporation is made flow enough, the chrystals are more perfect; and it may be eafily kept dry in the ordinary state of our atmofphere. This is the reafon why great falt may be more easily kept dry than small falt.

If, however, the alkali that is mixed with the falt had not been in a cauftic state, it is well known it would not have deliquefced; for the natrum of the antients, or the foffil alkali of the moderns in its native

* Common falt is a compound fubftance formed by the union of a particular acid with the foffil alkali.

ftallization; fo that it immediately acts upon, and diffolves another portion of the

quick

tive mild ftate, is a firm chryftalline falt, much refembling nitre, from whence it originally derives its name.

Exactly fimilar to thefe, are the changes produced upon common fugar, by the different proceffes it may be made to undergo. Sugar is a folid concrete, obtained by evaporating to drynefs the juice of the fugar cane. In the ordinary procefs for obtaining that substance, it is deprived of fome part of its fixed air, and is haflily concreted into an imperfect fort of chryftalline mass. In this ftate, it is poffeffed of a certain degree of acrid caufticity; and can be diffolved in water in any proportion, from the flightest degree of impregnation to perfect dryness. -But, when it is placed in proper circumstances, and is allowed time to abforb its air,-like the other substances above mentioned, it can only be diffolved in certain proportions, and therefore quits the water as it gradually unites with its air; and affumes a regular chrystalline form.

These chrystals are distinguished by the name of ugar candy, and are well known to be more difficult

ly

1

quick-lime that remained below, after the water was faturated. This is also chrystallized in its turn, and a fresh folution takes place; and fo on it continues, conftantly chryftallizing and diffolving a-new, as long as any cauftic lime remains in the water to be diffolved.

It is in consequence of this constant action of the water and air that lime-water always continues of an equal degree of strength, fo long as any cauftic lime remains in the veffel for the water to act upon, notwithstanding the large proportions of calcareous chryftals that are continually feparating from it.

$ 18.

ly foluble in water,-to be a milder and lefs acrid fweet, and to poffefs many other qualities different from the fugar of which they were originally formed.

$ 18.

From the foregoing induction it appears, that, when a large quantity of fand is mixed in the mortar, it will bear à great proportion to the whole mafs; fo that the water that may be mixed with the mortar will be much greater, in proportion to the quantity of lime contained in this mortar, than if the whole had confifted of pure calcareous matter.-And, as the fand abforbs none of that water, after a part of the lime is chryftallized and separated from the water, it is left at liberty to act once more upon those few particles of cauftic lime that may ftill remain in the mortar, which will be diffolved and converted into chrystals in their turn.

In this manner, it may happen, in fome circumftances, that a very large proportion of the lime may become chryftallized, fo

that