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In consequence of this peculiarity, it necessarily happens, that, in proportion as


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

When these salts are in a caustic state, they are soluble in water in any proportion. They have even such a tendency to unite with it when in this state, that it is extremely difficult, if not altogether impossible, to free them from the water till they are reduced to a mild state.——No art has ever yet been discovered by which a caustic volatile alkali could be exhibited in a solid form; and, although dry concretions of the fixed alkali are sometimes obtained while it is possessed of a certain degree of causticity, yet these are only effected in consequence of some part of it becoming mild in the operation, ; nor can they be kept in that state without the utmost care.

Ordinary pot-ash is an alkaline salt obtained from the ashes of burnt vegetables. This is, in some measure, deprived of its air in burning the plant; but, during the process, before the watery solution is thoroughly evaporated to dryness, the alkali has absorbed some part of its air, and is in some measure rendered mild, so as to admit of being reduced


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


to a dry state by the force of fire. But, as the salt is not in this state per/eSlly mild, the caustic part of the alkali attracts the moisture from the air with so much power, as soon 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 saline substance that attract* moisture from the air, and dissolves in it, is called a deliquescent salt.

But, if this alkali be exposed to the air for a sufficient length of time, till it has slowly 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 dissolved in water only in one certain proportion, like other salts \ and may be made to ihoot into regular chrystals, 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 case, the alkali, it is plain, leaves the water as soon as it has united with its air, in the fame manner as lime separates from water, and assumes a dry chry

stalline instantly capable of dissolving at much caustic lime as it had lost by the former chry


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

Common salt is, in like manner, in part decomposed * by the violent heat that takes place in our ordinary way of boiling it.—A part of its acid is dissipated; the alkali that remains is left in its caustic state. Hence it has a perpetual tendency to absorb water j—'-in consequence of which the whole becomes a deliquescent salt- If the evaporation is made slow enough, the chrystals are more perfect; and it may be easily kept diy in the ordinary state of our atmosphere.—This is the reason ^why great salt may be more easily kept dry than small salt.

If, however, the alkali that is mixed with the salt had not been in a caustic state, it is well known it would not have deliquesced: for the natrum of the antients, or thefefftl alkali of the moderns in its native

* Common salt is a compound substance formed by the union of a particular acid with the fosil alkali.

stallization; so that it immediately acts upon, and dissolves another portion of the


tive mild state, is a firm chrystalline sait, much resembling nitre, from whence it originally derives its nameExactly similar to these, are the changes produced upon common sugar, by the different processes it may be made to undergo. Sugar is a solid concrete, obtained by evaporating to dryness the juice of the sugar cane. In the ordinary process for obtaining that substance, it is deprived of some part of its fixed air,—and is hastily concreted into an imperfect fort of chrystalline mass. In this state, it is possessed of a certain degree of acrid causticity; and can be dissolved in water in any proportion, from the slightest degree of impregnation to perfect dryness. —But, when it is placed in proper circumstances, and is allowed time to absorb its air,—like the other substances above mentioned, it can only be dissolved in certain proportions, and therefore quits the water as it gradually unites with its air; and assumes a regular chrystalline form.

These chryslals are distinguished by the name of ugar candy, and are well known to be more difficultquick-lime that remained below, after the water was saturated. This is also chrystallized in its turn, and a fresh solution takes place; and so on it continues, constantly chrystallizing and dissolving a-new, as long as any caustic lime remains in the water to be dissolved.

It is in consequence of this constant action of the water and air that lime-water always continues of an equal degree of strength, so long as any caustic lime remains in the vessel for the water to act; upon, notwithstanding the large proportions of calcareous chrystals that are continually separating from it.

§ 18.

ly soluble in water,—to be a milder and les* acrid sweet,—and to possess many other qualities different from the sugar of which they were originally formed.

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