Salt may well be denominated a menstruum, as it is easily diluted with water; fixed alcaline salts we have already seen appear to be the produce of fire alone.—Such are never distinguished in the composition of vegetables in their natural state; though a volatile alcalious salt (the effect of heat equal or superior to that necessary for putrefaction) is found in many, and especially in such as are putrified.
The power of a fixed alcali as a solvent is great, applied (says Boerhaave) to animal, vegetable, or fossil concretions, so far as they are oils, balsams, gummy, resinous, or of gummy resinous nature, and therefore concreted from oily substances: these, this salt intimately opens, attenuates, and resolves: disposing them to be perfectly miscible with water: oils of alcohol leaving however the impression of taste naturally belonging to this salt.
Vegetable acid salt dissolves animal, vegetable, fossil, and metalline substances, except mercury, silver, and gold. In most terrestrial vegetables this salt is evident; ripe mealy corn has the least indication of it, yet extracts therefrom, when fermented, and sometimes before they are fermented, discover sensibly their acidity. Sea-plants in general have not their roots inserted in the earth at the bottom of the sea, and these in distillation yield an oily volatile alcali; but more subtil than the native acids of vegetables, are the vinous acids produced by fermentation; they dissolve equally most matters put into them, and render the whole homogene. Into a must or wort, when under this act, by means of an elæosaccharum, might be introduced the choicest flavors, and the aromatics of the Indies be applied to heighten the taste and flavor of our barley wines. The laws of England at present subsisting are indeed opposite to any improvement of this sort, from the apprehensions of abuse: but where elegance alone is intended, undoubtedly the merit of our beers and ales might thereby be increased. As such, this is a part of chymical knowledge well worth the enquiry and attention of the brewer.
Neutral salts have already been mentioned; these are very various, and very different when acting as menstruums. Resins and gum-resins are generally said to be most effectually dissolved by alcohol; but Boerhaave informs us, that sal-amoniac (a very salutary subject and a neutral salt) if boiled with gums, resins, or the gum-resins of vegetables, intimately resolves, and disposes them to be conveniently mixed in aqueous and fermenting spiritous menstruums. Of this class of salts thus much is sufficient. This observation perhaps is of too much consequence to escape the notice of the artist.
SECTION VI.
OF THE THERMOMETER.
This instrument is designed for measuring the increase or decrease of heat. By doing it numerically, it fixes in our minds the quantity of fire, which any subject, at any time, is impregnated with. If different bodies are brought together, though each possesses a different degree of heat, it teaches us to discover what degree of heat they will arrive at when thoroughly mixed, supposing effervescence to produce no alteration in the mixture.
The inventor of this admirable instrument is not certainly known, though the merit of the discovery has been ascribed to several great men, of different nations, in order to do them and their countries honor. It came to us from Italy, about the beginning of the sixteenth century. The first inventors were far from bringing this instrument to its present degree of perfection. As it was not then hermetically sealed, the contained fluid was, at the same time, influenced by the weight of the air, and by the expansion of heat. The academy of Florence added this improvement to their thermometers, which soon made them more generally received; but, as the highest degree of heat of the instrument, constructed by the Florentine gentlemen, was fixed by the action of the strongest rays of the sun in their country, this vague determination, varying in almost every place, and the want of a fixed universal scale, rendered all the observations made with such thermometers of little use to us.
Boyle, Halley, Newton, and several other great men, thought this instrument highly worthy of their attention. They endeavoured to fix two invariable points to reckon from, and, by means of these, to establish a proper division. Monsieur des Amontons is said to have first made use of the degree of boiling water, for graduating his mercurial thermometers. Fahrenheit, indeed, found the pressure of the air, in its greatest latitude, would cause a variation of six degrees in that point; he therefore concluded, a thermometer made at the time when the air is in its middle state, might be sufficiently exact for almost every purpose. Long before the heat of boiling water was settled as a permanent degree, many means were proposed to determine another. The degree of temperature in a deep cave or cellar, where no external air could reach, was imagined by many a proper one; but what that degree truly was, and whether it was fixed and universal, was found too difficult to be determined. At last the freezing point of water was thought of, and though some doubts arose, with Dr. Halley and others, whether water constantly froze at the same degree of cold, Dr. Martine has since, by several experiments, proved this to be beyond all doubt, and this degree is now received for as fixed a point as that of boiling water.