me the honour to affociate me with them, to these fubjects, fo far as to lead to any new experiments, by which the qualities and preparation of articles fo important may be more perfectly investigated and explained, and the great expence attending the prefent mode of preparing or importing them diminished.

The very curious procefs, difcovered by Mr. Birch, and related by you, Sir, with fo much perfpicuity in the Philofophical Tranfactions, for procuring the vegetable alkali from the water of dunghills, may be adduced as a proof, that very confiderable improvements have been lately made in the preparation of this falt. And as the fum paid by the nation to Ruffia, and other foreign ftates, is no less than one hundred and fifty thousand pounds per annum, for pot-afh,* every hint towards an improvement that can be fuggested in its preparation, is valuable; especially as it is not impoffible, that we may now also lofe, in a confiderable degree, the advantage of that, which has been hitherto prepared for the use of these islands in North America.

This alkali, having been long obtained folely by the combuftion of vegetable fubftances, was by most chemifts confidered, as a production of the fire, formed by fome fecret combination of the earthy, and perhaps fome portion of the in

Watfon's Effays, Vol. I. p. 135.

flammable

flammable part of the plant, with the native acid, the aqueous part being diffipated in the process.

On the other hand, it was by a few perfons conjectured, that the alkali exifts in vegetables perfectly formed previously to the combuftion; and that the operation of the fire is nothing more, than a perfect feparation and diffipation of the other principles, by which it was entangled and concealed.

In confirmation of this latter opinion, it was alledged, that Meffrs. Beaumé and Boulduc,† &c. had extracted nitre, and other falts, of which this alkali is the base, from several plants by infufion and decoction, without incineration. In anfwer to thefe obfervations it was advanced, that the nitre, &c. thus detected, muft be confidered as foreign, adventitious, and not effential to the plant, because from the fame fpecies the quantity obtained at different times is extremely variable. Its existence, therefore, at all in plants, must be ascribed to fome peculiar circumstances of culture and foil; and in proof of this, it was remarked, that it frequently happens, that the fame plants which contain much nitre, when these two circumstances favour its introduction, contain none at all in contrary circumstances.

Dict. of Chemistry, Article, Alkali Fixed Vegetable. + Encycloped. Art. Nitre and Mem. del Acad. des Sciences. 1734.

Of these two opinions, that which infinuates, that the alkali exifts in vegetables in its perfect ftate, can hardly be admitted, at leaft until we have accumulated a larger collection of facts relative to this point. It is much more probable, that the alkali, wherever it is found, whether in confequence of combuftion or otherwise, is formed by fome tranfmutation (if I may be allowed the expreffion) of the native acid of plants, or by a particular combination of it with the earthy and inflammable principles.

I am inclined to adopt this doctrine, from the three following circumftances, the two laft of which will alfo fhew, that this tranfmutation may be effected without combuftion, and therefore, that this alkali cannot be any longer confidered as the creature or offspring of fire.

First, Thofe vegetable fubftances, which contain the largest portion of the native acid, afford the largest quantity of alkali by incineration: and the quantity of alkali obtained is very confiderably increased by particular modes of applying the heat, which can only be understood to operate, by bringing the feveral component principles of the vegetable fübftance into closer contact, and within the sphere of each other's action.

Secondly, This alkali is produced in a very confiderable quantity by the procefs of fermentation, to which only the faccharine and acefcent parts of plants are liable. And,

Thirdly,

Thirdly, It is produced in the putrefaction both of animal and vegetable matters.

In the two laft of these cafes, we have not indeed hitherto obtained the alkali feparate, but in combination with acids forming nitrous and other neutral falts.

I. To confirm the first of the positions above stated, it is only requifite, before this Society, to observe, that no vegetable fubftances furnifh a larger quantity of alkali than the extracts of plants, or their native effential falts. And in the common mode of preparing this alkali by incineration, it has been often remarked, that even when thofe vegetable fubftances, which abound with acid, are burnt with a close smothering heat (as in making the alkaline falt of Tachenius) the greatest part of the acid is gra dually diffipated, and very little alkali is obtained; whereas, by a quick open fire, the acid feems to be intimately coagitated with the earthy part before fuch a diffipation can take place, and the produce of the alkali is vastly larger. It is a common obfervation, how minute a proportion of alkali the ashes of charcoal yield, compared to that which may be obtained by burning an equal quantity of the fame wood in an open fire.*

II. That the vegetable alkali is produced copiously in the natural procefs of vinous fer

Neumann's Chemistry, by Lewis, Vol. II. p. 290.

mentation, appears evinced by the celebrated Scheele's curious analyfis of the tartar of wine. *

This fubftance was long confidered fimply as a peculiar acid, debafed, perhaps, by a large admixture of earthy matter: but no one before Mr. Scheele ever conceived, that it was a neutral falt, with a vegetable alkaline base. Yet fuch his experiments have fhewn it to be, the alkali being, however, fuperfaturated with the vegetable acid.

His principal experiment (which I relate, not as fuppofing it unknown to the Gentlemen of the Society, but only to recal it to their remembrance) was as follows:

He diffolved cream of tartar in a fufficient quantity of boiling water, and added fine chalk in powder to it until the effervefcence ceafed : a copious white fediment fell to the bottom, which was the chalk, combined with a part of the acid of tartar; and the liquor, that floated above the precipitate, afforded, by careful evaporation, a crystallization of foluble tartar, which is a compound of the acid of tartar, with the vegetable alkali. Though perfectly fatisfied of the accuracy of Scheele, and of Dr. Black, who re

* The original account of thefe experiments is given in the Swedish Tranfactions for the year 1770, but as that work is not common in this kingdom, it may be useful to refer to an Abstract of this Memoir in the Medical Commentaries, Vol. I. p. 320.

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