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the first and readieft to impregnate a cold infufion (b). And this will affift us to account for the virtues of tar-water. That volatile acid in vegetables, which refifts putrefraction, and is their great prefervative, is detained in a fubtile oil mifcible with water, which oil is itfelf imprifoned in the refin or groffer part of the tar, from which it is eafily fet free and obtained pure by cold wa.

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124. The mild native acids are observed more kindly to work upon, and more thoroughly to diffolve, metallic bodies, than the strongest acid fpirits produced by a vehement fire; and it may be fufpected, they have the fame advantage as a medicine. And as no acid, by the obfervation of fome of the best chemifts, can be obtained from the fubftance of animals thoroughly affimilated, it fhould follow, that the acids received into a healthy body must be quite fubdued and changed by the vital powers: but it is easier to subdue and affimilate (e) the gentler than the stronger acids.

125. I am very fenfible, that on fuch fubjects arguments fall fhort of evidence: and that mine fall short even of what they might have been, if I enjoyed better health, or thofe opportunities of a learned commerce, from which I am cut off in this remote corner. I fhall nevertheless go on as I have begun, and proceed by reafon, by conjecture, and by authority, to caft the best light I can on the obfcure paths that lie in my way.

126. Sir Ifaac Newton, Boerhaave, and Homberg are all agreed, that the acid is a fine fubtile fubftance, pervading the whole terraqueous globe; which produceth divers kinds of bodies, as it is united to different fubjects. This according to (b) 1, 7: (e) 48 H 2 Homberg

Homberg is the pure falt, falt the principle, in it felf fimilar and uniform, but never found alone. And although this principle be called the falt of the earth, yet it fhould feem it may more properly be called the falt of the air, fince earth turned up and lying fallow receives it from the air. And it fhould feem that this is the great principle of vegetation, derived into the earth from all forts of manures, as well as from the air. The acid is allowed to be the caufe of fermentation in all fermented liquors. Why therefore, may it not be fuppofed to ferment the earth, and to conftitute that fine penetrating principle, which introduces and affimilates the food of plants, and is fo fugitive as to escape all the filtrations and perquifitions of the moft nice obfervers?

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127. It is the doctrine of Sir Ifaac Newton and Monfieur Homberg, that, as the watry acid is that which renders falt foluble in water, fo it is that fame which joined to the earthy part makes it a falt. Let it therefore be confidered, that the organs (d) of plants are tubes, the filling, unfolding, and diftending whereof by liquors, doth conftitute what is called the vegetation or growth of the plant. But earth itself is not foluble in water, fo as to form one vegetable fluid therewith. Therefore the particles of earth must be joined with a watry acid, that is, they must become falts in order to diffolve in water; that so, in the form of a vegetable juice, they may pass through the ftrainers and tubes of the root into the body of the plant, fwelling and diftending it's parts and organs, that is, increafing it's bulk. Therefore the vegetable matter of the earth is in effect earth changed into falt. And to render earth

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fertile, is to caufe many of it's particles to affume a faline form.

128. Hence it is obferved, there are more falts in the root than in the bark, more falts in vegetables during the fpring, than in the autumn or winter, the crude faline juices being in the fummer months partly evaporated, and partly ripened by the action and mixture of light. Hence alfo it appears, why the dividing of earth, fo as to enlarge it's furface, whereby it may admit more acid from the air, is of fuch ufe in promoting vegetation: And why afhes, lime, and burnt clay are found fo profitable manures, fire being in reality the acid, as is proved in the fequel (a). Marls alfo and fhells are useful, forafmuch as thofe alcaline bodies attract the acid, and raise an effervefcence with it, thereby promoting a fermentation in the glebe. The excrements of animals and putrid vegetables do in like manner contribute to vegetation, by increafing the falts of the earth. And where fallows are well broken, and lye long to receive the acid of the air into all their parts; this alone will be fufficient to change many terrene particles into falts, and confequently render them foluble in water, and therefore fit aliment for vegetables.

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129. The acid, faith Homberg, is always joined to fome fulphur, which determines it to this or that fpecies, producing different falts, as it is the vegetable, bituminous, or metallique fulphur. ven the alcaline, whether volatile or lixivial falts, are fuppofed to be nothing but this fame acid ftrictly detained by oil and earth, in fpight of the extreme force of fire, which lodgeth in them, without being able to dislodge some remains of the acid.

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130. Salts, according to fir Ifaac Newton, are dry earth and watery acid united by attraction, the acid rendering them foluble in water (f). He fuppofeth the watry acid to flow round the terrestrial part, as the ocean doth round the earth, being attracted thereby, and compares each particle of falt, to a chaos whereof the innermoft part is hard and earthy, but the furface foft and watery. Whatever attracts and is attracted moft ftrongly is an acid in his sense.

131, It feems impoffible to determine the figures of particular falts. All acid folvents together with the diffolved bodies are apt to shoot into certain figures. And the figures, in which the foffil falts crystallize, have been fuppofed the proper natural fhapes of them and their acids. But Homberg hath clearly fhewed the contrary: forafmuch as the fame acid diffolving different bodies, affumes different fhapes. Spirit of nitre, for inftance, having diffolved copper, fhoots into hexagonal crystals; the fame having diffolved iron, fhoots into irregular fquares; and again, having diffolved filver, forms thin crystals of a triangular figure.

132..Homberg nevertheless holds in general, that acids are fhaped like daggers, and alcalies like fheaths and that moving in the fame liquor, the daggers run into the fheaths fitted to receive them, with fuch violence as to raise that effervefcence obferved in the mixture of acids and alcalies. But it feems very difficult to conceive, how, or why the mere configuration of daggers and fheaths, floating in the fame liquor, fhould cause the former to rush with fuch vehemence, and direct their points fo aptly into the latter, any more than a parcel of fpigots and foffets floating together in the fame water, fhould rush one into the other.

(f) 127.

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-132. It fhould feem rather, that the vehement attraction which Sir Ifaac Newton attributes to all acids, whereby he supposeth them to rush towards, penetrate, shake, and divide the moft folid bodies, and to ferment the liquid of vegetables, could better account for this phænomenon. It is in this attraction, that Sir Ifaac placeth all their activity, and indeed it should feem, the figures of falts were not of fuch efficacy in producing their effects, as the ftrong attractive powers whereby they are agitated and do agitate other bodies. Efpecially if it be true (what was before remarked) that lixivious falts are alike purgative, whatever may be the fhape of their angles, whether more or less acute or obtufe.

134. Sir Ifaac Newton accounts for the watery acids making earthy corpufcles foluble in water, by fuppofing the acid to be a mean between earth and water, its particles greater than thofe of water, and less than those of earth, and strongly to attract both. But perhaps there is no neceffary reafon for fuppofing the parts of the acid groffer than the parts of water, in order to produce this effect; may not this as well be accounted for, by giving them only a strong attraction or cohefion with the bodies to which they are joined?

135. The acid fpirit or falt, that mighty inftrument in the hand of nature, refiding in the air, and diffused throughout that whole element, is difcernible alfo in many parts of the earth, particularly in foffils, fuch as fulphur, vitriol, and alum; it was already obferved from Homberg, that this acid is never found pure, but hath always fulphur joined with it, and is claffed by the difference of its fulphurs, whether mineral, vegetable, or animal.

136. Salts are vulgarly reckoned the most active of chemical principles. But Homberg derives all

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