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the first and readiest to impregnate a cold infufion (b). And this will assist us to account for the virtues of tar-water. That volatile acid in vegetables, which resists putrefraction, and is their great preservative, is detained in a subtile oil mis. cible with water, which oil is itself imprisoned in the resin or groffer part of the tar, from which iç is easily set free and obtained pure by cold wa.

ter.

124. The mild native acids are observed more kindly to work upon, and more thoroughly to diffolve, metallic bodies, than the strongest acid spirits produced by a vehement fire ; and it may be suspected, they have the same advantage as a medicine. And as no acid, by the observation of some of the best chemists, can be obtained from the substance of animals thoroughly afsimilated, it should 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 assimilate (e) the gentler than the stronger acids.

125. I am very sensible, that on such subjects arguments fall short of evidence : and that mine fall short even of what they might have been, if I enjoyed better health, or those opportunities of a learned commerce, from which I am cut off in this remote corner. I shall nevertheless go on as I have begun, and proceed by reason, by conjecture, and by authority, to cast the belt light I can on the obscure paths that lie in my way.

126. Sir Isaac Newton, Boerhaave, and Homberg are all agreed, that the acid is a fine subtile substance, pervading the whole terraqueous globe; which produceth divers kinds of bodies, as it is united to different subjects. This according to

(b) 1, 7. (e) 48 WH 2

Homberg

Homberg is the pure salt, salt, the principle, in it felf similar and uniform, but never found alone, And although this principle be called the salt of the earth, yet it should seem it may more properly be called the salt of the air, since earth turned up and lying fallow receives it from the air. And it should seem that this is the great principle of vegetation, derived into the earth from all sorts of manures, as well as from the air. The acid is allowed to be the cause of fermentation in all fermented liquors. Why therefore, may it not be supposed to ferment the earth, and to constitute that fine penetrating principle, which introduces and assimilates the food of plants, and is so fugitive as to escape all the filtrations and perquisitions of the most nice observers ? - 127. It is the doctrine of Sir Isaac Newton and Monsieur Homberg, that, as the watry acid is that which renders falt foluble in water, so it is that same which joined to the earthy part makes it a salt. Let it therefore be considered, that the organs (d) of plants are tubes, the filling, unfolding, and distending whereof by liquors, doth constitute what is called the vegetation or growth of the plant. But earth itself is not soluble in water, so 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 strainers and tubes of the root into the body of the plant, swelling and distending it's parts and organs, that is, increasing 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 cause many of it's particles to assume a faline form.

128. Hence it is observed, there are more salts in the root than in the bark, more salts in vegetables during the spring, than in the autumn or winter, the crude faline juices being in the summer months partly evaporated, and partly ripened by the action and mixture of light. Hence also it appears, why the dividing of earth, so as to enlarge it's surface, whereby it may admit more acid from the air, is of such use in promoting vegetation : And why ashes, lime, and burnt clay are found so profitable manures, fire being in reality the acid, as is proved in the lequel (a). Marls also and shells are useful, forafmuch as those alcaline bodies attract the acid, and raise an effervescence with it, thereby promoting a fermentation in the glebe. The excrements of animals and putrid vegetables do in like manner contribute to vegetation, by increasing the salts 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 sufficient to change many terrene particles into falts, and consequently render them soluble in water, and therefore fit aliment for vegetables.

129. The acid, saith Homberg, is always joined to some fulphur, which determines it to this or that species, producing different salts, as it is the vegetable, bituminous, or metallique sulphur. Eyen the alcaline, whether volatile or lixivial salts, are supposed to be nothing but this fame acid strictly detained by oil and earth, in spight of the extreme force of fire, which lodgeth in then, without being able to dislodge some remains of the acid. (a) 202.

130. Salts,

130. Salts, according to sir Isaac Newton, are dry earth and watery acid united by attraction, the acid rendering them foluble in water (f). He fupposeth the watry acid to flow round the terrestrial part, as the ocean doth round the earth, being attracted thereby, and compares each particle of salt, to a chaos whereof the innermost part is hard and earthy, but the surface soft and watery. Whatever attracts and is attracted most ftrongly is an acid in his sense.

131, It seems impossible to determine the figures of particular salts. All acid solvents together with the dissolved bodies are apt to shoot into certain figureș. And the figures, in which the folfil falts crystallize, have been supposed the proper natural shapes of them and their acids. But Homberg hath clearly shewed the contrary : forasmuch as the fame acid diffolving different bodies, assumes different shapes. Spirit of nitre, for instance, having diffolved copper, shoots into hexagonal crystals ; the same having diffolved iron, shoots into irreguJar squaress and again, having diffolved silver, forms thin crystals of a triangular figure.

132., Homberg nevertheless holds in general, that acids are shaped like daggers, and alcalies like sheaths : and that moving in the same liquor, the daggers run into the sheaths fitted to receive them, with such violence as to raise that effervescence obferved in the mixture of acids and alcalies. But it seems very difficult to conceive, how, or why the mere configuration of daggers and sheaths, floating in the same liquor, should cause the former to rush with such vehemence, and direct their points so aptly into the latter, any more than a parcel of spigots and foffets floating together in the same water, should rush one into the other. (f) 127.

133. It

· 133. It should seem rather, that the vehement attraction which Sir Isaac Newton attributes to all acids, whereby he fuppofeth them to rush towards, penetrate, shake, and divide the most folid bodies, and to ferment the liquid of vegetables, could better account for this phænomenon. It is in this attraction, that Sir Isaac placeth all their activity, and indeed it should seem, the figures of salts were not of such efficacy in producing their effects, as the strong attractive powers whereby they are agitated and do agitate other bodies. Especially if it be true (what was before remarked) that lixivious falts are alike purgative, whatever may be the shape of their angles, whether more or less acute or obtuse,

134. Sir Isaac Newton accounts for the watery acids making earthy corpuscles soluble in water, by supposing the acid to be a mean between earth and water, its particles greater than those of water, and less than those of earth, and strongly to attract both. But perhaps there is no necessary reason for fupposing 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 cohesion with the bodies to which they are joined?

135. The acid fpirit or salt, that mighty inftrument in the hand of nature, residing in the air, and diffused throughout that whole element, is discernible also in many parts of the earth, particularly in fossils, such as sulphur, vitriol, and alum ; it was already observed from Homberg, that this acid is never found pure, but hath always fulphur joined with it, and is classed by the difference of its sul-' phurs, whether mineral, vegetable, or animal.

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

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