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books, are much to be pitied. As they are debarred the free use of air and exercise, this I will ven.' ture to recommend as the best fuccedaneum to both, Though it were to be withed, that modern scholars would like the ancients, meditate and converse more in walks and gardens and open air, which, upon the whole, would perhaps be no hindrance to their learning, and a great advantage to their health. My own sedentary course of life had long since thrown me into an ill habit, attended with many ailments, particularly a nervous cholic, which rendered my life a burthen, and the more fo, be" cause my pains were exasperated by exercise. But since the use of tar-water, I find, though not a perfect recovery from my old and rooted illness, yet fuch a gradual return of health and ease, that I efteem my having taken this medicine the greatest of all temporal blessings, and am convinced that, under providence, I owe my life to it.

120. In the distilling of turpentine and other balsams by a gentle heat, it hath been observed, that there rifeth first an acid 1pirit (n) that will mix with water; which fpirit, except the fire be very gentle, is loit.

This grateful acid spirit that first comes over, is, as a learned cheinift and physician informs us, highly refrigeratory, diuretic, sudorific, balsamic or preservative from putrefaction, excellent in nephritic cases, and for quenching thirst, all which virtues are contained in the cold infusion, which draws forth from tar only it's fine flower or quintessence, if I may

fo say, or the native vegetable fpirit, together with a little volatile oil.

121. The distinguishing principle of all vegetables, that whereon their peculiar smell, taste, and specific properties depend, seems to be some

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extremely

extremely fine and subtile spirit, whose immediate vehicle is an exceeding thin volatile oil, which is itself detained in a groffer and more viscid refin or balsam, lodged in proper cells in the bark and feeds, and most abounding in autumn or winter, after the crude juices have been thoroughly concocted, ripened, and impregnated with Tolar light. The spirit itself is by fome fuppofed to be an oil highly subtilized, so as to mix with water. But such volatile oil is not the spirit, but only it's vehicle. Since aromatic oils, being long exposed to air, will lose their specific fmell and tafte, which fy off with the spirit or vegetable falt, without any sensible diminution of the oil.

122. Those volatile falts, that are set free and raised by a gentle heat, may justly be fupposed effential (a), and to have pre-existed in the vegetable; whereas the lixivial fixed: salts obtained by the incineration of the subject, whose natural constituent parts have been altered or destroyed by the extreme force of fire, are by later chemifts, upon very good grounds, supposed not to have pre-exifted therein ; all such falts appearing, from the experiments of signor Redi, not to preserve the virtues of the respective vegetable subjects ; and to be alike purgative and in an equal degree, whatsoever may be the shape of their points, whether sharp or obtuse. But although fixed or lixivious falts may not contain the original properties of the subject; yet volatile falts raised by a Night heat from vegetables are allowed to preserve their native virtues: and such salts are readily imbibed by water.

123. The most volatile of the falts, and the most attenuated part of the oil, máy be supposed (a) 6.

the

the first and readieft 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 milcible with water, which oil is itself imprisoned in the refin or groffer part of the tar, from which it 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 1pirits produced by a vehement fire; and it may be suspected, they have the same advantage as a medicine. And as no acid, by the obfervation of some of the best chemists, can be obtained from the substance of animals thoroughly affimilated, it should follow, that the acids received into a healthy body must be quite subdued and changed by the vital powers : but it is easier to fubdue and aflim.late (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 best 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

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Homberg

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Homberg is the pure salt, salt 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 should seem it may more properly be called the sale of the air, since earth turned up and lying fallow receives it from the air. And it should seemn 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 soluble in water, so it is that same which joined to the earthy part makes it a falt.! Let it therefore be considered, that the organs (d) of plants are tubes, the filling, unfolding, and diftending 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 fuid 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 fo, in the form of a vegetable juice, they may pass chrough the strainers and tubes of the root into the body of the plant, swelling and distending it's pares 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 saline form.

128. Hence it is observed, there are more falts in the root than in the bark, more salts in vegetables during the spring, than in the autumn or winter, the crude saline juices being in the summer' months partly evaporatect, ard partly ripened by the action and mixture of light. Hence

also it appears, why the dividing of earth, fo 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, faith 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. Even 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 fome remains of the

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