and changed by the vital powers: but it is easier to subdue and assimilate the gentler than the stronger acids (sect. 48).

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 agree that the Acid is a fine subtle substance, pervading the whole terraqueous globe; which produceth divers kinds of bodies, as it is united to different subjects. This, according to Homberg, is the pure salt, salt the principle, in itself 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 watery acid is that which renders salt soluble 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 of plants are tubes (sect. 30, 31, 35)—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 watery acid; that is, they must become salts, in order to dissolve 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 its parts and organs, that is, increasing its bulk. Therefore the vegetable matter of the earth is in effect earth changed into salt. And to render earth fertile is to cause many of its particles to assume a saline 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 saline 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 its 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 sequel (sect. 202). Marls also and shells are useful, forasmuch as those alkaline 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 lie long to receive the acid of the air into all their parts; this alone will be sufficient to change many terrene particles into salts, and consequently render them soluble in water, and therefore a fit aliment for vegetables.

129. The acid, saith Homberg, is always joined to some sulphur, which determines it to this or that species, producing different salts, as it is the vegetable, bituminous, or metallic sulphur. Even the alkaline, whether volatile or lixivial salts, are supposed to be nothing but this same acid strictly detained by oil and earth, in spite of the extreme force of fire, which lodgeth in them, without being able to dislodge some remains of the acid.

130. Salts, according to Sir Isaac Newton, are dry earth and watery acid united by attraction, the acid rendering them soluble in water (sect. 127). He supposeth the watery 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 strongly 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 figures. And the figures in which the fossil salts crystallize have been supposed the proper natural shapes of them and their acids. But Homberg hath clearly shewed the contrary: forasmuch as the same acid dissolving different bodies assumes different shapes. Spirit of nitre, for instance, having dissolved copper, shoots into hexagonal crystals; the same having dissolved iron, shoots into irregular squares; and again, having dissolved silver, forms thin crystals of a triangular figure.

132. Homberg, nevertheless, holds in general, that acids are shaped like daggers, and alkalies 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 observed in the mixture of acids and alkalies. 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 fossets floating together in the same water should rush one into the other.

133. It should seem rather that the vehement attraction which Sir Isaac Newton attributes to all acids, whereby he supposeth them to rush towards, penetrate, shake, and divide the most solid bodies, and to ferment the liquid of vegetables, could better account for this phenomenon. 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 active powers whereby they are agitated and do agitate other bodies. Especially if it be true (what was before remarked) that lixivious salts 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 acid's 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 supposing the parts of the acid grosser 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 spirit or salt, that mighty instrument 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 sulphur joined with it, and is classed by the difference of its sulphurs, whether mineral, vegetable, or animal.

136. Salts are vulgarly reckoned the most active of chemical principles. But Homberg derives all their activity from the sulphurs joined with them. From which also, as hath been said, he derives all their kinds and differences (sect. 129). Salt, water, oil, and earth seem to be originally the same in all vegetables. All the difference, according to the chemists, ariseth from a spirit residing in the oil, called the rector or archæus. This is otherwise called by chemists ens primum, or the native spirit; whereon depend, and wherein are contained, the peculiar flavour and odour, the specific qualities and virtues, of the plant.

137. These native spirits or vegetable souls are all breathed or exhaled into the Air, which seems the receptacle as well as source of all sublunary forms, the great mass or chaos which imparts and receives them. The air or atmosphere that surrounds our earth contains a mixture of all the active volatile parts of the whole habitable world, that is, of all vegetables, minerals, and animals. Whatever perspires, corrupts, or exhales, impregnates the air; which, being acted upon by the solar fire, produceth within itself all sorts of chemical operations, dispensing again those salts and spirits in new generations, which it had received from putrefactions.

138. The perpetual oscillations of this elastic and restless element operate without ceasing on all things that have life,

whether animal or vegetable, keeping their fibres, vessels, and fluids in a motion, always changing; as heat, cold, moisture, dryness, and other causes alter the elasticity of the air which accounts, it must be owned, for many effects. But there are many more which must be derived from other principles or qualities in the air. Thus iron and copper are corroded and gather rust in the air, and bodies of all sorts are dissolved or corrupted, which sheweth an acid to abound and diffuse itself throughout the air.

139. By this same air fire is kindled, the lamp of life preserved, respiration, digestion, nutrition, the pulse of the heart, and motion of all the muscles seem to be performed. Air therefore is a general agent, not only exerting its own, but calling forth the qualities or powers of all other bodies, by a division, comminution, and agitation of their particles, causing them to fly off and become volatile and active.

140. Nothing ferments, vegetates, or putrifies without air, which operates with all the virtues of the bodies included in it; that is, of all nature; there being no drug, salutary or poisonous, whose virtues are not breathed into the air. The air therefore is an active mass of numberless different principles, the general source of corruption and generation; on one hand dividing, abrading, and carrying off the particles of bodies, that is, corrupting or dissolving them; on the other, producing new ones into being; destroying and bestowing forms without intermission.

141. The seeds of things seem to lie latent in the air, ready to appear and produce their kind, whenever they light on a proper matrix. The extremely small seeds of fern, mosses, mushrooms, and some other plants are concealed and wafted about in the air, every part whereof seems replete with seeds of one kind or other. The whole atmosphere seems alive. There is everywhere acid to corrode, and seed to engender. Iron will rust, and mould will grow in all places. Virgin earth becomes fertile, crops of new plants ever and anon shew themselves; all which demonstrate the air to be a common seminary and receptacle of all vivifying principles.

141. Air may also be said to be the seminary of minerals and metals, as it is of vegetables. Mr. Boyle informs us that the exhausted ores of tin and iron being exposed to the air