side in them; but this is prevented by a motion in the parts of fire: if they are properly agitated, they are suspended in the fluid; and if they are suspended, it follows that they are agitated. From extraordinary cases we are to learn what happens in others which are ordinary. It is here evident, that fire, by its motion, separates and suspends the parts of a solid body in a fluid menstruum : where the motion of fire is violent, the solution is quickly accomplished, and a very large quantity of the solvend is sustained in the menstruum; therefore, in all ordinary cases, where the solution is slow and gradual, and the quantity suspended is but inconsiderable, the same effect is brought to pass by the imperceptible intestine motion of that ordinary degree of fire, by which the atmosphere and all things in it are constantly agitated: as the agitation increases, the cause generally betrays itself by a proportional increase of heat *. Other menstruums are rendered

VOL. IX:

Q

Let a piece of iron or copper be put into a glass phial with aquafortis: if it works tolerably well, place the phial under a receiver, and exhaust the air: it will then work with much more violence; so that if the air were exhausted to an high degree, it might possibly take fire and explode.

dered such, and derive all the activity they have, by means of fire, which co-operates and gives them their proper effect. When a stone is in the condition of lime, strongly impregnated with the fire it has acquired in the operation of burning, the application of that moisture which the air carries with it will dissolve it by slow degrees, but water poured upon it will do it at once; and then the fire it contains, entering into conflict with the water, rises to a violent heat, and, as it transpires from the pores of the stone, intimately dissolves the whole substance. The utility of fire, in the works of human art and labour, is no where more conspicuous

than

While it is boiling with this vehemence, contrive to drop it into a vessel of cold water within the receiver, which will very soon so check the operation, that if the glass be never so well exhausted, the aquafortis will not work with that violence so long as it is surrounded by the cold water. That the agent in this case is fire, appears very plainly, and that the motion does not make the heat, but that the fire and heat occasion the motion; because when the air, the natural antagonist of fire, is removed, the fire acts more freely, as it makes water boil under the same circumstances much sooner than under the pressure of the atmosphere. That the cold water, applied externally, should check the fire, is very natu ral; but no reason can be given why it should check the operation if it is supposed to commence upon any other prin

than upon this occasion: for, how is it possible for men to build with any effect, either for ornament or security, unless we suppose them possessed of this wonderful secret, (for such it is, however commonly practised,) of dissolving stone, by means of fire detained. and imprisoned in a cold body, and uniting it again in a more convenient manner by the mediation of water? But fire is the agent, without which nothing can be done from the beginning of this work to the end of it: for, as it changes the stone into lime at first by burning, it fixes the mortar into solidity by evaporation; of which we are to treat in due order.

Water is a menstruum to the several kinds of salts; but the power of solution is not in the water. For let some water boil over the fire in a vessel of glass, into which let seasalt be cast by a little at a time, and we shall find that after a large quantity hath been dissolved, the water will still be transparent as before; which shews the solution to be perfect. Then let the vessel be removed from the fire, and as the water begins to cool, some salt will fall to the bottom: as it approaches nearer to the temperature of the air, more and more of the salt will be depoQ2 sited:

sited: and hence we argue, that, as the fire by its greater motion keeps a larger quantity suspended, what remains at last suspended in the water is supported there by the ordinary effect of the remaining heat: so that if water could be found without fire, it would be without the power of solution. of solution. And indeed this power never fails to leave it at a certain period, when the water is frozen into ice; because a solid mass cannot act as a solvent. The same medium that gives it fluidity makes it a menstruum, and its dissolving power increases with its heat. Water with the common boiling heat will not act upon oils and sulphurs; but if wood is acted upon by water in Papin's digester above described, the smell will afterwards indicate that its oils and sulphurs are extracted; which is also clear from the friable state to which it is reduced; this being the necessary consequence when the oily parts are gone, which served as a vinculum to tie the earthy parts together. Water is therefore a solvent, so far as fire enables it to be; and therefore, in such solutions, not water, but fire is the proper agent.

When some bodies are wrought upon by some particular menstruums without the help

of

of any extraordinary fire, and other bodies are not affected by the same with all the ad-vantages we can give them from the same element; the difference must be imputed to a mechanical fitness in one case, and an unfitness in the other, in those minute parts which constitute the different bodies: but this difference we cannot account for, because it hath its foundation in what is invisible, and must always remain so. Yet, being inclined to explain every thing, we fall upon the fruitless expedient of attractions and repulsions, which bring us into a labyrinth more mortifying, because more perplexing than our former ignorance. may readily imagine, that the primary configurations of the parts of bodies must occasion many appearances, which would be natural and mechanical if such configurations were the objects of sense: but when we have recourse to attractions and repulsions, we introduce things which have in them nothing of a physical nature, and, when defined by different philosophers, are so full of contradiction and absurdity, that honest ignorance, which knows itself, is preferable to the knowledge of words which convey nothing to us but sound. An hungry stomach, sensible of

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its