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meter: and it is true, in all the experiments we can make, that a fluid will boil sooner, when it is less resisted by the air. When the barometer rises above the mean state of 29,5 inches, water will not boil but with an heat superior to 212 degrees; and when it falls below the mean state, the same water will boil with an heat below 212. But when the pressure of the atmosphere is almost entirely removed in the vacuum of an airpump, the difference is very observable indeed; for water will then boil with an heat not exceeding 95 degrees, which is 117 degrees below the heat required in the open air. And hence it appears that fire and air act as antagonists in this operation of boiling; fire on the internal substance of fluids. from beneath, and air upon their surface from above.

There is likewise a certain condition of the boiling fluid itself which occasions a great difference: for where the specific gravity is greater, the fire, having a denser matter to overcome, will not raise any heavy liquors to a boiling state but with a great degree of heat; of which I need produce no other examples at present than these two of alcohol (or pure spirit of wine) and mer

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cury; the former of which being one of the lightest fluids, boils at 176°; the mercury, which is the heaviest of all, at 700*. pressed vegetable oils are not reducible to this rule on account of their tenacity, and probably for some other reasons in the natural constitution of them, too obscure to be pointed out.

That fire, when it makes a liquor boil, introduces a vacuum against the pressure of the air, is very conspicuous in the following experiment on boiling mercury. Into a common tube for a barometer pour some mercury to the depth of about 6 or 8 inches, and heat it by degrees over a dish of charcoal. When it begins to boil, you will see a cavity made by the fire lifting up the fluid from the bottom of the tube; but as soon as this happens, the air immediately presses it down again with a violent shock; and these alternate

* Fahrenheit gives us the following short table of boiling heights and specific gravities; Ibid. p. 50.

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alternate efforts of fire and air, which are now a balance to each other, occasion a continual knocking against the glass at the bottom. To shew that the mercury does not occasion this knocking by its weight in falling, let the tube be inclined near to an ho rizontal position; notwithstanding which the effect will be the same as before; and therefore it arises from the atmospherical pressure restoring the vacuum as fast as the fire produces it. By a vacuum in this place, it is obvious that nothing can be meant but a space without air; because fire in strictnesscan never constitute a vacuum.

Of Solution.

The solution of bodies must follow as the natural effect of fire, for these two reasons; first, because it expels that mediating substance, whatever it may be, which serves as a cement to bind their own native parts together: and secondly, because it separates their parts, by its expansive action, to such a distance that they lose in some cases their solidity, in others their continuity.

Mere earth is a friable substance, which will not ordinarily cohere, unless it is connected

nected by the addition of water, salt, oil, sulphur, or something of the like kind; all of which answer this one purpose of filling up its pores, and excluding the air or ether from a free circulation within; in consequence of which, it presses without, and acts upon the solid parts, instead of acting between them.

When fuel of any kind is burnt in the fire, it is dissolved, on this principle, that all the ingredients which bound it together are driven out of it. When a log of wood begins to burn, the water is first expelled in a moist cloud, and ouzes out at the extremities: the oily, sulphureous, and saline parts, are next carried off in a denser smoke and flame; and the residue, having lost the matter by which its pores were filled, falls asunder in the form of ashes. But fire alone, without the concurrent force of the air, will not accomplish this particular separation. A piece of charcoal closely shut up in an iron box, and laid for a long time in the strongest fire, will be unchanged: whereas, if it had been exposed for a twentieth part of the time to the action of a naked fire, where the air has free access, it would have fallen into powder.

Any kind of body, which if applied to

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another

another body acts upon it so as to dissolve its parts, is called by the chemists à menstruum. The term itself is mysterious and insignificant; the application of it is very well known. Aquafortis is a menstruum to iron, and to all other metals except gold: spirit of wine is a menstruum to resinous bodies; and upon this principle all the common varnishes are made: water is a menstruum to gums and salts; and air is a menstruum to water. The two conditions requisite to the operation of a menstruum, are a certain accommodation in the parts of the solvent to those of the solvend, and a proper degree of motion or agitation for applying the parts of one to the parts of the other: of which motion fire is the general cause; and the whole affair may be thus illustrated. If a ball of clay is laid in cold water, it remains at rest, and the fluid continues pure as before: but if this water is set over the fire till it boils, the clay is soon diffused through it, and the whole continues turbid, so long as the parts of the water are agitated by the fire. when the water grows cold, the clay subsides to the bottom, and leaves the water clear. The parts of the clay being specifically heavier than those of the water, ought to sub

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