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nerally agree among themselves. We cannot always judge of compounds from the principles into which they are chemically resolvable; because the operation which should only open and unfold the subject under trial, has the power of changing it, by a fresh composition, into some new thing which did not before exist. When wood has undergone the action of fire, an alcaline salt is discovered in the ashes; but nothing of this kind is found to exist in the wood, so long as it remains in its natural state: so that when we aim at a solution, we have a new composition; and hence the tests of chemistry are often very deceitful : yet we are obliged to have recourse to them on occasion of our present inquiry, and shall find them of great service when used with discretion.
Whatever substance will stand the trial, so as not to be divided into any others; or, which cannot be composed by the union of any others; that substance may properly be taken as an element.
Earth, in its siinple state, is that white friable substance which remains in a bone, after all the other principles are burnt out of it; and will endure the utmost violence of fire, without being consumed or vitrified.
The same element is the basis of all stones and common earths; though in the different sorts it has very different appearances, from some new arrangement of the parts, and from the introduction of foreign principles. When lime-stone is burnt, we might reasonably expect to obtain a simple radical earth by the operation ; and indeed it is nearly such, being extremely white and pure, especially when it falls into powder as it slackens in the air : but we discover plainly by its effects, that fire is combined with it; and this element, in spite of all our care, will attach itself to such bodies as we attempt to analyze by exposing them to the action of it. When the alcaline or lixivial salt is all washed away from the ashes of burnt vegetables, we have a permanent, substance left, which will retain its simplicity under all farther trials; and therefore we admit it as another specimen of the element of earth in its simple form. But the purest earth is obtained by. the distillation of rain water, which leaves a powder at the bottom of the vessel, consisting of those fine terrestrial corpuscles, which are carried up and distributed in the atmosphere from the smoke of burnt vegetables.
Water in springs, rivers and seas, is a compound of many ingredients : but as none of the adventitious matter will rise with it into vapour,
may be procured in its simple form by evaporation or distillation : and if the fluid thus, obtained be rarefied into vapour never so often, it is capable of no farther purification, and will always return by condensation into the same fluid as before. So that here we have another homogeneous substance, which we can neither destroy nor decompose, and may therefore take it as another element.
When all humidity is extracted from the air, and it is cleared of all other extraneous particles that float in it, there remains an elastic fluid, capable of contraction and dilatation in a very high degree, and distinct from all other fluids in the world except fire, which no art can separate from it entirely: and therefore, though custom hath always taken air for a separate element, yet can it never be exhibited in its simple form; and it would perhaps be more agreeable to nature to class it with fire as a fluid of the same elementary nature. By its elastic force on the surfaces of bodies, and by its obscurity, it differs from fire, which becomes luminous with agitation, and can penetrate to the internal substance of all bodies : in other respects it is closely allied to it, so that it is hard to say where air ends and æther or elementary fire begins, unless we allow that air is always ascertained by its superficial pressure. Such is the relation between these two fluids, that many writers have insensibly fallen into the practice of calling them by the same name. Spirit is applied indifferently by the chemists to fire and air. The etherial part of cyder and beer is called. spirit, and comes forth from these liquors as a flatulent air. The etherial part of brandy is also called spirit, though we cannot extract any air from it, and its inflammability and heat shew it to be impregnated with fire. The ancients, by the word πνευμα, do not always hean gross tangible air, but that finer etheriál spirit which pervades the substance of bodies, and is to be found in the upper regions of the heaven. Dr. Hales, an English philosopher, speaks of the aerial particles of fire, as if there was a substance common in some degree to both fluids. So intimate is the relation between them in the experiments of electricity, that it is difficult in many cases to determine how far an effect is owing to
the air, and how far it is owing to fire: but this is a subject, the farther prosecution of which belongs to another place.
Though air cannot be obtained in a state of separation from the purer element of fire, fire itself may be found in a simple form, because fire can go where air cannot follow it: thus much at least is true, that we can remove the
grosser parts of the etherial fluid called air, and leave none remaining but that subtile part of the same fluid, which can pass through the pores of glass and all other solids. This may be effected either by the air pump, the Torricellian tube, or the intense heat of a furnace. When the gross air is extracted from a pneumatic receiver, the remaining medium will easily manifest itself upon any friction or agitation; and, as it was observed by Sir Isaac Newton, will affect a thermometer the same as in any other space where the air is present. In the space at the top of the tube of a good barometer, this fluid will appear when the mercury is made to vibrate; and it is said to be capable of being expanded, when heat is applied to it, so as to depress the mercury in the tube. In the heat of a furnace, the medium is so pure, notwithstanding the air hath ac