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ence, we have found that air has no power to diminish weight; for a blown bladder is not lighter than an empty and compressed one; nor is a sponge or a fleece of wool full of air, lighter than the same when empty, or with the air squeezed out. But the bodies of animals differ sensibly in their gravity before and after death: though not so much as is commonly conceived.* Therefore air seems not at all to diminish gravity; as the living spirit appears to do: and as weight is the criterion of density; so the diminution of weight should be the criterion of rarity.†

Flame comes last in this series; because flame manifestly ascends: and again, because the na

It should seem as if the author had made some experiments about the weight of animal bodies alive and dead; and that these experiments did not agree with those of Mr. Boyle; who upon weighing a live mouse and a kitten, then strangling them, and immediately weighing them again, found them a little lighter. See Abridgm. Vol. II. p. 527. This matter requires to be farther prosecuted; and at the same time it should be fully tried, whether eggs gain in weight upon their animation.

Let the distinction of specific and absolute gravity, be well remembered; and that in common weighing, the experiment is made in a gravitating fluid. Thus if the body collapse, or shrink, after death, it might when weighed in air, prove heavier upon the balance, than when alive. This affair seems subject to many contingencies and it requires great caution to make a valid experiment therein.

tures of pneumatical bodies differ not from those of the tangible bodies that supply them. And, therefore, as oil is rarer than water; so flame should be rarer than air, and spirit. Again, flame seems to be a thinner, softer, and more yielding body than air; since the lightest commotion of the air near a burning taper, will cause the flame to tremble.


How great soever the difficulty might be of discovering the expansion of a pneumatical, with regard to a tangible body; yet we have not despaired thereof: and it seems to us a very certain kind of proof, if any tangible body of a known expansion, could be converted into a pneumatical body and then the expansion of that be likewise observed; so as, from a comparison of the two proportions, an evident demonstration might be had of the dimensions upon the rarifaction.

We, therefore, took a small glass vial, capable of containing about an ounce; and poured into it half an ounce of spirit of wine; which being the lightest of liquors, comes nearest to a pneu→ matical nature: then taking a new and large bladder, capable of containing a gallon, and squeezing all the air out of it, as exactly as possible, till the sides came close together; and also rubbing its outside well with oil, to make it still

more close and pliable; we tied its neck tight, over the mouth of the vial, with a wax thread. We now placed the vial over warm embers in a chaffing-dish; when presently the vapour of the spirit of wine ascended into the bladder: and strongly inflated it every way. Then immediately removing the glass from the fire, and pricking a hole in the top of the bladder, that the vapour might rather get out, than fall back into drops; we took the bladder away from the vial, and examined by the balance what proportion of the half ounce of spirit was wanting, or turned into vapour; and found it to be not more than six pennyweight so that six pennyweight of spirit of wine, which in that state did not possess above one fortieth part of a pint, being turned into vapour occupied the space of eight pints.


The bladder began to grow somewhat flaccid upon being removed from the fire; so that notwithstanding such a considerable expansion; the vapour did not seem converted into a pure and. fixed pneumatical body; but inclined to recover itself. And this experiment may prove fallacious, if it be hence conjectured that common air is still rarer than this kind of vapour; because we conceive that spirit of wine turned pneumatical, though but imperfectly, does, by reason of the heat, exceed the rarity of cold air; as air it

self is by heat dilated very considerably, and greatly exceeds the expansion of cold air. Whence we judge that if the experiment were made with water, the degree of expansion would be much less though the body of the water contained more matter than spirit of wine.


If the fume proceeding from a wax-taper, newly put out, be viewed, and an estimate be formed of its dimensions by the eye; and again, if the body of the fume be afterwards set on fire, the expansion of the flame will appear to exceed that of the fume, as about two to one.


A few corns of gunpowder being set on fire, there appears to be a great expansion made, with respect to the body of the powder; though when the flame is extinguished, the body of the fume expands itself much more. But let it not hence be supposed, as if the tangible body were more expanded in fume than in flame; the reason of the phenomenon being this; that flame is an entire body, but fume a body mixed with a much larger portion of air: and therefore, as a little saffron tinges a large quantity of water, so a little fume diffuses itself in a large proportion of air. For a thick, dense fume, not diffused, appears less than the body of flame; as we before observed.

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A piece of fresh orange-peel being suddenly squeezed betwixt the fingers, and directed to the flame of a candle, there starts out a dewy, oily, aromatic matter, in fine drops. that makes a very large body of flame, in respect of those little drops.


Peripatetical fiction, as to the rarity of the ele→ ments being in a tenfold proportion to one another, is arbitrary and hypothetical: for it is certain that air is, at least, a hundred times rarer than water; and flame a hundred times rarer than oil; and that flame is, at least, ten times rarer than air.


This enquiry and speculation, about pneumatical bodies, should not be thought too subtile or too curious; because it is certain that an omission, and want of attending to it, has stupefied both philosophy and medicine; and rendered them, as it were, planet-struck in the true investigation of causes: whilst they have unprofitably attributed those things to qualities, which are owing to spirits.* And so much for the enquiry into the expansion of matter in bodies, according to their different consistencies, whilst at


* See the axioms to the History of Life and Death.

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