twice as much as in the foregoing experi

ment.

These considerations have made me easy in regard to the immediate cause of the air's elasticity. I heartily wish they may have the like influence upon others, and get the better of all their scruples. If fire, that universal and active element, can occasion such a property in the air, by a mechanical and impulsive motion, the thing is intelligible. By this means we become possessed of one important truth in natural philosophy, and such as may in time direct us to many others. Could it once be shewn, that air, totally emptied of its fire, has any of this elastic property in it*, it would then be time enough to call in the assistance of repulsion, which is but the same thing with giving up the point as unintelligible. But unless elastic air can be éxhibited under such circumstances, (which cannot be, unless the thermometer be first made to sink 290 of Fahrenheit's degrees below the freezing point,) there is no more occasion for this quality than for that of cohesion.

To

Non tantum aer in ignem transit, sed nunquam sine igne est. Detrahe illi calorem, rigescet, stabit, durabitur, Senec. Nat. Quæst. lib. 3,

To those who examine this matter impartially, these powers will appear incredible in their very nature. The effects which are explained by attraction and repulse, do so often. vary, that we are obliged to suppose the same particles of matter endued with both; with one virtue of drawing and alluring bodies, and another of terrifying and driving them away at the same time. But how, in the nature of things, can two principles, destructive of each other, reside in the same subject? Repulsion is as opposite to attraction, in my way of conceiving it, as darkness to light, or cold to heat; and it is equally impossible that the same particle should both attract and repel, as that it should be both black and white, hot and cold, at the same time. reconcile this, some have invented several concentric spheres, or involucra of contrary powers, surrounding the same atom of matter; so that when other matter approaches the atom, thus invested, as it were, with the coats of an immaterial onion, it meets first with a repelling sphere; being forced somewhat nearer, it falls in with an attracting sphere; and coming yet nearer, it meets with a repelling sphere, to keep it from immediate contact. What a complication of causes is

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To

here!

here! and all to make one poor atom vibrate through less than the 1000th part of an inch! This can never be agreeable to that simplicity of nature, so much boasted of by all philosophers; and if our own senses are to have any weight, the experience of every day might lead us to something more true, useful, and intelligible.

CHAP. VI.

The Application of Cohesion, as an unme chanical Principle, serves only to keep us in Ignorance. This is proved from a plain Example.

O bring the argumentative part of this

T%

book to a conclusion, I shall take the subject of vegetation, and compare it with the favourite principle which hath been applied of late years for the explanation of it by our systematical writers; in order to shew, by one example out of many, that these powers, which have unhappily been substituted in the place of the true agents, do not only not

explain

explain any effects, but tempt us to conceive falsely of nature, and to affirm what is contrary to experience. My only reason for driving the argument thus far, is to encourage those, who pretend to be guided by experiment, to endeavour at some farther progress, and not for ever assert their right to stop, when they might go farther on with pleasure and safety.

When a glass tube of a very small bore is dipped into water, or any other fluid except mercury, the fluid will be raised to a certain height within the tube above the surface of the liquor in the vessel; and its elevation, in several tubes of different sizes, will be reciprocally as the diameters of their bores.

The ingenious Mr. Rowning will have it"it is drawn up by a tendency it has by the "principle of attraction, till the surface is "loaded with as great a weight as that ten"dency can support*." Not to take advantage here of the author's terms, (which to be sure are unintelligible,) let us for a while. agree with him; and suppose the inner surface of the tube to attract the water.

Hence (says he) a RIGHT NOTION of "the ascent of sap in vegetables †."

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Pref. p. 17. 18.

I. Now,

+ Ibid.

I. Now, if the sap ascends in a vegetable, on the same principle that water rises in a capillary tube, of which he seems to have no doubt; let us take a capillary tube, in which, for example, water will be raised to the height of two inches. In the tubes of a plant, therefore, whose bores are of the same diameter, the sap should be raised to the same height, but no higher *. On the contrary, I have observed an heavy and viscous juice to issue plentifully at the height of two feet; and that out of vessels, considerably larger than the orifice of a capillary tube, within which pure water would not rise to the height of two inches. It is hardly worth while to relate the history of so easy an experiment: however, the plant I made this trial upon, was a very large specimen of the Tithymalus helioscopius, or common sun-spurge, the trunk of which is without a joint, and was cut transversely near the top of it; after which a section of its vessels was compared, in a microscope, with a section of the glass tube.

II. It

Dr. Grew has the same remark-" Although we see "that small glass-pipes, immersed in water, will give it an ascent for some inches; yet there is a certain period, accord, "ing to the bore of the pipe, beyond which it will not rise. Anat. of plants, p. 126.