themselves to be divided, or separated from others, on account of the mutual contact and connection they affect, which is a motion called by the schools the motion preventive of vacuity, as when water is forced upwards by suction, or by the pump, or when the flesh is raised by cup ping-glasses, or as when water runs through a siphon, with numerous other examples of the the same kind*.

Let the third motion be what we call motion of liberty, or freeing motion, whereby bodies endeavour to ease and free themselves from any preternatural pressure or tension, or restore themselves to the dimensions agreeable to their own natures. And of this motion also the examples are numerous. Thus water endeavours to free itself from pressure in the action of sailing and rowing; the air, in the action of flying, and in the undulations of the winds; the springs in watches; compressed air in pot-guns, which forces out the pellet, &c.

And, for this motion of liberty in the case of tension, we see the air frees itself therefrom, when it remains after suction in glass eggs t; we see it also in strings, leather, cloth, &c. which

* Let it be considered, whether all these motions are justly explained by the weight, spring, or pressure of the air.

restore themselves after tension, unless where they remained so long stretched, as to have their motion of liberty subdued. This motion is unscientifically called, by the schools, the motion of the elementary forms; for it does not only belong to air, water, and flame, but to all the diversities of consistent bodies, as wood, iron, lead, cloth, skins, &c. each body having its own measure of extent or dimension, from whence it is with difficulty stretched to any considerable distance*.

But as this motion of liberty is, of all others, the most obvious, and also extremely general, it requires to be well and clearly distinguished; for some very inadvertently confound it with the motion of resistance and the motion of connexion. Thus when, by this motion, matter frees itself from pressure, they confound it with the motion of resistance; and when matter frees itself from tension, they confound it with the motion of connexion; as if compressed bodies yielded or dilated themselves only to prevent a penetration of dimensions; and as if, when stretched, they restored or contracted themselves, to prevent a vacuum; whereas if compressed air were to become as dense as water

See the History of Condensation and Rarifaction; and the article Motion in the Sylva Sylvarum.

or wood as dense as stone, there would be no danger of a penetration of dimensions, even though these bodies could be much farther compressed than they any way admit of. So likewise, if water were to be dilated to the same degree of rarifaction with air, or stone to the same degree of rarifaction with wood, there is no necessity that a vacuum should ensue, even though these bodies could be extended much farther than their natures any way allow*. And therefore the case is not brought to a penetration of dimensions and vacuity, except in the utmost limits of condensation and rarifaction; whereas these motions stop far short of those limits, as being no more than appetites which bodies have of continuing themselves in their own consistencies or dimensions, and not suddenly departing from them, unless altered by gentle means and by consent.

But it is much more necessary, as being a thing of great consequence, to observe that violent motion, as it is called, is no other than this very motion of liberty, tending from compression to relaxation, For in all simple protrusion, or flight through the air, there is no tendency to motion or change of place before the parts of

*See the Author's History of Condensation and Rarifaction.

the body suffer preternaturally, and are compressed by the impelling force; whence it is that some parts successively pressing against the rest, the whole body is driven off, or protruded; and not only in a progressive, but at the same time in a rotatory or revolating motion, so as that by this means also the parts may free themselves, or suffer more equally.

Let the fourth motion be the motion of extension; which is, in some measure, the opposite to the motion of liberty; for in the motion of liberty, bodies avoid, dread, and fly from a new dimension, or a new degree of dilatation or contraction, and endeavour, with all their force, to recover their former state; whereas, on the contrary, in the motion of extensión, bodies affect a new dimension, and sometimes willingly and hastily aspire to it, even with a violent endeavour, as we see in gunpowder. And the most powerful, or at least the most common, though not the only instruments of this motion, are heat and cold.

For example, if air be dilated' by tension, as it is in suction, it has a great appetite of restoring itself; but if heat be applied thereto, it, on the contrary, tends to dilate, and occupy a new dimension, and goes willingly over thereto, as into a new state, and after some continuance of this dilatation, it has no great appetite to return, un

less solicited thereto by the application of cold, which is no spontaneous return, but a repeated change.

So likewise water, when compressed, resists and endeavours, as it were, to enlarge itself; but upon the continuance of intense cold, it spontaneously and readily concretes into ice; and if the cold continues very long, without interruption from any warmth, as perhaps sometimes happens in the deeper caverns, the water is turned into crystal, or matter approaching thereto, and never recovers the nature of water again

Let the fifth motion be the motion of continuity; by which we do not mean a simple and primary continuation with any other body, for that is motion of connexion, but of self-continuation in a certain body. For it is manifest, that all bodies resist a solution of their continuity; some indeed more, and others less, but all of them in a degree. And, as in hard bodies, such as steel and glass, the resistance of discontinuation is very strong; so likewise in liquors, where this motion seems either to cease, or at least to languish, yet it is not absolutely wanting, but it plainly resides in them, as it were, in the lowest degree, and discovers

See the Sylva Sylvarum, under the article Transmutation. See also Mr. Boyle, upon the Origin of Gems.