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has its own proper size, and is with difficulty stretched to any 

 other. Since, however, this motion of liberty is the most obvi- 

 ous of all, and to be seen in an infinite number of cases, it will 

 be as well to distinguish it correctly and clearly ; for some most 

 carelessly confound this with the two others of resistance and 

 connection; namely, the freedom from pressure with the for- 

 mer, and that from tension with the latter, as if bodies when 

 compressed yielded or expanded to prevent a penetration of 

 dimensions, and when stretched rebounded and contracted them- 

 selves to prevent a vacuum. But if the air, when compressed, 

 could be brought to the density of water, or wood to that of 

 stone, there would be no need of any penetration of dimensions, 

 and yet the compression would be much greater than they actu- 

 ally admit of. So if water could be expanded till it became 

 as rare as air, or stone as rare as wood, there would be no need 

 of a vacuum, and yet the expansion would be much greater than 

 they actually admit of. We do not, therefore, arrive at a pene- 

 tration of dimensions or a vacuum before the extremes of con- 

 densation and rarefaction, whilst the motion we speak of stops 

 and exerts itself much within them, and is nothing more than 

 a desire of bodies to preserve their specific density (or, if it be 

 preferred, their form), and not to desert them suddenly, but 

 only to change by degrees, and of their own accord. It is, how- 

 ever, much more necessary to intimate to mankind (because 

 many other points depend upon this), that the violent motion 

 which we call mechanical, and Democritus (who, in explaining 

 his primary motions, is to be ranked even below the middling 

 class of philosophers) termed the motion of a blow, is nothing 

 else than this motion of liberty, namely, a tendency to relaxation 

 from compression. For in all simple impulsion or flight through 

 the air, the body is not displaced or moved in space, until its 

 parts are placed in an unnatural state, and compressed by the 

 impelling force. When that takes place, the different parts 

 urging the other in succession, the whole is moved, and that 

 with a rotatory as well as progressive motion, in order that the 

 parts may, by this means also, set themselves at liberty, or more 

 readily submit. Let this suffice for the motion in question. 



Let the fourth be that which we term the motion of matter, 

 and which is opposed to the last ; for in the motion of liberty, 

 bodies abhor, reject, and avoid a new size or volume, or any new 

 expansion or contraction (for these different terms have the 

 same meaning), and strive, with all their power, to rebound and 

 resume their former density ; on the contrary, in the motion of 

 matter, they are anxious to acquire a new volume or dimension, 

 and attempt it willingly and rapidly, and occasionally by a most 

 vigorous effort, as in the example of gunpowder. The most 

 powerful, or at least most frequent, though not the only instru- 



