182 NOVUM ORGAN DM. 



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 penetration of 

 dimensions or a vacuum before the extremes of condensa 

 tion 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 sud 

 denly, but only to change by degrees, and of their own ac 

 cord. It is, however, 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 philo 

 sophers) 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 mo 

 tion of liberty, bodies abhor, reject, and avoid a new size or 

 volume, or any new expansion or contraction (for these dif 

 ferent 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 wil 

 lingly 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 instruments 

 of this motion, are heat and cold. For instance ; the air, 

 if expanded by tension (as by suction in the glass egg), 

 struggles anxiously to restore itself; but if heat be applied, 

 it strives on the contrary to dilate itself, and longs for a 

 larger volume, regularly passing and migrating into it, as 

 into a new form (as it is termed) : nor after a certain degree 

 of expansion is it anxious to return, unless it be invited 

 to do so by the application of cold, which is not indeed a 

 return, but a fresh change. So also water, when confined 



