10. Between the degrees of hard and soft are those things which we call _tough_, tough being that which may be bent without being altered from what it was; and the bending of a line is either the adduction or diduction of the extreme parts, that is, a motion from straitness to crookedness, or contrarily, whilst the line remains still the same it was; for by drawing out the extreme points of a line to their greatest distance, the line is made strait, which otherwise is crooked. So also the bending of a superficies is the diduction or adduction of its extreme lines, that is, their dilatation and contraction. [Sidenote: Dilatation & contraction suppose mutation of the smallest parts in respect of their situation.] 11. _Dilatation_ and _contraction_, as also all _flexion_, supposes necessarily that the internal parts of the body bowed do either come nearer to the external parts, or go further from them. For though _flexion_ be considered only in the length of a body, yet when that body is bowed, the line which is made on one side will be convex, and the line on the other side will be concave; of which the concave, being the interior line, will, unless something be taken from it and added to the convex line, be the more crooked, that is, the greater of the two. But they are equal; and, therefore, in flexion there is an accession made from the interior to the exterior parts; and, on the contrary, in tension, from the exterior to the interior parts. And as for those things which do not easily suffer such transposition of their parts, they are called brittle; and the great force they require to make them yield, makes them also with sudden motion to leap asunder, and break in pieces. [Sidenote: All traction is pulsion.] 12. Also motion is distinguished into _pulsion_ and _traction_. And pulsion, as I have already defined it, is when that which is moved goes before that which moveth it. But contrarily, in traction the movent goes before that which is moved. Nevertheless, considering it with greater attention, it seemeth to be the same with pulsion. For of two parts of a hard body, when that which is foremost drives before it the medium in which the motion is made, at the same time that which is thrust forwards thrusteth the next, and this again the next, and so on successively. In which action, if we suppose that there is no place void, it must needs be, that by continual pulsion, namely, when that action has gone round, the movent will be behind that part, which at the first seemed not to be thrust forwards, but to be drawn; so that now the body, which was drawn, goes before the body which gives it motion; and its motion is no longer traction, but pulsion. [Sidenote: Such things as being pressed or bent restore themselves, have motion in their internal parts.]