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Show , I . 122 COHESION. ledge of it. In order to that, there must be something which we can call ~ubstance, and that su~s~at~~~ must be of some extenswn, or measur~, or h.ul ' ·e is, it must occupy space, and space m ~h1Ch thei can be no other substance at the same time. That space must be of some shape or .fip;ure, too; and the shape of the space must be exactly the same as t~at of the body which fills it; and the substan.ce wh1~h thus occupies space must have some conszst~ncy, m order that we may know by muscular resistance that it exists; and its consistency ~nust depend, n~t only on what h.as been already notiC~~ as Its gra.vltation of quantity of matter and position, by w h1c!t it keeps its place among other substances, but It must have a consistency of its own, ?Y means of which it preserves its ~hape. That IS called the cohesion of the body, which means the tendency that the still smaller bodies, of which we mus~ suppose even the very smallest that we can ex3:mme to be made up, have to stick together. And .th~s last property, of which there are manr vanettes, ~as. no neces.sary connexion wi.th umversa~ grav1~atwn. The same bu1k of water Is, under ordmary c~rcumstances much heavier than cork, for cork swims on the surface of water; but water can be hel9 up in a vessel made of cork. Indeed, the cork IS really heavier than water; for it may be so soaked in water that it will sink like a stone, and at the same time be no lar<rer than before. But the water cannot possibly be h;avier than itself_; an.d so the c<!rk must be heavier than water. Qmcksllver too IS much heavier than glass; and yet it can be contained in. a glass vessel; but still quicksilver, though when la1.d or poured on the table, it sprea~s, yet shows that It has still the property of coheswn, though only to a limited extent. 'rhat principle or property cannot resist the action of a pound or an ounce; but the little beads of it are quite round, and they dance. about like small balls of polished steel. CONSISTENCY. 123 It is necessary that the observer of nature, if he is to be any thing higher than a mere "unmeaning gazer" (and those who are contented with that have little chance of reading these pa<res, even though ten times simpler than they ar~), should have accurate notions of the consistency of substances. It is that which the unaided senses immediately observe; and all the changes or phenomena ... that take place in nature are brought about through the medium of the consistency of bodies. Be the sensation what it may, taste, odour, sound, colour, warmth, or resistance, still it is in the consistency of the body that we find it out; and after mere motion, which though in conjunction with other agencies it often changes the appearances of things, is nothing but change of place (and place without substance cannot be known at all), all our knowledge of action or appearance in nature is change of con-sistency. · There may be changes of consistency brought about by motion and resistance : and the motion may be the result of any thing that can cause motion, and the resistance any thing that can oppose motion. Thus bodies that are compressible may be squeezed into less space by the gravitation of heavy weights placed on them. Cheese is generally pressed in that way; and smoothing-irons and mangles compress the linen by their weight. A less compressible body being made to approach a more compressible one very s]owly will squeeze it together with far greater force than any weight that is at all manageable. Screw-presses, which are used for so many purposes, and Bramah's hydraulic or water-press, act on that principle. The finer the threads of the screw are the more powerful is the press ; and so, as there are no particles or parts that the eye can find or the finger touch in water, so as to take note of their magnitude the column of water which moves in the cylinder of Bramah's press is |