66. =Particles Must be very Near to Each Other to Adhere.=--Why is it that when you have broken any thing made of glass, however accurately you may bring the two parts together, you can not make them unite in one again? It is simply because the particles of substances will not attract each other enough to be united unless they are brought very near together. Now it is impossible to bring the particles on the two surfaces of a broken piece of glass as near together as they were before it was broken. If you could do so, no crack would be visible. You can join them by some kind of cement. This is because the particles of the cement, while it is soft, can be insinuated among the particles of the glass; and thus, when it dries, it becomes a bond of union between the particles on each side of the breach. For the same reason you can make the cut surfaces of some yielding substances adhere. If you divide a piece of India-rubber with a clean cut, you can make the two surfaces adhere by pressing them together firmly. The particles in this case are not unyielding, as those of the glass are, and some of them are therefore brought into such near neighborhood as to attract each other sufficiently to unite together. So, too, if you cut two bullets so as to have a very smooth flat surface on each, you can make them adhere quite strongly by pressing them together, especially if you give a little turning motion at the same time that you press, for this will cause the particles on the two surfaces to be somewhat mingled together. If you have quite large balls of lead with handles, as represented in Fig. 14, it will require considerable force to separate them when they have been thus pressed together. 67. =Other Illustrations.=--Silver and gold may be made to adhere to iron by a very great and sudden pressure. The iron must be made very smooth, and the silver or gold plate very thin. A powerful blow brings the particles of the thin plate into such nearness to those of the iron that union is effected, or, in other words, that they attract each other sufficiently to be united. So, also, a sheet of tin and one of lead can be made to adhere so as to form one sheet by the pressure of the rollers of a flatting-mill. Two very smooth panes of glass laid one upon another may have so many particles brought into great nearness as to occasion some adhesion. It will be slight, however, few comparatively of all the particles coming near enough to adhere, for the smoothest glass is full of inequalities, as may be seen by the microscope.