Cohesion thus unites particles of a similar kind; chemical affinity, of a dissimilar nature. It is to cohesion that the existence of _masses_ of matter is owing, and its power increases as the squares of the distances diminish, in an inverse ratio to the squares of the distances of the particles on which it acts. The power exerted by cohesion may be exhibited in various ways. This is one: Procure two discs of glass about three inches in diameter, their surfaces being ground extremely smooth; fix each into a square piece of wood, taking care that they are placed accurately in the center; then put them together by sliding their edges very carefully over each other, so as to avoid any air getting between them, and you will find a great force necessary to separate them. A hook should be fixed into the center of each piece of wood, so that they may be suspended, and a weight hung to the lower one. It is almost impossible for any one to separate them by merely pulling them with both hands; a weight of many pounds is required for that purpose. In like manner two freshly-cut surfaces of caoutchouc will, on being squeezed together, cohere so perfectly, that it is difficult to tear them asunder, and it is in this way that tubes of caoutchouc may be rapidly prepared for experiments, where little or no pressure is exerted. Chemical affinity is sometimes called _elective_, or the effect of _choice_, as if one substance exerted a kind of _preference_ for another, and chose to be united to it rather than to that with which it was previously combined; thus, if you pour some vinegar, which is a weak acetic acid, upon some pearlash (a combination of potash and carbonic acid), or some carbonate of soda (a combination of the same acid with soda), a violent effervescence will take place, occasioned by the escape of the carbonic acid, displaced in consequence of the potash or soda preferring the acetic acid, and forming a compound called an acetate. Then if some sulphuric acid be poured on this new compound, the acetic acid will in its turn be displaced by the greater attachment of either of the bases, as they are termed, for the sulphuric acid. Again, if into a solution of blue vitriol (a combination of sulphuric acid with oxide of copper) the bright blade of a knife be introduced, the knife will speedily be covered with a coat of copper, deposited in consequence of the acid _preferring_ the iron, of which the knife is made, a quantity of it being dissolved in exact proportion to the quantity of copper deposited.