All solids, however, do not repel mercury. If any golden trinket be dipped in that liquid, or even be exposed for a moment to contact with it, the gold will be instantly intermingled with particles of quicksilver, the metal changes its colour, and becomes white like silver, and the mercury can only be extricated by a difficult process. Chains, seals, rings, &c. should always be laid aside by those engaged in experiments or other processes in which mercury is used. (103.) Of all the forms under which molecular force is exhibited, that in which it takes the name of _affinity_ is attended with the most conspicuous effects. Affinity is in chemistry what inertia is in mechanics, the basis of the science. The present treatise is not the proper place for any detailed account of this important class of natural phenomena. Those who seek such knowledge are referred to our treatise on CHEMISTRY. Since, however, affinity sometimes influences the mechanical state of bodies, and affects their mechanical properties, it will be necessary here to state so much respecting it as to render intelligible those references which we may have occasion to make to such effects. When the particles of different bodies are brought into close contact, and more especially when, being in a fluid state, they are mixed together, their union is frequently observed to produce a compound body, differing in its qualities from either of the component bodies. Thus the bulk of the compound is often greater or less than the united volumes of the component bodies. The component bodies may be of the ordinary temperature of the atmosphere, and yet the compound may be of a much higher or lower temperature. The components may be liquid, and the compound solid. The colour of the compound may bear no resemblance whatever to that of the components. The species of molecular action between the components, which produce these and similar, effects, is called _affinity_. (104.) We shall limit ourselves here to the statement of a few examples of these phenomena. If a pint of water and a pint of sulphuric acid be mixed, the compound will be considerably less than a quart. The density of the mixture is, therefore, greater than that which would result from the mere diffusion of the particles of the one fluid through those of the other. The particles have assumed a greater proximity, and therefore exhibit a mutual attraction. In this experiment, although the liquids before being mixed be of the temperature of the surrounding air, the mixture will be so intensely hot, that the vessel which contains it cannot be touched without pain.