If the two aeriform fluids, called oxygen and hydrogen, be mixed together in a certain proportion, the compound will be water. In this case, the components are different from the compound, not merely in the one being _air_ and the other _liquid_, but in other respects not less striking. The compound water extinguishes fire, and yet of the components, hydrogen is one of the most inflammable substances in nature, and the presence of oxygen is indispensably necessary to sustain the phenomenon of combustion. Oxygen gas, united with quicksilver, produces a compound of a black colour, the quicksilver being white and the gas colourless. When these substances are combined in another proportion, they give a red compound. (105.) Having noticed the principal molecular forces, we shall now proceed to the consideration of those attractions which are exhibited between bodies existing in masses. The influence of molecular attractions is limited to insensible distances. On the contrary, the forces which are now to be noticed act at considerable distances, and to the influence of some there is no limit, the effect, however, decreasing as the distance increases. The effect of the loadstone on iron is well known, and is one of this class of forces. For a detailed account of this force, and the various phenomena of which it is the cause, the reader is referred to our treatise on MAGNETISM. When glass, wax, amber, and other substances are submitted to friction with silken or woollen cloth, they are observed to attract feathers, and other light bodies placed near them. A like effect is produced in several other ways, and is attended with other phenomena, the discussion of which forms a principal part of physical science. The force thus exhibited is called electricity. For details respecting it, and for its connection with magnetism, the reader is referred to our treatises on ELECTRICITY and ELECTRO-MAGNETISM.