GEOLOGY OF CLAYS.
In order to understand the geology of clays it is necessary to think of the surface layers of the earth as an immense chemical laboratory in which exchanges are taking place on every hand, old compounds breaking down and being replaced by new ones continually. In this case it must be borne in mind that nature's laboratory differs from those with which we are familiar in that in hers the conditions are not constant but are continually changing, while in ours they are practically fixed and uniform.
Elementary Chemical Principles—§ 1. In considering whether a chem ical union will probably take place when two substances are brought to gether, it is necessary to think not only of the materials mixed but of the conditions under which they are placed. For example, oxygen and hydrogen may be mixed in a tube in the proper proportions to form water, and if kept in the dark will continue indefinitely as a mechanical mixture of gases, but if exposed to sunlight or to the passage of an electric spark union at once takes place. Again air and illuminating gas may be mixed in the cold without union, but once the temperature is raised to the proper point the mass bursts into flame. Certain substances will combine in the cold which refuse to do so when the temperature is elevated. Some unite under heavy pressure and separate again when the pressure is reduced. Some unite in the presence of a third substance, which, however, does not enter into the compound, and separate again as soon as that substance is withdrawn. In the ordinary operations of the chemical laboratory, little account is taken of these things because there the conditions are either uniform or are so easily controlled that the operator gives little thought to them—he learns the necessary control by practice rather than by precept—but in nature's laboratory where conditions are constantly changing they come to be of supreme im portance.
Silica and Silicic 2. if a piece of ordinary granite be care fully examined, glassy grains will be seen distributed through its mass. These are particles of quartz or silica, a compound of the element silicon with oxygen. The chemist designates this material by the symbol SiOz,
using the first two letters of the word silicon with the first of oxygen and adding a small figure 2 to indicate that the compound contains two atoms of oxygen for each one of silicon.
Under ordinary conditions quartz is considered one of the most inert substances with which we are acquainted. Water and the acids, other than hydrofluoric, seem to have little or no attraction for it, and very little affinity seems to exist between it and any of the other elements. If, however, finely-powdered quartz be mixed with pulverized compounds of the metals such as potash, soda, alumina, oxide of iron, etc., the mass saturated with water and the whole raised to high temperature under strong pressure, the quartz or silica will at once unite with water to form an acid called silicic acid and this will immediately unite with the compounds of the metals (bases) mentioned to form silicates.
So strong is the newly-formed acid that it is able to take the metallic elements or bases out of their combinations with other acids and convert them into silicates. Under conditions of high temperature and pressure we are accustomed to regard silicic acid as the strongest of all our acids and able to displace any of them. If a mass consisting of carbonate of soda, nitrate of potash, sulphate of alumina, sulphide of iron, oxide of magnesia, or other salts of the common bases, be mixed with finely powdered silica, suspended in water, and raised to a high temperature in a sealed vessel, the silica will unite with water, forming silicic acid, and this will crowd out the other acids and unite with their bases, forming simple or compound silicates of potash, soda, alumina, magnesia and iron, the acids which were originally united with these bases being set free and dissolved in the water. This, it is believed, would be the case with the salts of any acid which might be placed in the mixture; silicic acid being stronger than the other acids under these conditions will crowd them out and unite with their bases.