In carbon are exemplified the properties of the quadrivalent elements, forming RH 4 , RO 2 , RO(OH) 2 , R(OH) 4 , RHN, RCl 4 , RHCl 3 , &c. We meet with these forms of combination , or degrees of union of atoms, in all other elements, some being analogous to hydrogen, others to oxygen, and others to nitrogen or to carbon. But besides these quantitative analogies or resemblances, which are foretold by the law of substitution (Chapter VI.), there exist among the elements qualitative analogies and relations which are not fully seen in the compounds of the elements which have been considered, but are most distinctly exhibited in the formation of bases, acids, and salts of different types and properties. Therefore, for a complete study of the nature of the elements and their compounds it is especially important to become acquainted with the salts, as substances of a peculiar character, and with the corresponding acids and bases. Common table salt, or sodium chloride, NaCl, may in every respect be taken as a type of salts in general, and we will therefore pass to the consideration of this substance, and of hydrochloric acid, and of the base sodium hydroxide, formed by the non-metal chlorine and the metal sodium, which correspond with it.
Sodium chloride, NaCl, the familiar table salt, occurs, although in very small quantities, in all the primary formations of the earth's crust,2 from which it is washed away by the atmospheric waters; it is contained in small quantities in all waters flowing through these formations, and is in this manner conveyed to the oceans and seas.
The immense mass of salt in the oceans has been accumulated by this process from the remote ages of the earth's creation, because the water has evaporated from them while the salt has remained in solution. The salt of sea water serves as the source not only for its direct extraction, but also for the formation of other masses of workable salt, such as rock salt, and of saline springs and lakes.3
The extraction of salt from sea water is carried on in several ways. In southern climes, especially on the shores of the Atlantic Ocean and the Mediterranean and Black Seas, the summer heats are taken advantage of. A convenient low-lying sea shore is chosen, and a whole series of basins, communicating with each other, are constructed along it. The upper of these basins are filled with sea water by pumping, or else advantage is taken of high tides. These basins are sometimes separated from the sea by natural sand-banks (limans) or by artificial means, and in spring the water already begins to evaporate considerably. As the solution becomes more concentrated, it is run into the succeeding basins, and the upper ones are supplied with a fresh quantity of sea water, or else an arrangement is made enabling the salt water to flow by degrees through the series of basins. It is evident that the beds of the basins should be as far as possible impervious to water, and for this purpose they are made of beaten clay. The crystals of salt begin to separate out when the concentration attains 28 p.c. of salt (which corresponds to 28° of Baumé's hydrometer). They are raked off, and employed for all those purposes to which table salt is applicable. In the majority of cases only the first half of the sodium chloride which can be separated from the sea water is extracted, because the second half has a bitter taste from the presence of magnesium salts which separate out together with the sodium salt. But in certain localities—as, for instance, in the estuary of the Rhone, on the island of Camarga4—the evaporation is carried on to the very end, in order to obtain those magnesium and potassium salts which separate out at the end of the evaporation of sea water.