According to the above observations, an acid is nothing more than a salt of hydrogen. Water itself may be looked on as a salt in which the hydrogen is combined with either oxygen or the aqueous radicle, OH; water will then be HOH, and alkalis or basic hydrates, MOH. The group OH, or theaqueous radicle, otherwise calledhydroxyl, may be looked on as a haloid like the chlorine in table salt, not only because the element Cl and the group OH very often change places, and combine with one and the same element, but also because free chlorine is very similar in many properties and reactions to peroxide of hydrogen, which is the same in composition as the aqueous radicle, as we shall afterwards see in ChapterIV. Alkalis and basic hydrates are alsosalts consisting of a metal and hydroxyl—for instance, caustic soda, NaOH; this is therefore termedsodium hydroxide. According to this view,acid saltsare those in which a portion only of the hydrogen is replaced by a metal, and a portion of the hydrogen of the acid remains. Thus sulphuric acid (H2SO4) not only gives the normal salt Na2SO4, with sodium, but also an acid salt, NaHSO4. Abasic saltis one in which the metal is combined not only with the haloids of acids, but also with the aqueous radicale of basic hydrates—for example, bismuth gives not only a normal salt of nitric acid, Bi(NO3)3, but also basic salts like Bi(OH)2(NO3).
As basic and acid salts of the oxygen acids contain hydrogen and oxygen, they are able to part with these as water and to give anhydro-salts, which it is evident will be compounds of normal salts with anhydrides of the acids or with bases. Thus the above-mentioned acid sodium sulphate corresponds with the anhydro-salt, Na2S2O7, equal to 2NaHSO4, less H2O. The loss of water is here, and frequently in other cases, brought about by heat alone, and therefore such salts are frequently termedpyro-salts—for instance, the preceding is sodium pyrosulphate (Na2S2O7), or it may be regarded as the normal salt Na2SO4+ sulphuric anhydride, SO3.Doublesalts are those which contain either two metals, KAl(SO4)2, or two haloids.[57]
Inasmuch as oxygen compounds predominate in nature, it should be expected from what has been said above, that salts, rather than acidsor bases, would occur most frequently in nature, for these latter would always tend to combine forming salts, especially through the mediumof the all-pervading water. And, as a matter of fact, salts are found everywhere in nature. They occur in animals and plants, although in but small quantity, because, as forming the last stage of chemical reaction, they are capable of only a few chemical transformations. And organisms are bodies in which a series of uninterrupted, varied, and active chemical transformations proceed, whilst salts, which only enter into double decompositions between each other, are little prone to such changes. But organisms always contain salts. Thus, for instance, bones contain calcium phosphate, the juice of grapes potassium tartrate (cream of tartar), certain lichens calcium oxalate, and the shells of mollusca calcium carbonate, &c. As regards water and soil, portions of the earth in which the chemical processes are less active, they are full of salts. Thus the waters of the oceans, and all others (Chap.I.), abound in salts, and in the soil, in the rocks of the earth's crust, in the upheaved lavas, and in the falling meteorites the salts of silicic acid, and especially its double salts, predominate. Saline substances also make up the composition of those limestones which often form mountain chains and whole thicknesses of the earth's strata, these consisting of calcium carbonate, CaCO3.
Thus we have seen oxygen in a free state and in various compounds of different degrees of stability, from the unstable salts, like Berthollet's salt and nitre, to the most stable silicon compounds, such as exist in granite. We saw an entirely similar gradation of stability in the compounds of water and of hydrogen. In all its aspects oxygen, as an element, or single substance, remains the same however varied its chemical states, just as a substance may appear in many different physical states of aggregation. But our notion of the immense variety of the chemical states in which oxygen can occur would not be completelyunderstood if we did not make ourselves acquainted with it in the form in which it occurs in ozone and peroxide of hydrogen. In these it is most active, its energy seems to have increased. They illustrate fresh aspects of chemical correlations, and the variety of the forms in which matter can appear stand out clearly. We will therefore consider these two substances somewhat in detail.