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 the _aqueous radicle_, otherwise called _hydroxyl_, 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 Chapter IV. Alkalis and basic hydrates are also salts consisting of a metal and hydroxyl--for instance, caustic soda, NaOH; this is therefore termed _sodium hydroxide_. According to this view, _acid salts_ are 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 (H_{2}SO_{4}) not only gives the normal salt Na_{2}SO_{4}, with sodium, but also an acid salt, NaHSO_{4}. A _basic salt_ is 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(NO_{3})_{3}, but also basic salts like Bi(OH)_{2}(NO_{3}). 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, Na_{2}S_{2}O_{7}, equal to 2NaHSO_{4}, less H_{2}O. The loss of water is here, and frequently in other cases, brought about by heat alone, and therefore such salts are frequently termed _pyro-salts_--for instance, the preceding is sodium pyrosulphate (Na_{2}S_{2}O_{7}), or it may be regarded as the normal salt Na_{2}SO_{4} + sulphuric anhydride, SO_{3}. _Double_ salts are those which contain either two metals, KAl(SO_{4})_{2}, or two haloids.[57] [57] The above-enunciated generalisation of the conception of salts as compounds of the metals (simple, or compound like ammonium, NH_{4}), with the haloids (simple, like chlorine, or compound, like cyanogen, CN, or the radical of sulphuric acid, SO_{4}), capable of entering into double saline decomposition, which is in accordance with the general data respecting salts, was only formed little by little after a succession of most varied propositions as to the chemical structure of salts.