Many compounds are known which are at the same time chloride and oxide, or chloride and hydroxide of elements. Where the element with which the oxygen and chlorine is combined is one which forms a basic oxide, the compounds in question are termed basic chlorides. Similarly, there are basic bromides and iodides. For example, zinc oxide heated with zinc chloride forms oxychlo-rides, of which the simplest example is Cl~Zn-O-Zn^_CI; aluminium chloride, evaporated with water, has its chlorine gradually replaced by hydroxyl, forming successively Cl₂=Al(OH), Cl-Al=(OH)₂, and finally, Al(OH)₃, though at a temperature sufficient to complete the reaction, the aluminium would probably form the condensed hydroxide 0=A10H instead of the trihydroxide. We shall see later that other groups, playing a part analogous to that of the chlorine in a basic salt, may also exist in basic salts.

Acid Chlorides

Another class of double oxides and chlorides exists, most of which are easily volatile, and which therefore are of known molecular weight. These are the so-called " acid chlorides "-oxychlorides of those elements which form acids. These are related to acids, in as much as by replacement of their chlorine by hydroxyl, acids are formed. It will therefore be convenient to consider them along with the acids to which they are related.

A general idea has already been given of the nature of acids in describing the hydroxides of zinc and of aluminium. As a rule, acids are condensed hydroxides; that is* hydroxides which, having lost the elements of water, are partly oxides, partly hydroxides. They also possess the property of ionising into one or more hydrogen ions and an electro-negatively charged radical. In following the order of the periodic table, after such feebly acidic hydroxides as those of zinc and aluminium, hydroxide of boron claims attention.