ANHYDRIDES, is the term now commonly given to the compounds formerly known as anhydrous acids, which was a very unsatisfactory name, seeing that these bodies do not present any of the ordinary properties of acids. In some cases, they are the result of the dehydration of acids, and in all cases they represent in their composition the acid minus water. One of the most eminent French chemists, prof. Kurtz, lays down the following general principles: "(1) The anhydrides of monobasic acids (a) contain the elements of two molecules of a monobasic acid, minus 1 molecule (which corresponds with 2 atoms) of water; (2) the anhydrides of bibasic acids (b) contain the elements of a molecule of a bibasic acid, minus a molecule of water; (3) the anhydrides of tribasic acids (c) contain the elements of a tribasic acid, minus water." .Thus, using the modern forruulce and the type theory, we give a case of each form of acid: The reader who may not at once be able to interpret these formula', will readily see that N0II4 02) = the old formula for nitric acid, that S2H°. 1 04 = 2(S03.110), \ the of formula for sulphuric acid, which is now universally placed amongst the bibasic acids; and that 0.) = the old formula for tribasic phosphoric acid. According to the old system, the three anhydrides would be represented by NO., SOS,
and P06 respectively. We might have taken organic acids, as, for example, acetic acid, succinic acid, in place of nitric and sulphuric.
The anhydrides of the monobasic acids are formed in various ways; thus, hypochlo rous anhydride is formed by the action of chlorine on oxide of mercury; nitric anhy dride is formed by the action of chlorine on nitrate of silver, etc. By the action of ammonia, the anhydrides of monobasic acids are converted into amides; thus, benzoic anhydride (0,411603) + ammonia (NH.) = benzamide + water (HO). The anhydrides of tribasic acids are often formed by the mere action of heat on the acids, as is the case with lactic and tartaric acids.
The anhydrides present no uniformity of appearance; for example, carbonic anhydride (commonly known as carbonic acid, which in reality is CO.,110) is a gas; phosphoric i anhydride is a white powder; nitric anhydride occurs in crystals; sulphuric anhydride is a ductile wax-like substance; while the anhydrides of the organic acids are oily bodies heavier than water.
The most important property of this class is their conversion into the corresponding acids, under the influence of water.