When cellulose is acted upon by acid, alkali, and certain metallic salts, it enters into combination with one or more proportions of water, forming cellulose hydrates of varying complexity. This change is usually termed hydrolysis. With mineral acids like sulphuric and hydrochloric acids, cellulose, if boiled in weak solutions, is converted into a non-fibrous brittle substance having the composition C_{12}H_{20}O_{10} 2 H_{2}O to which the name _hydra-cellulose_ has been given. Similar changes occur, but at a much slower rate, when cellulose is in contact with free acids at ordinary temperatures. For this reason it is important that paper, when finished, should not be contaminated with free acid. The nature and extent of the chemical change can be varied by altering the strength of the acid and the conditions of treatment. The manufacture of _parchment_ paper is an example of the practical utility of the chemical reaction between cellulose and acid. A sheet of paper is dipped into a mixture of three parts of strong sulphuric acid and one part of water, when it becomes transparent. Left in the solution it dissolves, but if taken out and dipped into water in order to wash off the acid the reaction is stopped, and a tough semi-transparent piece of _parchment_ is obtained. The cellulose is more or less hydrated, having the composition C_{12}H_{20}O_{10} H_{2}O, a substance having the name _amyloid_. _Oxidation._--Cellulose is only oxidised to any appreciable extent by acid and alkali if treated under severe conditions. It is remarkable that the processes necessary for isolating paper pulp from plants when digested with these chemical reagents do not act upon or destroy the fibre, and this capacity for resisting oxidation has rendered cellulose extremely valuable to many of the most important industries. The resistant power of the cellulose is, however, broken down by the use of acid and alkali in concentrated form. Oxalic and acetic acids are obtained when cellulose is heated strongly at 250° C. with solid caustic soda. Oxy-cellulose, a white friable powder, is produced by means of strong mineral acids. Nitric acid at 100° C. attacks the fibre very readily and produces about 30-40 per cent. of the oxidised cellulose. CELLULOSE DERIVATIVES. The great number of compounds and derivatives, _i.e._, substances obtained by chemical treatment, may be judged from the following list. The substances of commercial importance are suitably distinguished from those of merely scientific interest by the printing of the names in small capitals. ACETIC ACID.--An important commercial product obtained by the destructive distillation of wood. The crude pyroligneous acid is first neutralised with chalk or lime, and the calcium acetate formed then distilled with sulphuric acid. Wood yields 5 to 10 per cent. of its weight of acetic acid according to the nature of the wood.