_Oxy-Cellulose._--A white friable powder produced by treating cellulose with nitric acid at 100° C. The oxidation of cellulose is brought about by several reagents such as chromic acid, hypochlorites of lime and soda, chlorine, and permanganates. The extent to which cloth has been damaged by overbleaching may be determined by a simple test with methylene blue solution, which is readily absorbed by oxy-cellulose present in such fabrics. PARCHMENT.--A tough paper prepared by the action of sulphuric acid on unsized paper. (See page 137.) _Pectins._--(See Pecto-Cellulose.) _Pecto-Cellulose._--A generic term applied to many important fibrous materials, such as flax, straw, esparto, bamboo, phormium, ramie, &c., which on alkaline treatment yield cellulose for paper-making, and a non-fibrous soluble residue of complex composition. These soluble derivatives are known as pectin (C_{32}H_{48}O_{32}), pectic acid (C_{32}H_{44}O_{30}), and metapectic acid (C_{32}H_{28}O_{36}). Although the soluble constituents of the pecto-celluloses amount to 50 per cent. by weight in most cases, no process for the recovery of the product in a commercial form has yet been devised. (See description of Soda recovery, page 78.) PYROXYLINE.--A substance prepared by nitrating cotton. The cotton is immersed in a mixture of nitric and sulphuric acids of carefully regulated strength, and subsequently washed free of the acid. Three volumes of nitric acid (sp. gr. 1·429) are diluted with two volumes of water and nine volumes of strong sulphuric acid (sp. gr. 1·839) added. To the solution when cool the cotton is added in small quantities at a time. The resultant pyroxyline is soluble in a mixture of equal quantities of alcohol and ether, and in the soluble form is utilised as _collodion_ for photography. SILK, ARTIFICIAL.--A remarkable substance made from wood or cotton cellulose, closely resembling silk in appearance and physical properties. Nitrated cellulose is dissolved in a mixture of equal parts of alcohol and ether.