Its composition is represented by the formula C_{6}H_{10}O_{5}. The celluloses obtained from various plants are not identical either in physical structure and chemical constitution, or as to their behaviour when employed for paper-making. In fact, the well-known differences between the raw materials used for paper-making, and also between the numerous varieties of finished paper, are to be largely accounted for and explained by a careful study of the cellulose group, particularly with reference to the microscopic characteristics and the chemical composition of the individual species. The only vegetable substance which may be regarded as a simple cellulose is cotton, all others being compound celluloses of varying constitution, the nature of which cannot be appreciated without a considerable knowledge of chemistry. The classification of such plants, therefore, in a book of this description must be limited to certain distinctions having some immediate practical bearing on the question of paper manufacture. _Cotton._--Regarded as the typical simple cellulose, containing 91 per cent. of cellulose, and remarkable for its resistance to the action of caustic soda. _Linen._--The cellulose isolated from flax by treatment with alkali or caustic soda cannot readily be distinguished from cotton cellulose by chemical analysis or reactions. The difference is almost entirely a physical one. Flax is a typical compound cellulose, to which has been given the name pecto-cellulose on account of certain properties. Other well-known plants of this class are ramie, aloe, "sunn hemp," manila. _Esparto._--The cellulose isolated from esparto differs in composition from cotton cellulose:-- Carbon 41·0 Hydrogen 5·8 Oxygen 53·2 ----- 100·0 ----- It is regarded as an oxycellulose, being readily oxidised by exposure to air at 100° C. Other oxycelluloses familiar to the paper-maker are straw, sugarcane, bamboo. _Wood._--The difference between wood and the plants already mentioned is expressed by the term lignified fibre or ligno-cellulose. This term is used to indicate that the wood is a compound cellulose containing non-fibrous constituents, to which has been given the name lignone. Jute is another example of this class. * * * * * These distinctions may be exemplified by reference to a simple experiment. If three papers, such as a pure rag tissue or a linen writing, an ordinary esparto printing, and a cheap newspaper containing about 80 per cent. of mechanical wood, are heated for twenty-four hours in an oven at a temperature of 105° C., the first will undergo little, if any, change in colour, while the others will be appreciably discoloured, the mechanical wood pulp paper most of all.