CELLULOSE. 191 



C 7 sH 12 oO ( ;3 -f H 2 = C 5 H 10 5 -f C 78 H 112 5<J ; 

 finally C 73 H 1M O M +13H a O = 10C 5 H 10 O 5 4-0.^^0^. 

 Some gums, on boiling with dilute sulphuric acid, yield not 

 only arabinose or xylose, as above, but galactose, C 6 H ll2 O fi , as 

 their main product. Hence gum is a name which includes 

 both pentosans and glucosans (i.e., polysaccharides, which yield 

 pentose arid hexose sugars). 



Cellulose is the substance which constitutes the main por- 

 tion of the frame-work of plants. It occurs in numerous 

 forms, and in plants is always mixed or combined with other 

 products of growth, from which it can usually be separated by 

 taking advantage of its inertness and resistance to most re- 

 agents. By treating the plant tissues with chlorine, boiling 

 with alkaline solutions, washing, treating with dilute acid, 

 water, alcohol, and ether, a residue of nearly pure cellulose is 

 usually obtained. Its composition corresponds to the empiric 

 formula C 6 H 10 5 . Ordinary air-dried specimens always con- 

 tain 7 to 9 % of hygroscopic moisture. It is insoluble in most 

 solvents, but can be obtained in solution by means of zinc 

 chloride or ammoniacal copper oxide. The solubility of cellu- 

 lose in these solutions is taken advantage of in the arts, the 

 former being used in the preparation of the " carbon filament" 

 of incandescent electric lamps, and the latter in the manufac- 

 ture of "WiUesden" waterproof paper. Cellulose appears to 

 have at the same time faint acid and basic properties ; it will 

 absorb and retain either bases or acids. 



By the action of caustic soda solutions (exceeding 10 % Na 2 0) 

 ordinary cotton fibre (practically cellulose) is converted into a 

 compound which, on treatment w r ith water, is decomposed, a 

 molecule of water apparently taking the place of Na 2 O. The 

 properties of the fibre are changed considerably, it swells 

 laterally or thickens with a corresponding shrinkage in length, 

 its tensile strength is increased, and it possesses greater affini- 

 ties for dyes. These facts were discovered by Mercer many 

 years ago, and cotton fabrics so treated have lately been 

 extensively made. They are sold as " Mercerised cotton." If 

 cotton be treated with a solution containing 15% of caustic 

 soda, squeezed, and placed in a bottle with about 40% of its 



