252 CELLULOSE, LIGNIN, ETC. 



rotatory fermentable sugar, so that this method may be adopted 

 for its estimation. Ammonia dissolves it with difficulty, and 

 it undergoes but little change when heated with potaah in 

 sealed tubes (§ 115). 



Gelose,^ the gelatinizing constituent of many algse, agrees with 

 lichenin in most of its properties, but is insoluble in ammonio- 

 sulphate of copper, and is less easily converted into sugar. By 

 decomposition with dilute acids, arabinose (lactose) is produced in 

 place of the glucose yielded by lichenin. The gelose appears to 

 be accompanied, at least in Sphserococcus lichenoides, by a carbo- 

 hydrate* soluble in dilute hydrochloric acid, but dififering from 

 pararabin (§ 112) in yielding glucose when boiled with aii acid. 



§ 246. Wood-gum. — Thomsen* found that when ligneous tissue, 

 previously exhausted with water, spirit, and very dilute alkali, 

 was macerated with caustic soda of sp, gr. !•!, a substance was ex- 

 tracted, the composition of which he ascertained to be CgHi^Oj, 

 and which he named wood-gum. It can be isolated from its 

 solution in soda by acidifying and adding alcohol. When once 

 dried, cold water will not redissolve it ; this is, however, efTected 

 by boiling. It is precipitated by basic acetate of lead, is converted 

 into glucose by boiling with a dilute acid, and is not coloured blue 

 by iodine. An alkaline solution is laevo-rotatory. It differs 

 from lichenin in not possessing the power of gelatinizing', from 

 metarabin in not being dissolved (when dry) by 01 per cent 

 solution of soda. 



A similar substance was obtained by Pfeil* from parenchymatous 

 tissue (agreeing, however, in composition better with the formula 

 CjjHj^O^,, a hydrocellulpse), by Treffner from mosses, and by 

 Greenish from algse. 



CEU.ULOSES, UGNIN, AND AU-IED SUBSTANCES., 



§ 247. Celluloses, etc, — Fr6my and Terreil* assume that woody 

 tissue is chiefly composed of three different substances, which they 

 distinguish as cellulose, incrusting substance, and cuticular sub- 



' Campare Morin and Fonunbaiu, Comptes rendus, xc. 924, 1081, 1880 

 (Year-book Pharra. 120, 121, 1881). 



= Greenish, Archiv d. Phann. [3], xx. 841. 



3 Journ. f. pract. Chem. [2], xix. 146, 1879 (Year-book Phann. 99, 1880). 



* Loc. cit. 



^ Journ, de Fharm. et de Ohim. vii. 241, 1868. 



