I 



PHOTO-SYNTHESIS 105 



till nearly, but not quite, neutral, and the calcium citrate 

 formed acidified with sulphuric acid. Citric acid forms an 

 insoluble calcium salt, which does not easily form without 

 boiling. The deposition of calcium citrate by boiling milk 

 in a saucepan is a well-known phenomenon, which produces 

 a crust on the bottom of the saucepan, rather difficult to 

 remove. 



The Carbohydrates. 



Fibre. — The members of the carbohydrate group, 

 which are pentosans, C5H8O4, that is five carbon gums, 

 are very common in all the fibrous parts of plants. 

 Straw may contain up to 20 per cent, of this material, 

 which is consequently often known under the name of 

 straw gum. The amount of pentosan present in most 

 plant products is roughly in proportion to the amount of 

 fibre. No satisfactory use has been made of straw gum as 

 yet, since its adhesive properties are too feeble. If the amount 

 of wheat grown in Great Britain is to be doubled, the straw 

 will also be doubled. It is hence important to discover new 

 uses for straw, and this subject seems worth}^ of further 

 inquiry. When heated with dilute acids the pentosans are 

 first converted into pentoses, and then condense into 

 furfur aldehyde, a volatile liquid which can be distilled with 

 steam and forms many coloured compounds, some of which 

 are dyestuffs. Straw is the best raw material for the 

 production of furfur aldehyde. The pentoses themselves 

 are not common materials in plant life. The pectins, gums, 

 and such substances, frequently yield substances of both 

 the Cs and Cg groups, and are, therefore, compound bodies 

 containing these two groups (see p. 131). The cellulose 

 group is a very common material to find in plants, most of 

 the stiffening parts of plants being due to this substance, 

 which, in its pure form, approaches CqKiqO^ in composition. 

 Cotton-wool and filter paper (see p. 128) may be taken 

 as practically pure specimens of cellulose. Cellulose is 

 insoluble in all common reagents, but is soluble in solutions 

 of copper hydroxide in ammonia, as well as in zinc chloride 



