356 SCIENCE PROGRESS. 



alkaline carbonates are mucilaginous and difficult to filter, 

 while when oxalate of ammonia is the solvent it is perfectly 

 fluid and filters readily, 



Metapectic acid. — This is a body with an acid reaction, 

 freely soluble in water, and forming soluble salts with all 

 bases, especially calcium and barium, which precipitate 

 pectic acid. Metapectates warmed with an excess of alkali 

 take a yellow colour. This body and its compounds approach 

 the gums in their composition. 



Metapectic acid can be prepared from either pectine or 

 pectic acid by boiling with excess of alkali. Acted upon by 

 sulphuric acid it splits up into a dextrorotatory crystallisable 

 sugar, apparently identical with arabinose, and into a little- 

 known organic acid, indicating by this behaviour some rela- 

 tionship to the group of the glucosides. 



The reactions just given clearly show that these pectic 

 bodies form a group quite distinct from the celluloses which 

 are known. The ease with which they are altered by the 

 reagents used for their extraction readily marks them off 

 from the latter, as do the products of their oxidation. If 

 warmed with nitric acid, instead of being oxidised to oxalic 

 acid like the carbohydrates, they give rise to mucic acid. 

 They are further all insoluble in Schweizer's reagent and 

 they give no blue coloration with iodine in any combination. 



In endeavouring to ascertain the various ways in which 

 these bodies are associated with cellulose in the cell-mem- 

 brane, Mangin relied partly upon staining reagents and 

 partly upon the action of various solutions in which one or 

 other of the constituents of the wall are soluble. His 

 investigations do not take into account the modifications 

 which lignified, suberised or mucilaginous walls present, 

 but only those in which the membrane has been considered 

 to consist of unchanged cellulose, 



A marked difference between cellulose and pectic bodies 

 soon comes out in studying the action of stains upon them. 

 Cellulose acts as a feeble base and takes up, therefore, acid 

 stains, particularly those containing nitrogen. Pectic com- 

 pounds, on the other hand, act as acids and require basic 

 stains. 



