THE VOLATILE PAET OF PLANTS. 59 



Flax-seed mucilage is procured by soaking unbroken flaxseed in cold 

 water, with frequent agitation, heating the liquid to boiling, strain- 

 ing, and evaporating, until addition of alcohol separates tenacious 

 threads from it. It is then precipitated by alcohol containing a little 

 hydrochloric acid, and washed by the same mixture. On drying, it 

 forms a horny, colorless, and friable mass. Fig. 13 represents a highly 

 magnified section of the ripe flaxseed. The external cells, a, contain 

 the dry mucilage. When soaked in water, the mucilage swells, bursts 

 the cells, and exudes. 



The Pectin Bodies. The flesh of beets, turnips, and 

 similar roots, and of most unripe fruits, as apples, 

 peaches, plums, and berries of various kinds, contain one 

 or several bodies which are totally insoluble in water, but 

 which, under the action of weak acids or alkaline solu- 

 tions, become soluble and yield substances having gummy 

 or gelatinous characters, that have been described under 

 the names pectin, pectic acid, pectosic acid, metapectic 

 acid, etc. Their true composition is, for the most part, 

 not positively established. They are, however, closely 

 related to the gums. The insoluble substance thus trans- 

 formed into gum-like bodies, Fremy termed pectose. 



The gums, as they occur naturally, are mostly mix- 

 tures. By boiling with dilute sulphuric or hydrochloric 

 acid they are transformed into sugars. 



In the present state of knowledge it appears probable 

 that the common gums, for the most part, consist of a 

 few chemically distinct bodies, some of which have been 

 distinguished more or less explicitly by such names as 

 Arabin, Metarabin, Pararabin, Galactin, Paragalactin, 

 etc. 



Arabin, or Arabic Acid, is obtained from some va- 

 rieties of Gum Arabic* by mixing their aqueous solution 

 with acetic acid and alcohol. It is best prepared from 

 sugar-beet pulp, out of which the juice has been ex- 

 pressed, by heating with milk of lime ; the pulp is 

 thereby broken down, and to a large extent dissolves. 



* Those sorts of commercial Gum Arabic which deviate the plane of 

 polarization of light to the, left contain arubin in largest proportion. 



