238 LEGUMINOS^. 



Chemical Characters and Composition — At ordinary tem- 

 peratures gum dissolves very slowly and without affecting the thermo- 

 meter in an equal weight of water, forming a thick, glutinous, slightly 

 opalescent liquid, having a mawkish taste and decidedly acid reaction. 

 At higher temperatures the dissolution of gum is but slightly accele- 

 rated, and water does not take up a much larger quantity even at 100°. 

 The finest gum dried at 100° C. forms with two parts of water a 

 mucilage of sp. gr. 1-149 at 15° C. 



This solution mixes with glycerine, and the mixture may be evapo- 

 rated to the consistence of a jelly without any separation taking place. 

 Solid gum in lumps, on the contrary, is but little affected by concen- 

 trated glycerine. In other liquids, gum is insoluble or only slightly 

 soluble, unless there is a considerable quantity of water present. Thus 

 100 parts of spirit of wine containing 22 volumes per cent, of alcohol, 

 dissolve 57 parts of gum ; spirit containing 40 per cent, of alcohol 

 takes up 10 parts, and spirit of 50 per cent, only 4 parts. Aqueous 

 alcohol of GO per cent, no longer dissolves gum, but extracts from it a 

 small quantity Q to ^ per cent, according to the variety) of resin 

 colourmg matter, glucose, calcium chloride, and other salts. 



Neutral acetate of lead does not precipitate gum arable mucilage ; 

 but the basic acetate forms, even in a very dilute solution, a precipitate 

 of definite constitution. 



Soluble silicates, borates, and ferric salts render gum solution turbid, 

 or thicken it to a jelly. It is not a compound of gum with any of these 

 substances which is formed, but in the cases of the first, basic silicates 

 separ-ate. No alteration is produced by silver salts, mercuric chloride 

 or lodme. Ammonium oxalate throws down the lime contained in a 

 solution of gum. Gum dissolves in an ammoniacal solution of cupric 

 oxide. Acted upon by nitric acid, mucic acid is produced. 



Small, air-dried lumps of gum lose by desiccation over concentrated 

 sulphuric acid (or by heating them in the water-bath) 12 to 16 per 

 cent of water. If gum independently of its amount of lime, be presented 

 by the formula C^^H^^Oh -^ 3 H^O, the loss of 3 molecules of water vviU 

 correspond to a decrease in weight of 13G per cent.; in carefully 

 selected colourless pieces, we have found it to amount tolSUper 

 cent. At a temperature of about 150° C, gum parts with another mole- 

 cule of water, and partly loses its solubility and assumes a brownish Hue 

 and empyreumatic taste. Gum already by keeping it for a week at a 

 temperature not exceeding 95° C. gradually acquires a decidedly empy- 

 reumatic taste. We have also observed, on the other hand, a fine white 

 gum affording an imperfect solution which was qlairy, like the mu^lag^ 

 of marsh-mallow, but in no other respect could we find that it differed 

 n -""n" • ""^^ gum. On exposing it for some days to a temperature 01 

 kil ^^o^^ed a solution of the usual character. , ,. ■, 



«i; i!;i ^?'? ''^'^^^^ ^« dissolved in cold water and the solution is 

 ^ligtitiy acidulated with hydrochloric acid, alcohol produces it in ^ 

 precipitate of ^l ra&m or ^raMcJc/d It may be also prepared by 



crura 



r ; . " "^^'^i "" a aiaiyser, w 

 behind a solution of arabin. 



bv XIT "^^^amn differs from one of gum in not being precipe- 

 by alcohol. Having been dried, it loses its solubility, merely swelling 



