238 ' LEGUMINOSiE. 



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 

 colouring 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 

 separate. No alteration is produced by silver salts, mercuric chloride 

 or iodine. 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^m^^O^^ + SW-0, the loss of 3 molecules of water will 

 correspond to a decrease in weight of 13 "6 per cent. ; in carefully 

 selected colourless pieces, we have found it to amount to 13'14 per 

 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 glairy, like the mucilage 

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

 from ordinary gum. On exposing it for some days to a temperature of 

 95° C., it afforded a solution of the usual character. 



When gum arable is dissolved in cold water and the solution is 

 slightly acidulated with hydrochloric acid, alcohol produces it in a 

 Y>rec\])ita>te of Arabin or Arabic Acid. It may be also prepared by 

 placing a solution of gum (1 gum + 5 water), acidulated with hydro- 

 chloric acid, on a dialyser, when the calcium salt will diffuse out, leaving 

 behind a solution of arabin. 



Solution of arabin differs from one of gum in not being precipitated 

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



