CHEMISTRY. 



109 



Compo^i- 

 tiun. 



Bifferent 

 kind* of 

 gum. 



of iron is poured into a strong mucilage, the whole is 

 converted into a brown nemitransparent jelly, which is 

 not readily dissolved by water. 



Neither the alkalies, alkaline earths, nor earthy salts, 

 occasion any precipitate in mucilage; except silicatcd 

 potash, which throws down a white flaky precipitate, 

 even though very much diluted. The liquid remains 

 transparent and colourless. Silicated potash is by far the 

 most delicate test of gum that we have yet met with. 



Liquid potash rirst converts gum into a substance not 

 unlike curd, and then dissolves it. The solution is of a 

 light amber colour, and transparent. When kept long, 

 the gum falls again in the state of curd. Alcohol throws 

 down the gum in white flakes, still soluble in water, but 

 it retains the potash obstinately, and is much more friable 

 than before. Lime water and ammonia likewise dissolve 

 <nim, and it may be afterwards separated little altered. 

 The vegetable acids dissolve gum without alteration, 

 the strong acids decompose it. When thrown into sul- 

 phuric acid it blackens, and is decomposed. Charcoal is 

 evolved, amounting to nearly one third of the gum ; some 

 artificial tannin may be detected, and water and acetic 

 acid are likewise formed. When gum is dissolved in 

 strong muriatic acid, a brown solution is obtained, which 

 becomes perfectly transparent when diluted with water, 

 while at the same time some charry matter falls. If the 

 solution be saturated with ammonia evaporated to dry- 

 ness, and the residue digested in alcohol, the alcohol as- 

 sumes a deep brown colour, and dissolves the whole ex- 

 cept a little sal ammoniac. The gum now bears some 

 resemblance to sugar in its properties, at least when 

 heated it m^Jts, and give* out a very strong smell of caro- 

 mel. 



If nitric acid be slightly heated upon gum till it has 

 dissolved it, and till a little nitrous gas has exhaled, the 

 solution on cooling deposites saclactic acid. Malic acid 

 is formed at the same time, and if the heat be continued, 

 the gum is at last changed into oxalic acid. 



Gum is insoluble in alcohol. It is precipitated from 

 water by alcohol. It is insoluble also in ether and in oils ; 

 but when triturated with a little oil, it renders the oil 

 miscible with water. 



Gum readily combines with sugar by mixing together 

 the solutions of both in water, and evaporating to dry- 

 ness. Alcohol digested on the residuum, dissolves most 

 of the sugar ; a matter remains which still has a sweetish 

 taste, anil resembles the substance of which the nests of 

 wasps are formed. 



When gum is distilled in a retort, the products are 

 water impregnated with acetic acid and oil, or pyromu- 

 MIIS acid, as it was formerly called, a little empyreumatic 

 oil, carbonic acid gas, and heavy inflammable air. There 

 remains in the retort charcoal containing lime and phos- 

 phate of lime. Gum yields also traces of iron when its 

 ashes are examined, but no fixed alkali or sulphur can be 

 detected. 



According to the analysis of Thenard and Gay-Lus- 

 sac, gum is composed of 



Oxygen .... 50.84 

 Carbon .... *2.23 

 Hydrogen . . . 6.93 



100.00 



The species of gum at present known are numerous, 

 and a more rigid examination of the vegetable king- 

 dom will doubtless discover a still greater number. The 

 most remarkable are gum arable, gum Senegal, gum tra- 

 gacanth, and cherry tree gums. 



tion of 



Nature. 



Gum tra- 

 gacanth. 



Prumu 

 avium. 



Gum arabic exudes from the uiiixosa nilolica. It is 

 the species described in the preceding part of tiiis Chap- 

 Gum Senegal, brought from the island of tlial name, 

 or, the coast of Africa, often supplies the place of gum 

 arabic in the shops. It is in larger masses than the ara- 

 bic, and its colour is darker, but in other respects its 

 properties are the same. 



Gum tragacanth is the produce of the astragalus 

 Iragacanlha, a thorny shrub which grows in Candia, 

 and other islands of the Levant. It exudes about the 

 end of June, from the stem and larger branches, and soon 

 dries in the sun. It is in the state of whitish vermiform 

 pieces, not nearly so transparent as gum arabic, and is 

 exceedingly different from it in many of its properties. 

 When put into water, it slowly imbibes a large quantity 

 of the liquid, and forms a soft but not fluid mucilage. 

 If the quantity of water be more than the gum can ira- 

 bibc, the mucilage forms an irregular mass, which doe's 

 not unite with the rest of the liquid. When tragacanth 

 is treated with nitric acid, it yields abundance of saclactic 

 acid, malic acid, and oxalic acid, but not the least trace 

 of artificial tannin. When the mucilage of gum traga- 

 canth is triturated in a mortar with water, it forms a ho- 

 mogeneous solution. This solution forms a precipitate 

 with acetate and superacetate of lead and oxymuriate of 

 tin. Nitrate oi' mercury throws down a slight precipi- 

 tate ; but neither oxysulphate of iron, nor silicated pot- 

 ash produce any effect. These properties show it to dif- 

 fer very materially from gum arabic in its properties. 



The primus ui'ium, the common cherry and plum-trees, 

 and the almond and apricot, likewise yield a gum which 

 exudes in great abundance from natural or artificial open- 

 ings in the stem. It is of a reddish brown colour, in 

 large masses ; at first much softer than gum arabic, but, 

 by keeping, it becomes very hard. When put into water 

 it gradually swells, and is converted into a semi-trans- 

 parent reddish brown jelly. Part of it dissolves, but a 

 part of it remains in the state of jelly, and refuses to dis- 

 solve even when boiled in water for some time. The 

 gum dissolved is not precipitated by alcohol nor by sili- 

 cated potash. Acetate of lead produces no immediate 

 effect, but on standing the whole becomes opake, and a 

 precipitate at last subsides. Stannane causes the liquid 

 to gelatinize immediately. The superacetate of lead and 

 the nitrate of mercury produce no effect. When treated 

 with nitric acid it yields a portion of saclactic acid. 

 These properties show a marked difference between cher- 

 ry-tree gum and the other species. 



SECT. VI. Of Mucus. 



The substances to which we give the name of mucus, Mucus. 

 have been hitherto considered as varieties of gum ; but, 

 from the recent experiments of Dr Bostock, it appears 

 that their properties differ so much from those of gum 

 as to entitle them to a separate place as vegetable priuci- 



Eles. They arc very numerous, existing in the roots, 

 aves, and seeds of a great variety of plants. They 

 scarcely ever separate spontaneously, but may be obtain- 

 ed artificially in a state of tolerable purity. Only a few 

 of them have been examined. The rest are classed with 

 these only from analogy. 



Linseed yields mucus in a state of tolerable purity. 

 When it is infused in ten times its weight of water, a 

 fluid is obtained of the consistence of white of egg, which 

 has the adhesive qualities of mucilage of gum arabic. 

 When mixed with alcohol, the mucus is separated in 

 white flocks, but the liquid does not become opake and 



