SIR H. Davy's agricultural chemistry. 



109 



I shall describe generally the properties 

 and composition of these bodies, and the 

 manner in which they are procured. 



1. Gum is a substance which exudes 

 from certain trees; it appears in the form 

 of a thick fluid, but soon hardens in the 

 air, and becomes solid; when it is white, 

 or yellowish, white more or less trans- 

 parent, and somewhat brittle, its specific 

 gravity varies from 1300 to 1490. 



There is a great variety of gums, but 

 the best known are gum arabic, gum 

 Senegal, gum tragacanth, and the gum of 

 the plum or cherry tree. Gum is soluble 

 in water, but not soluble in spirits of wine. 



If a solution of gum be made in water, 

 and spirits of wine or alcohol be added 

 to it, the gum separates in the form of 

 white flakes. Gum can be made to in- 

 flame only with difficulty ; much mois- 

 ture is given off" in the process, which 

 takes place with a dark smoke and feeble 

 blue flame, and a coal remains. 



The characteristic poperties of gum are 

 its easy solubility in water, and its inso- 

 lubility in alcohol. Diflferent chemical 

 substances have been proposed for ascer- 

 taining the presence of gum, but there is 

 reason to believe that few of them afford 

 accurate results ; and most of them (par- 

 ticularly the metallic salts,) which pro- 

 duce changes in solutions of gum, may 

 be conceived to act rather upon some sa- 

 line compounds existing in the gum, than 

 upon the pure vegetable principle. Dr. 

 Thomson has proposed an aqueous solu- 

 tion of silica in potassa as a test of the 

 presence of gum ; in solutions he states 

 that the gum and silica are precipitated 

 together : — this test, however, cannot be 

 applied with correct results in cases when 

 acids are present. 



Mucilage must be considered as a va- 

 riety of gum ; it agrees with it in its most 

 important properties, but seems to have 

 less attraction for water. 



According to Hermbstadt, when gum 

 and mucilage are dissolved together in 

 water, the mucilage may be separated by 

 means of sulphuric acid ; — mucilage may 

 be procured from linseed, from the bulbs 

 of the hyacinth, from the leaves of the 

 marsh-mallows, from several of the lich- 

 ens, and from many other vegetable sub- 

 stances. 



From the analysis of M.M. Gay Lus- 

 sac and Thenard, it appears that gum 

 arabic contains in 100 parts of carbon 

 42.23, of oxygen 50.84, and of hydrogen 

 6.93, with a small qantity of saline and 

 earthy matter, or of carbon 42.23, oxy- 

 gen and hydrogen in the proportions 

 necessary to form water — 57.77. This 

 estimation agrees very nearly with the 

 definite proportions of 11 of carbon, 10 

 of oxygen, and 20 of hydrogen. 



All the varieties of gum and mucilage 

 are nutritious as food. They either par- 

 tially or wholly lose their solubility in 

 water by being exposed to a heat of 500° 

 or 600° Fahrenheit, but their nutritive 

 powers are not destroyed unless they 

 are decomposed. Gum and mucilage are 

 employed in some of the arts, particu- 

 larly in calico printing ; till lately, in this 

 country the calico printers used gum ara- 

 bic, but many of them at the suggestion of 

 Lord Dundonald, now employ the mu- 

 cilaoe from lichens. 



2. Starch is procured from different 

 vegetables, but particularly from wheat 

 or potatoes. To make starch from wheat, 

 the grain is steeped in cold water till it 

 becomes soft, and yields a milky juice 

 by pressure ; it is then put into sacks of 

 linen, and pressed in a vat filled with 

 water : as long as any milky juice ex- 

 udes, the pressure is continued ; the fluid 

 gradually becomes clear and a white pow- 

 der subsides, which is starch. 



Starch is soluble in boiling water, but 

 not in cold water, nor in spirits of wine. 

 According to Dr. Thomson, it is a charac- 

 teristic property of starch to be soluble in 

 a warm infusion of nut-galls, and to form 

 a precipitate when the infusion cools. 



Starch is more readily combustible than 

 gum ; when thrown upon red-hot iron, 

 it burns with a kind of explosion, and 

 scarcely any residuum remains. 



According to Gay Lussac and Then- 

 ard, 100 parts of starch are composed 

 of,— 

 Carbon, with a small quantity of 



saline and earthy matter, 43.55 



Oxygen, - - - - 49.68 

 Hydrogen, - - - - 6.77 



Or Carbon, - - - - 43.55 

 Oxygen and hydrogen in the pro- 

 portions necessary to form water, 56.45 



