596 



FARMERS' REGISTER 



[No. 11 



ihe largest part of the vegetable ; but in all cases 

 it seems to contain the sap, or solid materials de- 

 posited from the sap. 



The slender and comparatively dry leaves of the 

 pine and the cedar perform the same Junctions as 

 the large and juicy leave of the fig-tree, or the 

 walnut. 



Even in the cryptogamia class, where no flowers 

 are distinct, still there is every reason to believe 

 that the production of the seed is effected in the 

 same way as in the more perfect plants. The 

 mosses and lichens, which belong to this family, 

 have no distinct leaves, or roofs, but they are 

 furnished with filaments which perform the same 

 functions ; and even in the fungus and the mush- 

 room there is a system for the absorption and aera- 

 tion of the sap. 



It was stated in the last lecture, that all the 

 different parts of plants are capable of being de- 

 composed into a few elements. Their uses as 

 food, or for the purposes of the arts, depend upon 

 compound arrangements of those elements which 

 are capable of being produced either from their 

 organized parts, or from the juices they contain; 

 and the examination of the nature of these sub- 

 stances is an essential part of agricultural che- 

 mistry. 



Oils are expressed from the fruits of many 

 plants: resinous fluids exude fi-om the wood; sac- 

 charine matters are afforded by the sap; and d}''- 

 ing materials are furnished by leaves, or the petals 

 of flowers: but particular processes are necessary 

 to separate the different compound vegetable sub- 

 stances from each other; such as maceration, in- 

 fusion, or digestion in water, or in spirits of wine: 

 but the application and the nature of these pro- 

 cesses will be better understood when the chemi- 

 cal nature of the substances is known; the consi- 

 deration of them will therefore be reserved for 

 another place in this lecture. 



The compound substances found in vegetables 

 are, 1. gum, or mucilage, and its different modifi- 

 cations; 2. starch; 3. sugar; 4. albumen; 5. ffluten; 

 6. gum elastic; 7. extract; 8. tannin; 9. indigo: 

 10. coloring principles; 11. bitter principles; 12. 

 wax; 13. resins; 14. camphor; 15. fixed oils; 16. 

 volatile oils; 17. woody fibre; 18. acids; 19. alka- 

 lies, earths, metallic oxides, and saline compounds. 



I shall describe generally the properties and 

 composition of these bodies, and the manner in 

 which they are procured. 



1. Gam is a substance which exudes from cer- 

 tain 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 while, more or less 

 irausparent; and somewhat brittle, its specific 

 gravity varies from 1300 to 1490. 



There is a great variety of gums, but the best 

 known arc gum arable, irum senetjal. gum traga- 

 canth, 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 cum 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 inflame only with difficulty; much 

 moisture is given off in the process, which takes 

 place with a dark smoke and feeble blue flame, 

 and a coal remains. 



The characteristic properties of gum are its 

 easy solubility in water, and its insolubility in al- 

 cohol. Different chemical substances have been 



proposed for ascertaining the presence of gum, 

 but there is reason to believe that lew of them af- 

 ford accurate results; and most of them (particu- 

 larly the metallic salts), which produce changes 

 in solutions of gum. may be conceived lo act 

 rather upon some saline compounds existing in 

 the gum, than upon the pure vegetable principle.* 



Mucilage must be considered as a variety of 

 gum; it agrees with it in its most important pro- 

 perties, but seems to have less attraction for wa- 

 ter. According to Hermbstadt, when gum and 

 mucilage are dissolved together in water, the mu- 

 cilage may be separated by means of sulphuric 

 acid. Mucilage may be procured from linseed, 

 fi-om the bulbs of the hyacinth, from the leaves of 

 the marsh-mallows, from several of the lichens, 

 and from many other vegetable substances. 



From the analysis oi MM. Gay Lussac and 

 Thenard, it appears that gum arable contains in 

 100 parts, 



Of Carbon . . . - 42.23 



Oxygen .... 50.84 



Hydrogen . . . - 6.93 



With a small quantity of saline and earthy 

 matter. 



Or, of carbon - - - - 42.23 



Oxygen and hydrogen in the proper- P f,~ y-, 

 tions necessary to Ibrm water ^ 



This estimation agrees very nearly with the de- 

 finite proportions of 11 of carbon, 10 of oxygen, 

 and 20 of hydrogen. 



All the varieties of gum and mucilage are nu- 

 tritious as food. They either partially or wholly 

 lose their solubility in water by being exposed to 

 a heat of 500° or 600^ Fahrenheit, but their nu- 

 tritive powers are not destroyed unless they are 

 decomposed. Gum and mucilage are employed 

 in some of the arts, particularly in calico-printing: 

 till lately, in this country, the calico-printers used 

 gum arable; but many of them, at the suggestion 

 of Lord Dundonald, now employ the mucilage 

 from lichens. 



2. Starch is procured from different vegetables, 

 but particularly from wheat or from potatoes. To 

 make starch from wheat, the grain is steeped in 

 cold water till it becomes soil, 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 exudes the pressure is continued: 

 the fluid gradually becomes clear, and a white 

 powder subsides, which is starch. 



Starch is soluble in boiling water, but not in 

 cold water, nor in spirits of wine.f It is a cha- 

 racteristic property of starch to be rendered blue 

 by iodine. 



Starch is more readily combustible than gum; 

 when thrown upon red-hot iron, it burns with a 

 kind of explosion, and scarcely any residuum re- 

 mains. According to MM, Gay Lussac and 

 Thenard, 100 parts of starch are composed of 



* The subacetate of lead appears to be the most 

 delicate test of gum; accordinnj to the experiments of 

 Berzeliiis, the precipitate which it occasions is com- 

 posed of 38.25 oxide of lead, and 61.75 gum. — J. D. 



t Exposed to a temperature a little exceeding 212°, 

 it is rendered soluble in cokl water, it becomes very 

 similar to gum; is what De Saussure has called ami- 

 dine, and is best fitted for the use of the calico-print- 

 er. Boiling water has an analogous effect on starch; 

 boiled?starch, rendered gelatinous, has the properties 

 of amidine. — J. D. 



