SIR H. DAVY S AGRICULTURAL CHEMISTRY. 



123 



tlic misletoe, from gum mastic, opium, 

 and from the berries of the smilax cadii- 

 ca, in which last plant it has been lately 

 discovered by Dr. Barton. 



Gum elastic, when distilled, affords 

 volatile alkali, water, hydrogen and car- 

 Ijon, in diflerent combinations. It there- 

 fore consists principally of azote, hydro- 

 u;cn, oxygen and carbon ; but the propor- 

 tions in which they are combined, have 

 not yet been ascertained. Gum elastic is 

 an indigestible substance, not fitted for the 

 food of animals ; its uses m the arts are 

 well known. 



' 7. Extract, or the extractive princi- 

 ple, exists in almost all plants. It may 

 be procured in a state of tolerable purity 

 from saffron, by merely infusing it in 

 water, and evaporating the solution. It 

 may likewise be obtained from catechu, or 

 Terra Japanica, a substance brought 

 from India. This substance consists prin- 

 cipatly of astringent matter, and extract ; 

 by the action of water upon it, the astrin- 

 gent matter is first dissolved, and may be 

 separated from the extract. Extract is 

 always more or less colored ; it is solu- 

 ble in alcohol and water, but not soluble 

 in ether. It unites with alumina when 

 that earth is boiled in a solution of extract, 

 and it is precipitated by the salts of 

 alumina, and by many metallic solutions, 

 particularly the solution of muriate of tin. 



From the products of its distillation, it 

 seems to be composed principally of hy- 

 drogen, oxygen, carbon, and a little azote. 



There appears to be almost as many 

 varieties of extract as there are species of 

 plants. The diflference of their proper- 

 ties, probably in many cases depends upon 

 their being combined with small quanti- 

 ties of other vegetable principles, or, to 

 their containing diflerent saline, alkaline, 

 acid, or earthy ingredients. Many dying 

 substances seem to be of the nature of ex- 

 tractive principle, such as the red color- 

 ing matter of madder, and the yellow dye, 

 procured from weld. 



Extract has a strong attraction for the 

 fibres of cotton or linen, and combines 

 with these substances, when they are 

 boiled in a solution of it. The combina- 

 tion is made stronger by the intervention 

 of mordants, which are earthy or metallic 

 combinations, that unite to cloth, and ena- 



ble the coloring matter to adhere more 

 strongly to its fibres. 



Extract, in its pure form, cannot be 

 used as an article of food, but it is proba- 

 bly nutritive when united to starch, mu- 

 cilage, or sugar. 



8. Tannin, or the tanning principle, 

 may be procured by the action of a small 

 quantity of cold water, on bruised grape 

 seeds, or pounded gall-nuts ; and by the 

 evaporation of tlic solution to dryness. It 

 appears as a yellow substance, possessed 

 of a highly astringent taste. It is difficult 

 of combustion. It is very soluble, both 

 in water and alcohol, but insoluble in 

 ether when a solution of glue, or isinglass 

 {gelatine) is mixed with an aqueous solu- 

 tion of tannin, the two substances, i. e. 

 the animal and vegetable matters, fall down 

 in combination, and form an insoluble pre- 

 cipitate. 



When tannin is distilled in close ves" 

 sels, the principal products are charcoal? 

 carbonic acid, and inflammable gases, 

 with a minute quantity of volatile alkali. 

 Hence its elements seem the same as those 

 of extract, but probably in diflerent propor- 

 tions. The characteristic property of tan- 

 nin is its action upon solutions of isinglass 

 or jelly ; this particularly distinguishes it 

 from extract, W'ith which it agrees in most 

 other chemical qualities. 



There are many varieties of tannin, 

 which probably owe the diflference of their 

 properties to combinations with other 

 principles, especially extract, from which 

 it is not easy to free tannin. The purest 

 species of tannin is that obtained from the 

 seeds of the grape; this forms a white 

 precipitate, with a solution of isinglass. 

 The tannin from gall-nuts resembles it in 

 its properties. That from sumach aflbrds 

 a yellow precipitate ; that from kino a 

 rose colored ; that from catechu a fawn 

 colored one. 



The coloring matter of Brazil wood? 

 which Mr. Chevreul considers as a pecu- 

 liar principle, and which he has called 

 Hematine, differs from other species of 

 tannin, in affording a precipitate with gela- 

 tine, which is soluble in abundance of hot 

 water. Its taste is much sweeter than 

 that of the other varieties of tannin, and 

 it may perhaps be regarded as a substance 

 intermediate between tannin and extract. 



