SIR H. DAVY S AGRICULTURAL CHEMISTRY. 



125 



substances are employed, the difference of 

 the weights of the papers when dried, 

 will indicate with tolerable accuracy, the 

 quantities of tannin contained by the sub- 

 stances, and their relative value, for the 

 ]iurposes of manufacture. Four-tenths of 

 the increase of weight in grains must be 

 taken, which will be in relation to the 

 weights in the table. 



Besides the barks already mentioned? 

 there are a number of others which con- 

 tain the tanning principle. Few barks 

 indeed are entirely free from it. It is 

 likewise found in the wood and leaves of 

 a number of trees and shrubs, and is one 

 of the most generally diffused of the vege- 

 table principles. A substance very simi- 

 lar to tannin has been formed by Mr. 

 Hatchett, by the action of heated diluted 

 nitric acid on charcoal, and evaporation 

 of the mixture to dryness. From 100 

 grains of charcoal, Mr. Hatchett obtained 

 120 grains of artificial tannin, which, like 

 natural tannin, possessed the property of 

 rendering skin insoluble in water. 



Both natural and artificial tannin form 

 compounds with the alkalies and the alka- 

 line earths; and these compounds are not 

 decomposible by skin. The attempts 

 that have been made to render oak bark 

 more efficient as a tanning material by in- 

 fusion in lime water, are consequently 

 founded on erroneous principles. Lime 

 forms with tannin a compound not solu- 

 ble in water. The acids unite to tannin, 

 and produce compounds that are more or 

 less soluble in water. It is probable that 

 in some vegetable substances tannin exists, 

 combined with alkaline or earthy matter; 

 and such substances will be rendered more 

 efficacious for the use of the tanner; by the 

 action of diluted acids. 



9. Indigo may be produced from wood 

 {Isatis tinctoria,) by digesting alcohol on 

 it, and evaporating the solution. White 

 crystalline grains are the substance in 

 question. The indigo of commerce is 

 principally brought from America. It is 

 procured from the Indigo/era argentea, 

 or wild indigo, the Indigofera disperma, 

 or Gautimala indigo, and the Indigofera 

 tinctoria, or French indigo. It is pre- 

 pared by fermenting the leaves of those 

 trees in water. Indigo in its common 

 form appears as a fine, deep blue powder, 



it is insoluble in water, and but slightly 

 soluble in alcohol; its true solvent is sul- 

 pliuric acid: 8 parts of sulphuric acid dis- 

 solve 1 part of indigo; and the solution 

 diluted with water forms a very fine blue 

 clye. 



Indigo, by its distillation, affords car- 

 bonic acid gas, water, charcoal, ammonia, 

 and some oily and acid matter: the char- 

 coal is in very large proportion. Pure 

 indigo, therefore, most probably consists 

 of carbon, oxygen, hydrogen, and azote. 



Indigo owes its blue color to combina- 

 tion with oxygen. For the uses of the 

 dyers it is partly deprived of oxygen, by 

 digesting it with orpiment and lime water, 

 when it becomes soluble in the lime water, 

 and of a greenish color. Cloths steeped 

 in this solution combine with the indigo; 

 they are green when taken out of the 

 liquor, but become blue by absorbing 

 oxygen when exposed to air. 



Indigo is one of the most valuable and 

 most extensively used of the dying ma- 

 terials. 



10. The narcotic principle is found 

 abundantly in opium, which is obtained 

 from the juice of the white poppy [Papu- 

 rea album.) 



To procure the narcotic principle, water 

 is digested upon opium: the solution ob- 

 tained is evaporated till it becomes of the 

 consistence of a syrup. By the addition 

 of cold water to this syrup, a precipitate 

 is obtained. Alcohol is boiled on this 

 precipitate ; during the cooling of the 

 alcohol, crystals fall down. These crystals 

 are to be again dissolved in alcohol, and 

 again precipitated by cooling : and the 

 process is to be repeated till their color is 

 white; they are crystals of narcotic prin- 

 ciple. The narcotic principle has no taste 

 nor smell. It is soluble in about 400 

 parts of boiling water ; it is insoluble in 

 cold water: it is soluble in 24 parts of 

 boiling alcohol, and in 100 parts of cold 

 alcohol. It is very soluble in all acid 

 menstrua. 



It has been shown by De Rosne, that 

 the action of opium on the animal economy 

 depends on this principle. Many other 

 substances besides the juice of the poppy, 

 possess narcotic properties; but they have 

 not yet been examined with much atten- 

 tion, — The Lactuca sativa, or garden let- 



