600 



FARMERS' REGISTER 



[1838 



ration is most perfect when they are tanned slow- 

 ly. When sitins are introduced into very strong 

 infusions of tannin, the exterior parts immediately 

 combine with that principle, and defend the inte- 

 rior parts from the action of the solution : such 

 leather is liable to crack and to decay by the action 

 of water. 



The precipitates obtained from infusions con- 

 tainint^ tannm by isinglass, when dried, contain at 

 a medium rate about 40 per cent, of vegetable 

 matter. It is easy to obtain the comparative value 

 of different substances for the use of the tanner, 

 by comparing the quantities of precipitate afforded 

 by infusions ofgiven weights mixed with solutions 

 of glue or isinglass. 



To make experiments of this kind, an ounce or 

 180 grains of the vegetable substance, in coarse 

 powder, should be acted upon by half a pint of 

 boiling water. The mixture should be frequently 

 stirreil, and suffered to stand 24 hours ; the fluid 

 ehouid then be passed through a fine linen cloth, 

 and mixed with an equal quantity of solution of 

 gelatine, made by dissolving glue, jelly, or isin- 

 glass, in hot water, in the proportion of a drachm 

 of glue or isinglass, or six table-spoonfuls of jelly, 

 to a pint of water. The precipitate should be 

 collected by passing the mixture of the solution 

 and infusion through Iblds of blotting-paper, and 

 the paper exposed to the air till its contents are 

 quite dry. If pieces of paper of equal weights are 

 used, in cases in which different vegetable sub- 

 stances are employed, the diffi^rence of the weights 

 of the papers, when dried, will indicate with tole- 

 rable accuracy the quantities of tannin contained 

 by the substances, and their relative value, for the 

 purposes of manufacture. Four-tenths of the in- 

 crease 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 contain the tanning 

 principle. Few barks, indeed, are entirely free 

 irom 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 vegeta- 

 ble principles. 



A substance very similar to tannin has been 

 formed by Mr. Hatchett, by the action ol" heated 

 ililuted nitric acid on charcoal, and evaporation of 

 the mixture to dryness. From 100 grains of char- 

 coal 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 com- 

 pounds with the alkalies and the alkaline earths ; 

 and these compounds are not decomposable by 

 skin. The attempts that have been made to ren- 

 der oak bark more eiiicient as a tanning material 

 by infusion in lime water, are consequently found- 

 ed on erroneous principles. Lime forms with tan- 

 nin a compound not soluble in water. 



The acids unite to tannin, and produce com- 

 pounds that are more or less soluble in water. It 

 is probable that in some vejjetable 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. Iiidign may be procured from woad ( fsalis 

 tlnctoria,) by digcstinir alcohol on it, and evapo- 

 rating the solution. White crystalline grains are 

 obtained, which gradually become blue by the 



action of the atmosphere: these grains are the 

 substance in question. 



The indigo of commerce is principally brought 

 from America. It is procured from the Jndigo- 

 fcra argentea, or wild indigo, the Indignfera di- 

 spcrma, or Guatimala indigo, and the Indigofera 

 tinctoria, or French indigo. It is prepared by 

 fermenting the leaves of those trees in water. In- 

 digo, 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- 

 phuric acid: 8 parts of sulphuric acid dissolve 1 

 part of indigo ; and the solution diluted with water 

 forms a very fine blue dye. 



Indigo by its distillation affords carbonic acid 

 gas, water, charcoal, ammonia, and some oily 

 and acid matter : the charcoal is in very large 

 proportion. Pure indigo, therefore, most proba- 

 bly consists of carbon, hydrogen, oxygen, and 

 azote. 



Indigo owes its blue color to combination 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 materials. 



10. There are a number of coloring principles 

 found in different vegetable productions, the pro- 

 perties of which are less marked than those of in- 

 digo, and the separation more difficult. The co- 

 loring matters of carthamus and madder are the 

 most fixed amongst the red vegetable colors. A 

 number of vegetable substances are rendered red 

 by the action of acids, and green by that of alka- 

 lies. They all seem to be composed of different 

 proportions of hydrogen, oxygen, and carbon ; but 

 are so liable to change, that few distinct experi- 

 ments have been made upon their nature. In 

 dying, they are usually applied to cloths prepar- 

 ed for receiving them by combination with certain 

 saline or metallic preparations called mordants ; 

 and, in consequence of the triple union formed 

 between the cloth, the mordant, and the coloring 

 matter, the tint is modified, or changed, and ren- 

 dered more permanent. 



11. The bitter principle is very extensively dif- 

 fused in the vegetable kingdom; it is found abun- 

 dantly in the hop (Humulus Lupulus'), in the 

 common broom (Spartium scoparrwni), in the 

 chamomile (^Anthemis nobilis), and in Quassia 

 amara and excelsa. It is obtained from those 

 substances by the action ol' water or alcohol, and 

 evaporation. It is usually of pale yellow color; 

 its taste is intensely bitter. It is very soluble, 

 both in water and alcohol; and has little or no 

 action on alkaline, acid, saline, or metallic solu- 

 tion. 



An artificial substance, similar to the bitter 

 principle, has been obtained by digesting diluted 

 nitric acid on silk, indigo, and the wood of the 

 white willow. This substance has the property 

 of dying cloth of a bright yellow color; it differs 



* By a carefully regulated temperature, it may be 

 sublimed without decomposltioiij and obtained in a 

 pure crystalline state.— J D. 



