VEGETABLE ACIDS, 161 



clolhs. In this process the whole fabric is covered with a 

 mordant of iron, which is afterwards removed by means of 

 this acid combined with gum, so that the color applied ad- 

 heres firmly only to those parts where the mordant has not 

 been destroyed: this process is conducted with far more ease 

 than that which was formerly practised, of applying the 

 mordant with the block, reserving those parts untouched 

 which were not to receive a fixed color. 



The oxalic acid is better than any other for removing 

 ink spots from cloth : it is only necessary for this purpose 

 to put a little upon the spot, and to moisten it with a drop 

 of water, after which a slight rubbing with the hand and 

 a little rinsing in pure water removes every vestige of the 

 stain. 



Messrs. Gay-Lussac and Thenard obtained, from aq an- 

 alysis of oxalic acid, carbon, oxygen, and hydrogen, in the 

 following proportions. 



100 parts of oxalic acid, 



Carbon 26.566 



Oxygen 70.689 



Hydrogen .... 2.745* 



Tartaric acid may be extracted from the juice of th« 

 mulberry, grape, currant, &c. This acid is almost always 

 found in vegetables combined with potassa, with which it 

 forms a nearly insoluble salt : it is this union which occa- 

 sions it to be so easily precipitated from the liquors in which 

 it is contained, especially when they ferment. The coats of 

 tartar which are found deposited upon the sides of casks 

 are a combination of tartaric acid, potassa, and extractive 

 matter. 



When tartar and the lees of wine are burned together, 

 they leave alight, grayish, alkaline residuum, known in com- 

 merce under the name of tartarated ashes ; this product has 

 its particular use in the arts. 



The crystallized substance known in commerce, and 

 extensively used, under the name of cream of tartar^ is 

 prepared by dissolving tartar in water containing pipe- 

 clay ; this solution, after having been filtrated, is carefully 

 evaporated till crystallization takes place ; a part of the 

 extractive matter of the tartar is separated and falls to the 

 bottom of the vessel, the rest remains in solution. The 

 crystals thus obtained are composed of potassa with an 



[• By the best analysis no hydrogen is found in oxalic aeids. — Tb.] 

 14* 



