OXALIC ACID. 287 



the tartar employed. By redissolving these and sub- 

 jecting them to a careful evaporation, pure crystals 

 may be obtained. 



Tartaric acid is used in many of the arts, where 

 the acidulous tartrate of potash could not produce the 

 effect desired. It is employed in several processes 

 in dyeing 1 , especially in calico-printing, in which it is 

 used to discharge false prints. This acid is extremely 

 soluble in water, and is very safe in its application. 



Oxalic acid is another vegetable acid, which is also 

 much used in the arts. It is an article of great con- 

 sumption in topical dyeing, both in the crystalline 

 state and in that of super-oxalate of potash. It has 

 been attempted to use the latter instead of citric acid; 

 but it is so insoluble, that there is some difficulty in 

 making solutions of sufficient strength. This salt is 

 used for removing ink spots, and is an excellent test 

 for discovering the presence of lime, for which sub- 

 stance oxalic acid has so great an affinity as to sepa- 

 rate it from any of the other acids, forming with it a 

 pulverulent insoluble salt, which is not easily decom- 

 posed, except by the agency of fire. 



Oxalic acid, combined with potash, is found in the 

 leaves of the wood sorrel, or oxalis acetosella, and in 

 those of the common sorrel, or rumex acetosa. The 

 expressed juice of the leaves of either of these wild 

 plants is diluted with water, and set aside for subsi- 

 dence: in a few days the feculent parts precipitate, 

 and the supernatant liquor becomes clear. It is then 

 strained off, evaporated, and placed in a cool situation 

 favourable to crystallization ; or sometimes the juice, 

 as soon as expressed, is clarified with white of eggs, 

 and being thus cleansed from its feculencies is at once 

 evaporated. 



The first product of crystals being taken out, the 

 liquid is still farther evaporated and crystallized, until 



