166 Chemistry. [Lecture 29. 



plied to use in calico-printing ; for, being mixed 

 in paste and stamped on cotton stuffs in various 

 patterns, they either prevent the colour with 

 which the ground is formed from fixing itself 

 in that part, or dislodge it, after dyeing, at plea- 

 sure, thus leaving the imprinted part in its 

 original whiteness. This effect results from 

 their attraction for the metallic basis of the co- 

 lour, while, unlike the mineral acids, they leave 

 the fibre of the cloth untouched. 



The component parts of the citric acid are 

 oxygen, hydrogen, and carbon. 



The malic and oxalic acids nearly resemble 

 those which we have now specified. 



Gallic acid exists ready formed in galls. It 

 produces a black dye, with solutions of iron, by 

 uniting with that metal into an insoluble com- 

 pound called gallat of iron. Hence, besides the 

 utility of galls in writing ink and dyeing, the 

 infusion of galls in water or spirit is one of the 

 best tests for the presence of iron in solutions 

 where it is suspected, but not otherwise appa- 

 rent. 



Prussic or hydrocyanic acid is also employed 

 by the chemists for this purpose. 



This, which may be regarded as an animal 

 acid, is wholly a creature of art ; and the disco- 

 very of Prussian blue (whose colouring basis is 

 prussiat or hydrocyanat of iron) was made long 

 before we knew any thing of this acid. For the 

 preparation of Prussian blue, a quantity of pot- 

 ass is calcined with dried blood till it ceases to 



