286 ANNUAL OF SCIENTIFIC DISCOVERT. 



solution of logwood, the first two remain colorless because they con- 

 tain no mordant, while the stuff dyed with pencil-blue becomes red on 

 account of the tin which it contains. 



The blue of soluble indigo, and that obtained with cyanide of po- 

 tassium, agree in being destroyed by alkalies; at the same time, how- 

 ever, the blue of indigo leaves a white ground, while that of the 

 cyanide leaves a rusty yellow ground on account of the iron mordant 

 employed. In order to remove all doubt, a few drops of acidulated 

 solution of cyanide of potassium should be added, which, if iron is 

 present, reproduces the blue color. This confirmatory test should al- 

 ways be used in the case of green colors. 



Prussian blue may be recognized by its being decolorized by alka- 

 lies, but not by chloride of lime, while the latter re-agent destroys in- 

 digo blue. The appearance alone is sufficient to indicate whether the 

 blue is ordinary prussian or bleu de France, prepared with stannate 

 of potash. 



Logwood-blue may be recognised from the fact that it is destroyed 

 by weak acids, and becomes red ; in most cases this is a sufficient 

 ground for inferring the presence of logwood, &c. When the color to 

 be examined is a mixed one, for example, logwood blue, with prussian 

 blue or indigo, the color of the logwood is first destroyed by diluted 

 acid, the stuff washed, and treated with chlorine to ascertain whether 

 the ground-color is indigo or prussian blue. 



Ultramarine may usually be recognised by its peculiar tint; after 

 incinerating the stuff, it remains unaltered in the ash. Hydrochloric 

 acid decomposes it, disengaging at the same time an unpleasant odor 

 of sulphuretted hydrogen. When the ultramarine is imprinted with 

 varnish, the stuff must be moistened with ether before the hydrochlo- 

 ric acid will act. 



lied Color*. With the exception of safflower, the red coloring 

 matter requires a preparation of alumina or tin. 



Safflower may be easily recognised by its color being discharged by 

 caustic, potash, or soda. Madder colors, when treated with hydro- 

 chloric acid, acquire a yellow or orange tint without any shade of 

 puce; upon then being treated with milk of lime, the color becomes 

 violet at those places where the hydrochloric acid has acted. The 

 violet is permanent, and by boiling with soap, passes into rose color. 



The madder-red colors are less susceptible of alteration by acids 

 the more they have been brightened by soap and the higher the tem- 

 perature at which this took place. The great durability of the Turk- 

 ish-red is owing to this fact. 



The red and rose colors from madder are separable into several 

 kinds Turkish-red and rose, ordinary madder-red and rose, the 

 true topical red, and the colors from garancine and garanceux. 



Turkish red may be known by its brightness and indestructibility 

 by acids. Ordinary madder-red, when brightened, scarcely (lifters 

 in any particular from a true topical color. The only difference is in 

 the mode of preparation. As the topical color is prepared before 

 printing with tin, and after printing the stuff is steamed, the white is 



