Chemical Science, 21S 



pass by it ; the operation should be conducted slowly ; the sulphur 

 will burn into sulphurous acid and escape, and the arsenic into 

 arsenious acid, which will condense in the upper cool part in crys- 

 tals. The tube is then to be softened in the lamp, and drawn out 

 below the arsenious acid ; a little piece of charcoal is to be intro- 

 duced, and then the arsenious acid passed across it in vapour, to the 

 narrow elongated part of the tube ; it will be reduced by the charcoal 

 in its passage, and metallic arsenic will appear. This process never 

 fails. — Annalen der Physikf 1828, 158. 



29. Oil a new use of the Chromate of Potash, hy M. Kaechlin- 

 Schouch. — This use of the chromate is to print a white pattern on 

 a blue or green ground. A blue colour is first given to the cloth 

 by means of the indigo-vat, more or less deep according to the 

 green required ; the cloth is then prepared with the aluminous 

 mordant, and passed through hot water ; it is then again prepared 

 with an uugummed solution of bi-chromate of potash, consisting of 

 2 J ounces of salt to 4 pints of water. It is then printed with the 

 following preparation ; 



Water thickened by roasted starch . 4 pounds 

 Tartaric acid . . .10 ounces 



Oxalic acid , . , . 6 ounces 



Nitric acid ... .2 ounces 



The nitric acid is unnecessary, except for delicate designs. The 

 moment this substance is printed, the blue colour is destroyed ; the 

 cloth is instantly put into running water, and afterwards dyed in 

 quercitron, or other dye stuffs. 



This destruction of vegetable colour arises fi'om the following 

 general fact : whenever chromate of potash is mingled with tartaric 

 or oxalic acid, or with a neutral vegetable substance and a mineral 

 acid, as the sulphuric or the nitric, a strong action takes place, 

 accompanied with the disengagement of heat and gaseous sub- 

 stances. The principal product of this reciprocal action is a new 

 body having acid properties. During the effervescence which takes 

 place, the mixture has the power of destroying vegetable colours. 

 Carbonic acid is evolved during the decomposition ; and when the 

 mixture is made in a retort, there comes over a colourless liquid, 

 slightly acid, having the odour of weak acetic acid, and reducing 

 the nitrates of silver or mercury if heated with them (formic acid?). 



When 9 parts of tartaric acid and 10 parts of chromate of potash 

 are boiled with water, a neutral green liquid is obtained, which, 

 being evaporated, does not crystallize, but become a brittle green 

 mass. When acetate of lead is added to the solution, a precipitate 

 is formed, which being well washed and then carefully decomposed 

 by sulphuric acid, yields a very acid green fluid, uncrystallizable, 

 and with alkalies forming either acid greenish violet salts or neutral 

 green salts. Cold sulphuric or nitric acids do not act upon this 

 pubstance ; but, being heated, they decompose it. When the acid 



