358 Chemical Observations. 



acidulated in the slightest degree by pure nitric or acetic acid, 

 and divided into several portions on watch-glasses, may yield 

 the most distinct and characteristic indications with the three 

 best liquid tests, ammonio-nitrate of silver, ammonio-sulphate 

 of copper, and hydro-sulphuret of ammonia. 



The best proof I can give that there is no difficulty in this 

 application of liquid tests is, that it has succeeded with every 

 one of my practical pupils, setting out with the reduction of a 

 quantity of the sulphuret of arsenic which never exceeded one- 

 tenth of a grain, although it was in general their first attempt 

 to test for arsenic. 



The process for demonstrating the presence of arsenic seems, 

 therefore, to be finally perfected by this unexceptionable 

 application of the liquid tests, devised by Mr. Clark. 



4. — Chrome Orange. 



It is singular, that, although no other colour has been so 

 much run upon for a couple of years in cotton yarns, no 

 account of the mode of raising this beautiful tint, so far as I 

 can learn, has hitherto been published ; yet the process is uni- 

 versally known, and followed by dyers. 



The first object is to procure upon the yarns a good body of 

 chrome yellow, of the ordinary and familiar tint of chromate of 

 lead. For this purpose, the goods are well charged with prot- 

 oxide of lead, which is done by dipping them in solution of 

 acetate of lead, and then decomposing the salt by lime-water, 

 of which the lime takes the acetic acid, and leaves the oxide of 

 lead in the cloth. Every trace of lime must then be got rid of 

 by washing. 



It is necessary to have nothing but oxide of lead in the 

 cloth ; for, with acetate or nitrate of lead as the mordant, the 

 colour will be uneven. The goods are then passed through a 

 bath of bichromate of potash, which instantly strikes the 

 chrome yellow with the oxide of lead. 



The orange is raised by throwing the goods so prepared into 

 lime-water at or near a boiling heat. Lime, at that tempera- 

 ture, appears to be capable of partially decomposing the chro- 

 mate of lead, taking half the chromic acid from a greater or 

 less portion of that salt, and reducing it to the state of dichro- 

 mate of lead. 



