1839.] 



On Chemical Tests. 



301 



salts with protoxide of antimony, at least we are not acquainted at pre- 

 sent with any such combinations. 



39. arsenic is detected (.7), by nitrate of silver ;(&), sulphate of cop- 

 per; (c), sulphuretted hydrogen; (d), soda; (e), iron. It is also pre* 

 cipitated by salts of lead and by lime water; the precipitates being 

 white. 



a. By nitrate of silver, which gives a yellow precipitate in arsenitcs, 

 and h brown precipitate in arseniates, bat, the solution ought to be 

 slightly alkaline. This test however must not by itself be considered 

 decisive, as it is acted on by many other substances. 



b. By sulphate of copper or by acetate of copper, both of which give 

 green precipitates, but in this case a little carbonated alkali must be 

 present in solution. 



c. By sulphuretted hydrogen, which produces a bright yellow colour 

 in acidulated solutions of arsenic, but a precipitate does not form till 

 afrer long standing. 



d. By soda. If arsenic mixed with a little soda be submitted to the 

 action of the blow-pipe, a smell of garlic occurs. 



e. The red-acetate of iron detects arsenic by forming a bright yellow 

 deposit. Pure hydrate of iron recently made and suspended in water, 

 completely precipitates arsenious acid. 



f. Note. There is mu h difficulty in detecting small quantities of 

 arsenic, and much caution is required lest false conclusions should 

 be formed; especially in investigations connected with cases of 

 poisoning. The garlic like smell by heat is a good criterion, and 

 still better if confirmed in the fallowing manner. Mix the substance sup- 

 posed" to contain arsenic with fresh made charcoal, or with black flux, in 

 a small glass tube ; submit it to heat, and if arsenic be present it will 

 sublime to the upper part of the tube, where it will be deposited and 

 may be recognized by its steel coloured lustre. The open end of the 

 tube must be loosely stopped with a piece of cotton, or other porous 

 substitute, see 2, a : and the remainder of this article. 



g. The following method for detecting arsenic has been published 

 by professor O'Shaughnessy at Calcutta. See Journal of the Asiatic 

 Society of Bengal, February 1839. Also Jameson's Journal No. 42, ok 

 the Arcana of Science for 1837. 



h. The beautiful process invented by Mr. Marsh of Woolwich, for 

 the detection of minute quantities of arsenical poison consists in placing 



