314 



On Chemical Tests. 



[Oct. 



united action of these three tests. Hydrosulphureis produce a dark 

 brown, or black precipitate. See the tables cf re-agents. 



a. Protonitrate of mercury precipitates /old as a mixture of calomel 

 and metallic gold. Sulphurous acid precipitates gold in the metallic 

 state. Gold is easily reduced by soda on charcoal before the blow-pipe. 

 It is refined by cupellation, and by quartation. 



60. GOLD, muriate of, detects tin producing a purple precipitate, 

 but a little carbonate of sqda should be present in solution . The colour 

 of the precipitate varies in proportion to the relative quantities of the 

 tin and the test. The colour produced may be violet, or even rose 

 coloured, see 122. 



61. HYDSOGSN sulphuretted, acts on most of the metals. See 

 the tables of re-agents. Tins test is useful in analysis either by itself 

 in the form of gas, or in combinetion with water, it precipitates many 

 of the metals in the form of sulphurets. See 62 for detecting this sub- 

 stance. 



62. hydrosulphurets in solution are detected by acetate of 

 lead producing a black precipitate; by nitrate of silver, or nitrate of 

 mercury producing also a black precipitate; by arsenious acid produ- 

 cing a yellow precipitate, and by silver leaf, or quicksilver which be- 

 comes tarnished, see the tables of re-agents. 



63. IODINE is detected by starch which produces an indigo blue 

 colour; but this ty.ue colour is discharged when the solution is made 

 hot. Starch and iodine are reciprocal lest 5 ?. 



a. In testing for iodine the iodine must be free, if not, a minute quan- 

 tity of acid must be added. The solution of starch, (i. e. the test) must 

 be used cold. The precipitate, ioduret of starch, is soluble in dilute 

 sulphuric acid. Iodine is~soluble in alcohol. A sr,ei«m of s^phuret- 

 ted hydrogen destroys the blue colour, after starch has pioduced it in 

 any solution that may be under examination, see 9 and 113. 



64. iridium is detected by sulphuretted hydrogen which produ- 

 ces a dark brown or black precipitate, but this precipitate is not dis- 

 tinctive of the metal. Ammonia and the fixed alkalies produce a yel- 

 low precipitate ; and tincture of galls instantly destroys the red colour 

 of the solution. Iridium may be precipitated (reddish brown) by mu- 

 riate of ammonia, or the chloride of potassa, but both the test and the 



