340 M. Berzelms on the Detection 



drop into it small portions of the mixture, which burn without 

 deflagrating, when too Httle saltpetre is not used. The mass is 

 dissolved in some drops, or in as small a quantity of water as 

 possible, then lime-water added in excess, and heated to boiling, 

 by which the arseniate of lime is more easily collected and washed. 

 The precipitate is collected, mixed with fresh burned charcoal 

 powder, and put into a glass tube of the following form ; 



7. a 



a 



so that the mixture comes to lie at a. The tube is first gently 

 heated to drive off any moisture the mixture may have ab- 

 sorbed, and then the under part of a is kept in the flame of the 

 blowpipe until the glass begins to melt. The arsenic is now re- 

 duced and collected in the neck b, where it is spread over so 

 small a surface that the smallest quantity may be detected. One- 

 tenth of a grain of sulphuret of arsenic is sufficient to afford a 

 satisfactory and decisive reduction test. Even the arseniate of 

 lime, which is obtained from one- sixth part of a grain of sul- 

 phuret of arsenic, can, if carefully collected, serve for three dif- 

 ferent reduction tests *. 



In using these delicate tests, we must be sure that our re- 

 agents contain no arsenic. All the sulphuric acid which is not 

 obtained from volcanic sulphur, but either from the sulphur from 

 iron-pyrites, or immediately from iron-pyrites, contains arsenic, 

 and affords, when it dissolves zinc or iron, an arseniuretted hy- 

 drogen gas. If the same acid is used in the preparation of sul- 

 phuretted hydrogen gas, we have to dread a mixture of arseniu- 

 retted hydrogen gas with the sulphuretted hydrogen gas, by 



• The following more simple mode of obtaining metallic arsenic from sul- 

 phuret of arsenic has been lately proposed by Berzelius. A very small por- 

 tion of sulphuret of arsenic is introduced into a tube, like that on page 338, 

 and brought up to a. Then a piece of steel piano-forte wire (No. 11.), an inch 

 in length, is inserted into the tube, so far as the surface of the sulphuret. The 

 steel-wire is next to be heated in a spirit of wine lamp, and the heat gradually 

 raised in such a manner that the sulphuret, in the state of vapour, passes along 

 the surface of the glowing iron. In this way, sulphuret of iron, and sublimed 

 metallic arsenic, are obtained. The operation ought to be conducted slowly- 

 Shavings of iron will not answer, because the arsenic combines with them, 

 without any sublimation. 



