78 ARSENIC AND TIN. 



phide of arsenic is filtered from the sulphide of lead, 

 which is collected upon a weighed filter, washed, first 

 with weak sulphide of ammonium, then with water, 

 dried and weighed, and treated as in No. 39. 



The sulphide of arsenic is precipitated from the 

 solution by dilute hydrochloric acid, the sulphuretted 

 hydrogen expelled by a gentle heat, the precipitate 

 filtered off, washed, and gently heated, together with 

 the filter, in a beaker, with concentrated hydrochloric 

 acid, with gradual addition of chlorate of potassa, until 

 all the arsenic and part of the sulphur are oxidized 

 and dissolved. The solution is diluted with water, 

 passed through a filter, which must be well washed, 

 and the arsenic acid precipitated, as in No. 51, with 

 sulphate of magnesia and ammonia. 



53. ARSENIC AND TIN. 



The finely-divided compound is gradually and care- 

 fully oxidized with nitric acid, which is dropped upon 

 it in a weighed vessel. When it is converted into a 

 dry white mass, more nitric acid is added, and the 

 whole evaporated to perfect dryness in a water-bath. 

 The mass dried at 100 is weighed. A portion of it 

 is then weighed in a bulb-tube, one limb of which is 

 bent downwards, and dips into caustic ammonia con- 

 tained in a small flask. A stream of sulphuretted 

 hydrogen is passed through the bulb-tube, and when 

 it is filled with gas, the mass is heated, gently at first, 

 afterwards more strongly, until a sublimation takes 

 place of sulphide of arsenic and sulphur, which dissolve 

 in the ammonia. When no fresh sublimate is formed, 

 the apparatus is allowed to cool, and the piece of tube 

 cut off in which any sublimate still remains. This 

 tube is placed in warm solution of potassa, which easily 



