358 Purification of Zinc. 



a mixture of arfenic, copper, and lead which the difoxidating adion of the zinc precipitates in 

 the metallic flate. This alfo happens to tliefe metals when afolution is made of tin with which 

 they are alloyed. Arfcnical lead, fuchasis made ufe of at the mines of Limares for the making 

 fmall (hot, depofit on the immerfion of a (heet of lead, part of the metallic arfenic they con- 

 tained. Thefe are not the only metals which adulterate the purity of the zinc. Iron and 

 manganefe are fometimes found in great quantities : w hence it is not furprizing that the 

 art of watch- making hasfo much reafon to complain of the bad qualities of brafs made in the 

 common way. The following fa£ls will (hew the enormous difference between purified 

 zinc, and that of commerce. To purify zinc, Mr. Proud propofes diftilling it in a 

 ftone retort, the neck of which mull be inclined at leaft 45 degrees, that the metal may 

 flow with greater eafe as foon as it becomes volatalized. A mixture of fand, and of the 

 oxides of iron, lead, copper, and zinc remain in the retort, the oxidation of which is attri- 

 buted by M. Prouft to the porofity of the veflels. 



The zinc thus purified does not differ either in colour or in weight from the zinc of 

 commerce : 55 decigrams ( 100 Fr. grains) of the latter yield by folution in fulphuric acid, 

 in lefs than hour, 3440 centimetres (from 172 to 174 Fr. inches) of hydrogen gas; whereas 

 more than 8 days are requifite to obtain the fame refult from diftilled zinc; This is the 

 difference which M. Prouft obferved between the two metals, The black matter which 

 feparates from the zinc, during its folution in fulphuric acid, is foluble in nitric acid ; and 

 Its folution on the addition of hydro-fulphurated water affords arfenic; and if that arfenic 

 had been mixed with copper or lead, the fame re-agent would have equally difcovered it. 

 For by adding it gradually to the folution of thofe three metals, the copper is feen to preci- 

 pitate itfelf firft of a brown colour the lead, the next of a black colour, and the arfenic 

 laft of a yellow colour ; and by a careful management they may be feparated by fil- 

 tration. 



M. Prouft juftly confiders the purification of zinc by fulphur as an abfolute deception. 



55 decigrams ( 100 Fr. grains) of zinc, dilTolved in nitric acid, leave, after the decompo- 

 fition of the nitre by fire, 69 decigrams (from 125 to 126 Fr. grains) of oxide of a light 

 yellow colour. A folution of the fame quantity of zinc, decompofed by carbonate of pot- 

 afh, furnifhed 99 decigrams (180 Fr. grains) of carbonate of zinc, which alfo left 125 grains 

 of oxide after calcination. 



The folution of this metal by fulphuric acid gave the fame refults ; which proves that 

 zinc, in all cafes, combines itfelf with a fpecific quantity of oxygen. M. Prouft obferves 

 how fingular it is that copper and zinc, which attraft the oxygen with fuch different degrees 

 of ftrength, fhould, neverthclefs, in their combination with acids, abforb exadly the fame 

 proportion of this principle. 



M. Prouft, in the next place, propofes the following formula to feparate zinc from cop- 

 per. Suppofe the fame folvent to contain lead, copper, and zinc. Sulphate of pot-afh will 

 precipitate the lead, if the (nitric) folution does not contain too much acid. Hepatic water 

 feparates the copper a long time before the zinc. The filtered liquid may be examined by 



fulphurated 



