OF ARKANSAS. 187 



removed by sulphhydrate of ammonia); the carbonate of zinc washed with 

 hot water, and ignited. 



IV. The filtrate from the sulphuret of zinc is acidified with hydrochloric 

 acid, the sulphur separated by filtration, the lime precipitated with oxalate 

 of ammonia, and in the filtrate the magnesia determined as phosphate of 

 magnesia-ammonia. 



For practical purposes it is unnecessary to ascertain the relative quan- 

 tities of all the different impurities, and the following method, which was 

 employed in the analysis of Nos. 4, 6, and 7, and which recommends 

 itself by great simplicity, may be advantageously followed: 



I. The mineral is ignited, and the amount of carbonic acid and water 

 ascertained by the loss in weight. 



II. The ignited substance is digested in a beaker, covered with a watch- 

 glass, with a mixture of ammonia and carbonate of ammonia, the residue 

 collected on a filter, ignited, and weighed. The difference in weight 

 expresses the amount of oxide of zinc extracted. 



III. The residue consists of silica, alumina, iron, lime, and magnesia, 

 and also contains silicate of zinc, if this compound was present in the ore. 

 Its presence is easily detected, by treating a portion of the residue before 

 the blowpipe on charcoal. 



From the fact that silicate of zinc is insoluble in ammonia, it follows 

 that this mode of analysis is not admissible for ores containing more than 

 a few per cent, of this compound. 



Schwarz (see Mohr " Lehrbuch der Titrirmethodc, part I, p. 231, and 

 part II, p. 74,' 1 ) recommends to precipitate the zinc from the ammoniacal 

 solution (II) by means of sulphhydrate of ammonia, to treat the sulphuret 

 of zinc with sesquichloride of iron which is thus reduced to protdchloride, 

 and to determine the amount of the latter with chamaeleon mineral; 2 

 equivalents of iron correspond to 1 equivalent of zinc. I have not suc- 

 ceeded in obtaining satisfactory results by means of this method. The 

 reasons for my failure will appear from the following considerations: It is 

 of the utmost importance that the excess of sulphhydrate of ammonia, 

 employed in precipitating the zinc, be thoroughly removed by washing; if 

 this is neglected, a portion of the sesquichloride of iron will become 

 reduced at the expense of this compound, and the amount of zinc, conse- 

 quently, be found too high. Now, it is extremely difficult to remove the 

 last trace of the precipitant by washing; even in using boiling water it 

 required ntarhj two days' icasliing, and I have no doubt that during this 

 time a perceptible quantity of sulphuret of zinc becomes oxidized and 

 passes into the washings as sulphate of zinc. If dilute ammonia is used 



