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 zine 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. 
Ill. The residue consists of silica, alumina, iron, lime, and magnesia, 
and also contains silicate of zine, if this compound was present in the cre. 
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 Titrirmethode, part I, p. 231, and 
part II, p.74,°) recommends to precipitate the zinc from the ammoniacal 
solution (II) by means of sulphhydrate of ammonia, to treat the sulphuret 
of zine with sesquichloride of iron which is thus reduced to protochloride, 
and to determine the amount of the latter with chameleon 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: Itis 
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 nearly two days’ washing, and | 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 
