276 Dr Colquhoun on the Assay 



oxide of iron generally contains a very small quantity of silica, 

 but seldom or never any notable trace of alumina. Muriatic 

 acid, assisted by heat, dissolves out the oxide of iron, and 

 leaves behind the silica in the state of insoluble gelatinous 

 flocks. 



Should the presence of alumina be suspected, the oxide, 

 previously to its desiccation, ought to be digested with some 

 caustic potash or soda ; and the earth may be subsequently 

 precipitated by supersaturating the alkaline solution with an 

 acid, and then decomposing it, in a boiling temperature, with 

 an excess of carbonate of soda. 



After having thus obtained the oxide of iron contained by 

 the ore, the next step is to separate the lime which may also 

 exist in it. It is necessary, for this purpose, that the alkaline 

 liquid separated from the oxide of iron should be rendered 

 exactly neutral : which may be done, either by adding to it 

 dilute muriatic acid until it ceases to affect vegetable colours, 

 or by supersaturating it slightly with the acid, evaporating to 

 dryness, and redissolving the sahne residue in water. Into 

 this neutralized liquid pour a solution of oxalate of ammonia 

 drop by drop so long as a precipitate continues to fall, and 

 when this is done, set the mixture aside for six or eight hours, 

 in a situation where it will be maintained at a temperature of 

 about 100°. The whole of the lime will by that time be pre- 

 cipitated in the state of an oxalate. Let this be transferred 

 upon a filter, washed and dried in a very gentle heat. 100 

 parts of it, dried in this manner, are equivalent to 38.36 parts 

 of lime or to 68.49 parts of carbonate of lime. If this oxalate 

 be cautiously calcined in a very low red heat, in a platinum 

 crucible, it will be converted into pure carbonate of lime, 100 

 parts of which represent 56 parts of lime. 



accuracy, to make the precipitation in a stoppered bottle, and to draw off 

 the clear liquid with a syphon, after the oxide has had time to subside. 

 The bottle should then be filled up with water, agitated, and set aside un- 

 til the oxide has again subsided to the bottom, after which the liquid should 

 be drawn off as before. In this manner the oxide should be washed seve- 

 ral times, while it is kept from all access of the atmospheric air, until the 

 greater portion of the soluble matter is separated. It may then, with suf- 

 ficient safety, be collected upon a filter, and subjected to the treatment de- 

 scribed above. 



