PRELIMINARY EXAMINATION. 207 



colors the bead more or less intensely, according to the quantity 

 of the metallic compound used. 



Some metals (copper, for instance) forming two series of 

 compounds, give different colors to the bead when present in 

 either the higher or lower state of oxidation. 



By modifying the blowpipe flame so as either to oxidize (by 

 supplying an excess of atmospheric oxygen) or deoxidize (by 

 allowing some unburnt carbon to remain in the flame), the 

 metallic compound in the bead may be made to assume the 

 higher or lower state of oxidation. A copper bead may thus 

 be changed from blue to red or red to blue, the blue bead con- 

 taining the copper in the cupric, the red bead in the cuprous 

 form. 



8. Liquefaction of solid substances. Most solid substances have 

 to be dissolved for analysis. The solvents used are water, or for 

 substances insoluble in that liquid the mineral acids, especially 

 diluted, or, if necessary, strong hydrochloric acid. The dissolv- 

 ing action of the acid should be facilitated by the aid of heat. 

 Nitric or even nitrohydrochloric acid may have to be used in 

 some cases. 



Three mistakes are frequently made by beginners in dissolv- 

 ing substances in acids, viz. : The substance is not powdered as 

 finely as it should be; sufficient time is not given for the acid to 

 act; too large an excess of the acid is used. 



If a substance is partly dissolved by water and partly by one 

 or more other solvents, it may be well to examine the different 

 solutions separately. 



Substances insoluble in water and acids have to be rendered 

 soluble by fusion with a mixture of potassium and sodium car- 

 bonate, or with potassium acid sulphate, or by the action of 

 hydrofluoric acid. 



The insoluble sulphates of the alkaline earths, when fused 

 with the alkaline carbonates, are converted into carbonates, 

 while the sulphates of the alkalies are formed. The latter com- 

 pounds may be eliminated by washing the fused mass with 

 water and filtering; the solid residue upon the filter contains 

 the carbonates of the alkaline earths, which may be dissolved 

 in hydrochloric acid. 



