248 



MINERALOGY. 



[Sect. VIII. 



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Hth. LitmuS'paper, blue and red, for detecting the presence of acids 

 aud alkalies. 



The experiments on an unknown mineral mast be made systematicalh 

 and referred for comparison to some list or table of minerals in ■which 

 their behaviour before the blowpipe is described, as VonKobell's tables.* 



The first point to examine is, whether it be fusible; and if so, in what 



degree. The various grades of fusibility may be conveniently divided 

 into six ; as representatives of which it is convenient to take the follow- 

 ing minerals^ species which are everywhere easy to obtain, and which 

 may therefore be often practised upon ; 



1. Antimony-glance^ or snlphuret of antimony, which melts at the 



caddie, 



2. Natrolite, or mesotype, jBne splinters of which may be rounded 



by the candle-flame, 



3. Aimandine, or precious garnet, which fuses in large pieces before 



the blowpipe, 



4. Actinolite (hornblende), fusible only in smaller portions. 



5. Orthoclase (felspar) offers some difficulty ; and 



6. Bronzite can only be rounded by the flame in the finest splinters. 



According to this scale, the mineral in question may be referred to 

 either of the above numbers, or placed half-way between any two of 

 them ; as for instance, apophyllite, being more easily fused than natrolite, 

 and yet more refractory than antimony-glance, will have its comparative 

 fusibility represented by 1 '5. 



The fragment to be experimented upon is generally held in the pla- 

 tinum forceps, but it is necessary to guard against the melting of the test 

 upon the points, since the platinum, though infusible, is by that means 

 rendered brittle. 



In other cases the mineral may be supported upon charcoal ; but what- 

 ever be the means of holding it^ the phenomena exhibited by the action 

 of the flame must be noted, as 



Ut. The manner in which it fuses, whether quietly, or with decre- 

 pitation, exfoliation^ intumescence, or phosphorescence ; whether it loses 

 or retains colour and transparency. 



2nd. The appearance of the product, whether a glass, an ena7nel, or a 

 dag ; or, as in the case of ores reduced upon charcoal, a metallic bead 



or reoidus, 



3rd. The separation of volatile substances, and the colour of the 

 deposit on the charcoal, by which we may recognise 



* Yon Kobell, ' Tafcln zur Bestimmung der Mineraliea, Miinchen, 

 and the same translated into English by K Campbell. 



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