28 



MINERALS AND GEOLOGY 



mon candle. The wick of the candle should be kept rather short 

 (but not so as to weaken the flame), and it should be turned slightly 

 to the left, or away from the point of the blowpipe, the stream of 

 air being blown along its surface. A lamp flame, or that of coal 

 gas, gives a higher temperature, and is in many respects preferable. 

 The wick-holder (or jet, if gas be used) should be of a rectangular 

 form, with its upper surface sloping towards the left at a slight angle. 

 Either good oil, or, better, a mixture of about 1 part of spirit of 

 turpentine, or benzine, with 7 or 8 parts of strong alcohol, may be 

 used with the lamp. If the latter mixture is used, equal volumes 

 of the two ingredients must be first well shaken up together, and 

 then the rest of the alcohol added. If the wick crust rapidly, the 

 turpentine will be in excess, in which case another volume of alcohol 

 may be added to the mixture. For stationary use, a Bunsen burner 

 provided with an additional tube to slip into the main tube, and so 

 cut off the supply of air, is still more convenient. The projecting 

 top of this additional tube should have the sloping form of the rec- 

 tangular wick-holder described above. 



The following are some of the more general operations required in 

 the examination of minerals by the blowpipe. A few others of 

 special employment are referred to under the Reactions given on a 

 subsequent page. 



(1) The Fusion Trial: In order to ascertain the relative fusi- 

 bility of a substance, we chip off a small particle, by the hammer or 

 cutting pliers, and expose it, either in the platinum-tipped forceps or 

 on charcoal, to the point of the blue flame (Fig. 26, above). If the 

 substance be easily reduced to metal, or if it contain arsenic, it must 

 be supported on charcoal (in a small cavity made by the knife-point 

 for its reception), as substances of this kind attack platinum.* In 

 other cases, a thin and sharply pointed splinter may be taken up by 

 the forceps, and exposed for about half-a-minute to the action of the 

 flame. It ought not to exceed, in any case, the size of a small carra- 

 way seed and if smaller than this, so much the better. If fusible, 

 its point or edge (or on charcoal, the entire mass) will become 

 rounded into a bead or globule in the course of ten or twenty seconds. 



* In order to prevent any risk of injury to the platinum forceps, it is advisable to use char- 

 coal as a support for all bodies of a metallic aspect, as well as for those which exhibit a dis- 

 tinctly coloured streak or high specific gravity. 



