CHEMICAL COMPOSITION OF MINERALS. 



6=^ 



,y immersion in an acid. The screw at b is for the purpose of 

 shortening the tube one-half so as to make it more convenient 

 for the pocket of the field mineralogist. It is un- 

 screwed for this purpose, and the smaller part put 

 within the larger. 



In using the blowpipe it is necessary to breathe 

 and blow at the same time, that the operator may 

 not interrupt the flame in order to take breath. 

 Though seemingly absurd, the necessary tact may 

 easily be acquired. Let the student first breathe a 

 few times through his nostrils while his cheeks are 

 inflated and his mouth closed. After this practice 

 let him put the blowpipe to his mouth and he will 

 find no difficulty in breathing as before while the 

 muscles of the inflated cheeks are throwing the air 

 th^y contain through the blowpipe. When the air 

 is nearly exhausted the mouth may again be filled 

 through the nose without interrupting the process 

 of blowing. 



The flame of a candle, or a lamp with a large 

 wick may be used, and when so it should be bent 

 in the direction the flame is to be blown. But it is far better, 

 when gas can be had, to use a Bunsen's burner. 



The flame has the form of a cone, yellow without and blue 

 within. The heat is most intense just beyond the extremity of 

 the blue flame. In some trials it is necessary that the air 

 should not be excluded from the mineral during the experiment, 

 and when this is the case the outer flame is used. The outer is 

 called the oxidizing flame (because oxygen, one of the consti- 

 tuents of the atmosphere, combines in many cases with some 

 parts of the assay, or substance under experiment), and the in- 

 ner the reducing flame. In the latter the carbon and hydrogen 

 of the flame, which are in a high state of ignition, and which are 

 enclosed from the atmosphere by the outer flame, tend to unite 

 with the oxygen of any substance that is inserted in it. Hence 

 Eiibstances are reduced in it. 



The mineral is supported in the flame either on charcoal; or 

 by means of steel forceps (as in the annexed figure) with plati- 



num extremities (a 6), opened by pressing on the pins p p\ ov 

 on platinum wire or foil. 



