82 CHEMICAL PEOPEETTES OF MINEBAL8. 



dered and treated with strong hydrochloric acid, are decon> 

 posed and deposit the silica in a state of a jelly. The experi- 

 ment may be performed in a test tube, or small glass flask. 

 Sometimes the evaporation of the liquid nearly to dryness ia 

 necessary in order to obtain the jelly. Some silicates do not 

 afford the jelly unless they have been previously ignited before 

 the blowpipe, and some gelatinizing silicates lose the power on 

 ignition. 



3. Decomposaoility of Minerals by Acids. — To ascertain 

 whether a mineral is decomposable by acids or not, it is very 

 finely powdered and then boiled with strong hydrochloric acid, 

 or, in case of many metallic minerals, with nitric acid. In 

 some cases where no jelly is formed there is a deposit of silica 

 in fine flakes. With the sulphides and nitric acid there is often 

 a deposit of sulphur, which usually floats upon the surface of 

 the fluid as a dark spongy mass. Some oxides, and also some 

 sulphates and many phosphates, are soluble entirely without 

 effervescence. But many minerals resist decomposition. It is 

 sometimes difficult to t.,11 whether a mineral is decomposed with 

 the separation of the silica or whether it is unacted upon. In 

 such a case a portion of the clear fluid is neutralized by soda 

 (sodium carbonate), and if anything has been dissolved it w T ill 

 usually be precipitated. 



Test for Fluorine. — Most fluorides are decomposed by strong 

 heated su'phuric acid, give out fluorine which will etch a glass 

 plate in r ach of the fumes. The trial may be made in a lead 

 cup and the glass put over it as a loose cover. 



2. Trials with the Blowpipe. 



The blowpipe, in its simplest form, is merely a bent tube of 

 small size, eight to ten inches long, terminating at one end in a 

 minute orifice. It is used to concentrate the Hame on a min- 

 eral, and this is done by blowing through it while the smaller 

 end is just within the flame. 



The annexed figure represents the form commonly employed, 

 except that the tube is usually without the division at b. It 

 contains an air chamber (o) to receive the moisture which is 

 condensed in the tube during the blowing; the moisture, unless 

 thus removed, is often blown through the small aperture and 

 inlerferes with the experiment. The jet, ef is movable, and 

 it is desirable that it should be made of platinum, in order that 

 it may be cleaned when necessary, either by high heating or 



