82 CHEMICAL PROPERTIES OF MINERALS. 
dered and treated with strong hydrochloric acid, are decom- 
posed and deposit the silica in a state ofa jelly. The experi- 
ment may be performed in a test tube, or small glass flask. 
Sometimes the evaporation of the liquid nearly to dryness is 
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. Decomposability 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 fiuid 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 tell 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 will 
usually be precipitated. 
Test for F’luorine.—Most fluorides are decomposed by strong 
heated sulphuric acid, give out fluorine which will etch a glass 
plate in reach 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 flame 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 (0) 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 
interferes 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 
