44 MINERALS AND GEOLOGY 



Experiment 5. Heat a small portion of the substance, in powder, 

 at the bottom of a test-tube, with a few drops of strong sulphuric acid. 



This experiment serves for the detection of Fluorides and Nitrates. 



a. The inside of the tube is more or less corroded, and also covered, 

 where damp, with a deposit of silica : Fluorides. The results are 

 best seen by washing out the tube, and then drying thoroughly in the 

 flame of the spirit lamp. The corrosion arises from the formation of 

 a compound of fluorine and hydrogen which readily attacks silica, pro- 

 ducing a volatile compound of fluorine and silicon. This is decom- 

 dosed by water, with deposition of silica. The latter re-action may 

 be seen on the damp sides of the glass, and still more distinctly if a 

 piece of narrow tubing with a drop of water at the end (kept there 

 by the pressure of the finger at the other extremity) be brought 

 within the mouth of the test-tube. The deposit of silica adheres to 

 the glass with great tenacity. 



b. Brownish or orange-coloured fumes are evolved : Nitrates. 

 The fumes possess the peculiar sweetish smell of nitrous acid. All 

 nitrates of natural occurrence are more or less soluble in water. 

 They deflagrate when ignited < on charcoal or in contact with other 

 organic bodies. 



B. REACTIONS OF THE MORE COMMON MINERAL BASES. 



In many minerals, the so-called base lead, for example, in sul- 

 phide of lead (galena), copper in red or black oxide of copper, baryta 

 in carbonate of baryta, and so forth may be easily recognized by the 

 use of the blowpipe. This is especially the case, when the base con- 

 sists of a single and easily reducible metal or metallic oxide, 

 such as silver, lead, copper, tin, &c., or where it imparts a colour to 

 borax or other reagents, as in the case of copper, iron, cobalt, nickel, 

 manganese, &c.; or forms a deposit on charcoal, communicates a 

 colour to the flame, or exhibits other characteristic reactions. Even 

 when several bodies of this kind are present together in the base, 

 their recognition, as a general rule, is easily effected. Earthy and 

 alkaline bases, when in the form of carbonates, sulphates, phosphates,, 

 fluorides, &c., can also be made out, in general, without difficulty, 

 unless several happen to be present together, in which case it is not 

 always possible, by the simple aid of the blowpipe, to distinguish them 

 individually. When these bases are combined with silica, on the 



