558 MINERALOGY 



bituminous odor. If the black residue burns or glows in the air 

 when heated, the presence of carbon or organic matter may be 

 assumed. 



e. Change of color. Characteristic of some oxides. 



Open tube. Pieces of hard glass tube 15 cm. long and 7 mm. 

 inside diameter are used to heat coarsely powdered minerals in a 

 current of air, oxidizing the volatile compounds as they are driven 

 off by the heat. Substances to be tested are placed about 3 cm. 

 from the end. The tube is held in the matrass holder ; the O. F. is 

 directed on the glass directly under the coarse powder, at the same 

 time the tube is held diagonally across the flame and 45 from the 

 perpendicular. The object is to obtain a draft of air to furnish the 

 oxygen and to also carry the vapors up through the tube, where 

 they may condense as sublimates or pass out as gases. Heat is 

 applied gently at first, increasing the temperature to full redness. 



a. The escaping gas smells like a burning sulphur match, due to 

 sulphur dioxide. Arsenic and selenium also yield characteristic 

 odors. 



Illustration. Heat some coarsely powdered pyrite in the open 

 tube. Here a sublimate of sulphur is not obtained, as in the closed 

 tube, but the sulphur combines with the oxygen of the air passing 

 through the tube, forming SO 2 , which escapes at the upper end, 

 yielding the odor. 



6. Sublimates. They may be white or colored as in the closed 

 tube. 



Illustration. Heat a small fragment of arsenic in an open tube. 

 The sublimate formed is not a metallic mirror as in the closed 

 tube, but a white sublimate of trioxide of arsenic (A 2 O 3 ) which is 

 composed of octahedral crystals. Examine with the magnifying 

 glass. 



Watch glasses to hold powdered mineral, soda, borax, etc., and 

 one or more porcelain evaporating dishes will be required to con- 

 centrate solutions apd to gelatinize powdered minerals. If the 

 under sides of these porcelain dishes are unglazed, they will serve as 

 streak tablets also. 



Blue Glass. A darkly colored piece of cobalt glass 5 cm. square 

 used for the absorption of sodium light in testing for potassium. 

 Bottles of clear glass with plane parallel sides, and filled with 

 solutions of potassium permanganates or chrome alum are much 

 better than the blue glass but less convenient for use in the 

 potassium test, see page 563. 



