378 ANALYTICAL CHEMISTRY. 



stances should be heated in a dry test-tube to expel the water and 

 then be examined on platinum foil. 



In many cases it is preferable to heat the substance in a bent glass 

 tube, as shown in Fig. 53, instead of on platinum foil, because vola- 

 tile products evolved during the process of heating may become re- 

 condensed in the cooler part of the tube, and thus saved for further 

 examination. 



The presence of water, sulphur, mercury, arsenic, etc., may often 

 be readily demonstrated by this mode of operating. 



4. Heating 1 on charcoal by means of the blowpipe. This test 

 reveals the presence of chlorates and nitrates by the vivid combus- 

 tion of the charcoal (known as deflagration), which takes place in 

 consequence of the oxidizing action of these substances. 



Arsenic is indicated by a characteristic odor of garlic. 



5. Heating- on charcoal with sodium carbonate and potas- 

 sium cyanide. A small quantity of the finely powdered substance 

 is mixed with twice its weight of potassium cyanide and dry sodium 

 carbonate. This mixture is placed in a small hole made in a piece 

 of charcoal, and heat applied by means of the blowpipe (see Fig. 54). 

 Many metallic compounds may be recognized by this test, the metals 

 being liberated and found as metallic globules or shining particles in 

 the fused mass after this has been removed from the charcoal and 

 washed with water in a small mortar. (See Fig. 55.) 



FIG. 55. 



A characteristic incrustation is formed by some metals, due to the 

 precipitation of a metallic oxide around the heated spot on the char- 

 coal. 



If sulphur as such, or in any form of combination, be present in 

 the substance examined by this test, the fused mass contains a sulphide 

 of the alkali (hepar), which may be recognized by placing it en a 

 piece of bright silver (coin) moistened with a drop of water, when the 



