228 ANALYTICAL CHEMISTRY. 



be evaporated in a small porcelain dish to see whether a solid residue 

 be left or not. If a residue be left, it should be treated like a solid.) 

 The heating of a small quantity of a solid substance upon platinum 

 foil held over the flame of a Bunsen burner or of an alcohol lamp, is 

 a test which should never be omitted, as it discloses in most cases the 

 fact whether the substance is of an organic or inorganic nature. Most 

 organic (non- volatile) substances, when thus heated, will burn with 

 a luminous flame, leaving in many cases a black residue of carbon, 

 which, upon further heating, disappears. In cases where the organic 

 nature of a compound is not clearly demonstrated by heating on plati- 

 num foil, the substance is heated with an excess of cupric oxide in a 

 test-tube or other glass tube, provided with a delivery -tube, which 

 passes into lime-water. Upon heating the mixture, the carbon of the 

 organic matter is converted into carbon dioxide, which renders lime- 

 water turbid. 



The analytical processes by which the nature of an organic sub- 

 stance is determined, are not considered in this part of the book, but 

 will be mentioned when considering the carbon compounds. 



An inorganic substance, heated on platinum foil, may either be 

 volatilized, fused, change color, become oxidized, suifer decomposition, 

 or remain unchanged. (See Table I., page 232.) 



PIG. 22. FIG. 23. 



Heating of solids in bent glass tube. Heating on charcoal by means of blowpipe. 



Some substances, containing small quantities of water enclosed 

 between the crystals (common salt, for instance), decrepitate when 

 heated, the small fragments being thrown from the foil; such sub- 



