INTRODUCTORY REMARKS. 377 



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, or upon a piece of mica, held over the flame of a Bunsen burner, 

 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 platinum 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 substance is 

 determined are not considered in this part of the book, but will be mentioned 

 when considering the carbon compounds. Some substances ruin platinum when 

 heated on it. Thus, salts of easily reducible metals, as lead, bismuth, antimony, 

 tin, especially their organic salts, are apt to do so, because these metals form 

 fusible alloys with the platinum. Thiosulphates corrode and hypophosphites 

 destroy platinum. Should the presence of any of the substances be suspected, 

 heating on platinum should be omitted. Indeed, tests 4 and 5 can be applied 

 first, as they may show the presence of these objectionable substances. 



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

 change color, become oxidized, suffer decomposition, or remain unchanged. 

 (See Table I., page 381.) 



FIG. 53. FIG. 54. 



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- 



