PRELIMINARY EXAMINATION. 205 



burner or an alcohol lamp, is a test which should never be 

 omitted, as it discloses in most cases the fact whether the sub- 

 stance 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 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, suffer de- 

 composition, or remain unchanged. (See Table I., page 208.) 



Some substances, containing small quantities of water en- 

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

 when heated, the small fragments being thrown from the foil ; 

 such substances should be heated in a test-tube to expel the 

 water and then examined on platinum foil. 



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

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

 bustion 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 potassium 

 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. Many metallic compounds may be recognized by 

 this test, the metals being liberated and found as metallic glob- 

 ules or shining particles in the fused mass after this has been 



