INTRODUCTORY REMARKS. 443 



There are, however, for many organic substances such character- 

 istic tests that these substances may be recognized by them ; these 

 reactions will be mentioned in the proper places. 



An analysis by which different organic substances, when mixed 

 together, are separated from each other is frequently termed proximate 

 analysis. Such an analysis includes the separation and determination 

 of essential oils, fats, alcohols, sugars, resins, organic acids, albuminous 

 substances, etc., and is one of the most difficult branches of analytical 

 chemistry. 



Qualitative analysis of organic substances. The presence of 

 carbon in a combustible form is decisive in regard to the organic 

 nature of a compound. If, consequently, a substance burns with 

 generation of carbon dioxide (which may be identified by passing the 

 gas through lime-water), the organic nature of this substance is 

 established. (See Chapter on Carbon.) 



The presence of hydrogen can be proven by allowing the gaseous 

 products of the combustion to pass through a cool glass tube, when 

 drops of water will be deposited. 



It is difficult to show by qualitative analysis the presence or 

 absence of oxygen in an organic compound, and its determination is 

 therefore generally omitted. 



The presence of nitrogen is determined by heating the substance 

 with dry soda-lime (a mixture of two parts of calcium hydroxide and 

 one part of sodium hydroxide), when the nitrogen is converted into 

 ammonia gas, which may be recognized by its odor or by its action 

 on paper moistened with solution of cupric sulphate, a dark-blue 

 color indicating ammonia. 



Ultimate or elementary analysis. While the student must be 

 referred to books on analytical chemistry for a detailed description of 

 the apparatus required and the methods employed for elementary 

 analysis, it may here be stated that the quantitative determination of 

 carbon and hydrogen is generally accomplished by the following pro- 

 cess : A weighed quantity of the pure and dry substance is mixed 

 with a large excess of dry cupric oxide, and this mixture is introduced 

 into a glass tube, the open end of which is connected by means of a 

 perforated cork and tubing with two glass vessels, the first one of 

 which (generally a U-shaped tube) is filled with pieces of calcium 

 chloride, the other (usually a tube provided with several bulbs) with 

 solution of potassium hydroxide. The two glass vessels, containing 

 the absorbents named, are weighed separately after having been 



