376 SMITH'S INTERMEDIATE CHEMISTRY 



a. practical problem is before us for solution, we use physical prop- 

 erties as the basis of our reasoning and classification. 



The Recognition of a Single Substance. The majority of 

 the substances we have met with are acids, bases, or salts. In 

 identifying a substance of one of these classes, it is convenient to 

 attempt the recognition of the positive and of the negative radicals 

 (or ions) as two almost separate problems. In other words, we 

 investigate one radical at a time. On the other hand, substances 

 which do not belong to any of these classes, such as simple sub- 

 stances (sulphur, carbon, chlorine, etc.), oxides (sulphur dioxide, 

 carbon monoxide, etc.), and many organic substances (e.g. car- 

 bon disulphide, ethyl alcohol), are investigated as a single prob- 

 lem. 



Scope of this Chapter. In discussing the recognition of a 

 single substance we shall, for the present, limit ourselves to the 

 non-metallic elements. We shall consider the elementary sub- 

 stances themselves (sulphur, oxygen, etc.), the oxides of such 

 elements, the few organic compounds described, the non-metallic 

 negative ions, and ammonium-ion. We shall leave out of con- 

 sideration until later (Chap. XL VI) the metallic elements (in- 

 cluding As, Sb, Bi, Na and K). We shall also ignore the metallic 

 positive radicals, although one of these (or hydrogen-ion) must 

 inevitably be combined with the negative radical under consider- 

 ation. 



External Examination (Solids). The specimen may be a 

 solid, a liquid, or a gas. We should note in which of these STATES 

 it exists under room conditions. What follows applies only to 

 SOLIDS the liquids and gases will be taken up later (pp. 380, 

 381). 



Without training in crystallography we can tell little about the 

 crystalline form (p. 94) of the specimen. But anything con- 



