CHAPTER XXXII 



THE RECOGNITION OF SUBSTANCES, I. A REVIEW OF THE 

 NON-METALLIC ELEMENTS 



IN the chapters preceding we have classified our substances 

 under one of the chemical elements they contained. Thus, 

 chloroform, alcohol, and ether were put under carbon. Hydrogen 

 sulphide and sulphuric acid were considered as compounds of 

 sulphur. Now this classification is of a theoretical nature. That 

 chloroform, alcohol, and ether all contain carbon can not be told 

 by mere inspection. We have to make experiments, and to reason 

 about the results, before we reach this inference. Thus we put 

 our inference as the title of the chapter, and distributed the ob- 

 servations and data through it. There is, however, another way 

 of classifying the facts, which is just the opposite of this one. 

 It is the practical classification. When we obtain a specimen, or 

 when a substance appears in the course of an experiment, we must 

 be able to tell what particular substance it is. If it is a white 

 powder, it may contain almost any of the whole list of elements. 

 It may be any one of several thousands of substances. We can 

 recognize it only by its physical properties (p. 6) and by the 

 physical properties of other substances that we can get from it 

 by interaction with known chemical compounds. We need, 

 therefore, a plan of operation, and this plan must be based upon 

 a classification by physical properties, not by constituents. 



One benefit of the discussion of such a plan is that it will afford 

 us a review of some of the facts already mentioned, by presenting 

 the same facts from a different view-point, and by showing the 

 uses to which they may be put. To avoid unnecessary repeti- 



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