SUBSTANCES AND PROPERTIES 3 



Substances. Upon considering these operations, we can 

 discover a general plan which the chemist has devised and employs 

 in his work. Different samples of cloth are of different colors 

 and appearance, even when they contain the same proportions of 

 cotton and wool. A casual inspection is, therefore, of little 

 value. But in certain respects all samples of wool are alike, 

 such as in dissolving in caustic soda, in " taking " certain dyes 

 and in possessing a scaly surface. Those respects in which all 

 samples of wool are alike are called the specific properties of wool. 

 Similarly all samples of cotton are alike in certain respects, which 

 are called the specific properties of cotton. And these properties 

 of cotton and of wool are different, many of them very different 

 indeed. For the purpose of stating what he means, the chemist 

 calls a kind of material, all specimens of which possess a certain 

 set of specific properties, a substance. Wool is one substance,* 

 and cotton another substance. Every part of a specimen of a 

 substance has the same specific properties as any other part. If 

 any portions can be found which have different specific properties, 

 these are portions of another substance, accidentally or inten- 

 tionally mixed with the first. The chemist calls the foreign mat- 

 ter an impurity, and the specimen an impure sample of the sub- 

 stance of which it is mainly composed. 



A substance, then, is a species or kind of matter, and all speci- 

 mens of it show the same set of specific properties. Any par- 

 ticular substance is recognized by the specific properties which 

 it exhibits when exposed to various tests. Specific properties, 

 conversely, are those qualities which are characteristic of any 

 particular substance. 



The plan which the chemist uses in his experimental tests, 

 therefore, is that of ascertaining whether a given material exhibits 



* In point of fact, wool contains several substances, but they are all alike 

 in respect to the three properties mentioned above. They differ slightly in 

 respect to other properties, and so can be distinguished from one an- 

 other. 



