OF NATURAL PHILOSOPHY. 99 



then, under this kind of mental convention, frame a 

 definition or statement of one of them, in such words 

 that it shall apply equally to them all, such state- 

 ment will appear in the form of a general proposi- 

 tion, having so far at least the character of a law of 

 nature. 



(90.) For example: a great number of trans- 

 parent substances, when exposed, in a certain par- 

 ticular manner, to a beam of light which -has been 

 prepared by undergoing certain reflexions or refrac- 

 tions, (and has thereby acquired peculiar properties, 

 and is said to be "polarized"} exhibit very vivid 

 and beautiful colours, disposed in streaks, bands, 

 &c. of great regularity, which seem to arise within 

 the substance, and which, from a certain regular 

 succession observed in their appearance, are called 

 " periodical colours." Among the substances which 

 exhibit these periodical colours occur a great 

 variety of transparent solids, but no fluids and no 

 opaque solids. Here, then, there seems to be suf- 

 ficient community of nature to enable us to use a 

 general term, and to state the proposition as a law, 

 viz. transparent solids exhibit periodical colours by 

 exposure to polarized light. However, this, though 

 true of many, does not apply to all transparent 

 solids, and therefore we cannot state it as a general 

 truth or law of nature in this form ; although the 

 reverse proposition, that all solids which exhibit 

 such colours in such circumstances are trans- 

 parent, would be correct and general. It becomes 

 necessary, then, to make a list of those to which it 

 does apply ; and thus a great number of substances 

 of all kinds become grouped together, in a class 

 H 2 



