100 DISCOURSE ON THE STUDY 



linked by this common property. If we examine 

 the individuals of this group, we find among them 

 the utmost variety of colour, texture, weight, hard- 

 ness, form and composition ; so that, in these re- 

 spects, we seem to have fallen upon an assemblage 

 of contraries. But when we come to examine 

 them closely, in all their properties, we find they 

 have all one point of agreement, in the property of 

 double refraction, (see page 30.) and therefore we 

 may describe them all truly as doubly refracting 

 substances. We may, therefore, state the fact in the 

 form, " Doubly refracting substances exhibit pe- 

 riodical colours by exposure to polarized light ;" 

 and in this form it is found, on further examination, 

 to be true, not only for those particular instances 

 which we had in view when we first propounded it, 

 but in all cases which have since occurred on further 

 enquiry, without a single exception ; so that the 

 proposition is general, and entitled to be regarded 

 as a law of nature. 



(91.) We may therefore regard a law of nature 

 either, 1st, as a general proposition, announcing, in 

 abstract terms, a whole group of particular facts re- 

 lating to the behaviour of natural agents in proposed 

 circumstances ; or, 2dly, as a proposition announcing 

 that a whole class of individuals agreeing in one 

 character agree also in another. For example : in 

 the case before us, the law arrived at includes, in 

 its general announcement, among others, the particu- 

 lar facts, that rock crystal and saltpetre exhibit pe- 

 riodical colours ; for these are both of them doubly 

 refracting substances. Or, it may be regarded 

 as announcing a relation between the two pheno- 



