378 THE PRINCIPLES OF SCIENCE. 



It is a peculiar property of equilateral triangles that 

 they are equiangular, or, vice versd, it is a peculiar pro- 

 perty of equiangular triangles that they are equilateral. 

 It is a property of crystals of the regular system that 

 they are devoid of the power of double refraction, but 

 this is not a property peculiar to them, because vitreous 

 and other amorphous transparent solids, such as glass, 

 together with all liquids and gases, are also devoid of the 

 same property. 



An Accident, the fifth and last of the Predicables, is any 

 quality, which may or may not belong to certain objects, 

 and which has no connexion with the classification adopted. 

 The particular size of a crystal does not in the slightest 

 degree affect the nature of the crystal, nor does the 

 manner in which it may be grouped with other crystals ; 

 these, then, are Accidents as regards a crystallographic 

 classification. With respect to the chemical composition 

 of a substance, again, it is an accident whether the sub- 

 stance be crystallized or not, or whether it be organized 

 or not. As regards botanical classification the absolute 

 size of a plant is an accident, due to external circum- 

 stances. Thus we see that a logical accident is any 

 quality or circumstance which is not known to be cor- 

 related with those qualities or circumstances forming 

 the definition of the species. 



The use of the Predicables can be very concisely ex- 

 plained by our symbols. Thus, let A be any definite 

 group of qualities and B another quality ; then A will 

 constitute a genus, and AB, A6 will be species of it, B 

 being the difference. Let C, D and E be other qualities, 

 and on examining the combinations in which A, B, C, D, E 

 occur let them be as follows : 



ABODE A&CWE 



ABCDe AbCde. 



