314 THE GRAMMAR OF SCIENCE 



they are, the more nearly we can describe our perceptual 

 experience of them by the conceptual symbols of 

 " particles of different mass." Thus in conception each 

 independent substance must be looked upon as indi- 

 vidualised for the purposes of our mechanical model of 

 the universe by a special mass for its fundamental 

 particle. If we take any homogeneous substance as a 

 standard substance, then if we take small and equal 

 volumes of any given homogeneous substance and of the 

 standard substance, the ratio of the masses of the particles 

 by which we represent conceptually these volumes as they 

 become smaller and smaller is termed the density of the 

 given homogeneous substance.^ It follows, from the 

 above statement as to the masses of two portions of the 

 same homogeneous substance being proportional to their 

 volumes, that : The density of a given homogeneous sub- 

 stance is the ratio of the masses of equal volumes of it and 

 of the staJidard substance. 



If a body be not such that two portions, anywhere 

 taken, present to us the same groupings of physical and 

 chemical sense-impressions, then the body is said to be 

 heterogeneous. If we take small and equal volumes of 

 this body from different parts, then the smaller we take 

 them the more nearly we find that our perceptual ex- 

 perience of them can be described by particles of different 

 masses. If we take small and equal volumes "from a 

 given point " of a heterogeneous body and from the 

 standard homogeneous substance, then the smaller we 

 take them the more nearly our perceptual experience can 

 be described by the mutual action of two particles. The 

 ratio of the mass of this particle of the heterogeneous 

 substance to that of the particle of the standard substance 

 is termed the density of the heterogeneous substance at 

 the given point. The density of such a substance is 

 therefore not, as in the case of a homogeneous substance, 

 the ratio of the masses of finite volumes of the given and 



1 The name adopted in the text-books is " specific gravity," but I think 

 this term imfortunately chosen and I prefer to use the word density in this 

 sense. 



