APPENDIX 353 



Material things are extended, that is, they have 

 form, and they exclude each other, so that they cannot 

 occupy the same place. They appear to us to be 

 aggregates of different nature : they may be solid and 

 homogeneous, like a piece of metal ; or solid and porous, 

 like a piece of pumice-stone ; or loose and granular, 

 like sand ; or viscous or liquid, like pitch or water. 

 They may have colour. They are opaque, or transparent 

 in various degrees. They may have odour. Material 

 things, as they are perceived by the distance sense- 

 receptors, appear to have qualities. 



Material things are aggregates of molecules. The 

 aggregates may possess essential form, like that of a 

 crystal, or an organism. The form of the aggregate 

 may be essential and homogeneous, so that it consists 

 of molecules, all of which are of the same kind, like a 

 crystal. It may be heterogeneous and essential, like 

 the body of the organism, when it consists of molecules 

 which are not all of the same kind. The aggregates 

 may have accidental form, like that of a river valle}'-, 

 or a delta, or a mountain, and the form in these, and 

 similar cases, is not a part of the essential nature of the 

 aggregate. 



The molecules are selections (in the mathematical 

 sense) of some of about eighty different kinds of atoms. 

 A molecule is a small number of atoms arranged to- 

 gether in a definite way, and its nature depends, not only 

 on the kinds of atoms of which it is composed, but also 

 on the arrangement of these atoms. Two or more 

 different arrangements of the same atoms are, in 

 general, different molecules. 



MASS 



When matter is perceived by the tactile and 



muscular sense organs, we have the intuition of mass. 

 z 



