COHESION. 59 



the same. Thus, chalk and marble are very different 

 looking substances, but they are composed of precisely 

 the same elements ; the difference between them is 

 not caused by heat, like the difference between the 

 three states of water, but is wholly of a mechanical 

 nature. The particles composing a piece of chalk are 

 much smaller than those composing a piece of marble, 

 which are in fact compound particles, consisting of 

 many joined together, and hence a piece of marble 

 appears made of many little grains, whilst chalk is 

 composed of particles so small that we are unable to 

 distinguish them, and therefore appears to be a uni- 

 form substance. 



89. The power which holds together the little par- 

 ticles composing the piece of marble or chalk, or any 

 other substance, is called cohesion, and this power 

 varies in strength in different substances ; thus it is 

 far stronger in marble than in chalk, and hence a 

 piece of marble requires a much harder blow to break 

 it, than a piece of chalk. In the same way, we say 

 that the particles composing the diamond are held 

 together more firmly by cohesion than the particles 

 composing a piece of charcoal. Cohesion is, of course, 

 quite independent of chemical attraction, for it holds 

 the different particles of a substance together, con- 

 stituting its mechanical strength ; whilst chemical 

 affinity binds together particles of two different sub- 

 stances, forming a compound substance, but does not 

 in any way affect the strength of the compound to 

 resist mechanical force applied to it. 



