186 THEORY OF COHESION. 



agent can operate as a bond of cohesion between 

 the particles of ponderable matter, it may be 

 proper to shew by a few familiar examples, that 

 the force with which liquids hold together the 

 particles of solid bodies, is not in proportion to 

 the attraction of their own particles for each 

 other, but in proportion to their affinity for the 

 solids. 



It is well known that at ordinary tempe- 

 ratures of the atmosphere, the particles of water 

 cohere with very slight force. It is also known, 

 that when granite, lime, clay, and marie, to- 

 gether with all other rocky and earthy bodies, 

 are reduced to the condition of a perfectly dry 

 and impalpable powder, their particles glide 

 freely over one another, having little or no at- 

 traction for each other. On adding to this pow- 

 der a due proportion of water, they are found to 

 unite together by a very strong attraction, form- 

 ing dense and highly tenacious mortar. The 

 less water such ingredients contain, the stronger 

 is their attraction for it. Hence it is, that the 

 cohesion of mortar augments in proportion as it 

 is deprived of water by drying ; or that when 

 the earth has been parched by a long drought, 

 it attracts and absorbs more rapidly a shower of 

 rain, than when in a moist state, in the same 

 way that all bodies attract and absorb caloric 

 with a force and rapidity in proportion as they 

 are deprived of it. In like manner, the cohesion 



