624 Of the Excellent Qualities of Coffee. 



If the liquid were perfectly at rest, the volatile par- 

 ticles disseminated in' it could not escape, or at least 

 not with the same facility as when it is agitated. 

 Those at the surface of the liquid might fly off, but 

 those below the surface would be confined and pre- 

 served. 



Now all liquids that are either heated or cooled 

 are necessarily disturbed and agitated, and the internal 

 motions into which their particles are thrown do not 

 cease till the heating or cooling process has ceased. 



As the particles of fluids are much too small to be 

 visible, the motions which take place among them 

 cannot be seen ; but means have, nevertheless, been 

 found to render these motions quite evident. 



If a small quantity of any solid substance, in the 

 form of a coarse powder, and having the same spe- 

 cific gravity as any transparent liquid, be mixed with 

 it, and the liquid be either heated or cooled, the cur- 

 rents formed in the liquid in consequence of the 

 change of its temperature will carry along with them 

 the visible particles of the powder disseminated in 

 the liquid, and the directions and velocities of those 

 currents will become apparent. 



The cause of these motions among the particles of 

 liquids that are heated or cooled is perfectly known. 



When a hot liquid is cooled, those of its particles 

 which are the first exposed to the cooling influence, 

 on losing a part of their heat, become specifically 

 heavier than they were before ; consequently they be- 

 come specifically heavier than the surrounding hotter 

 particles, which causes them to descend towards the 

 bottom of the containing vessel. 



This descent of the particles which are cooled neces- 



