SECT, xiv.] CLEAVAGE. 127 



far the division may be carried ; therefore the ultimate par- 

 ticle of carbonate of lime is presumed to have that form. 

 However this may be, it is certain that all the various crystals 

 of that mineral may be formed by building up six-sided solids 

 of the form described, in the same manner as children build 

 houses with miniature bricks. It may be imagined that a 

 wide difference may exist between the particles of an un- 

 formed mass and a crystal of the same substance between 

 the common shapeless limestone and the pure and limpid 

 crystal of Iceland spar ; yet chemical analysis detects none ; 

 their ultimate atoms are identical, and crystallisation shows 

 that the difference arises only from the mode of aggregation. 

 Besides, all substances either crystallise naturally, or may be 

 made to do so by art. Liquids crystallise in freezing, vapours 

 by sublimation (N. 167) ; and hard bodies, when fused, crys- 

 tallise in cooling. Hence it may be inferred that all sub- 

 stances are composed of atoms, on whose magnitude, density, 

 and form, their nature and qualities depend ; and, as these 

 qualities are unchangeable, the ultimate particles of matter 

 must be incapable of wear the same now as when created. 



The oscillations of the atmosphere, and the changes in its 

 temperature, are measured by variations in the heights of the 

 barometer and thermometer. But the actual length of the 

 liquid columns depends not only upon the force of gravitation, 

 but upon the cohesive force, or reciprocal attraction between 

 the molecules of the liquid and those of the tube containing 

 it. This peculiar action of the cohesive force is called capil- 

 lary attraction or capillarity. If a glass tube of extremely 

 fine bore, such as a small thermometer tube, be plunged into 

 a cup of water or spirit of wine, the liquid will immediately 

 rise in the tube above the level of that in the cup ; and the 

 surface of the little column thus suspended will be a hollow 

 hemisphere, whose diameter is the interior diameter of the 

 tube. If the same tube be plunged into a cupful of mercury, 

 the liquid will also rise in the tube, but it will never attain 

 the level of that in the cup, and its surface will be a hemi- 



