MATTER AND ITS PHYSICAL PROPERTIES. 



27 



ures, terminated by plane surfaces, the figures being always the same for the 

 same species of salt, but different for different species. There are several 

 circumstances in the formation of these crystals which merit attention. 



If one of the crystals be detached from the others, and the process of its 

 formation observed, it will be found gradually to increase, always preserving 

 its original figure. Since its increase must be caused by the continued acces- 

 sion of saline particles disengaged by the evaporation of the water, it follows 

 that these particles must be so formed, that, by attaching themselves successively 

 to the crystal, they maintain the regularity of its bounding planes/and preserve 

 their mutual inclinations unvaried. 



Suppose a crystal to be taken from the liquid during the progress of crystal- 

 lization, and a piece broken from it so as to destroy the regularity of its form ; 

 if the crystal thus broken be restored to the liquid, it will be observed gradu- 

 ally to resume its regular form, the atoms of salt successively dismissed by the 

 vaporizing water filling up the irregular cavities produced by the fracture. 

 Hence it follows that the saline particles which compose the surface of the 

 crystal, and those which form the interior of its mass, are similar, and exert 

 similar attractions on the atoms disengaged by the water. 



All these details of the process of crystallization are very evident indications 

 of a determinate figure in the ultimate atoms of the substances which are crys- 

 tallized. But besides the substances which are thus reduced by art to the form 

 of crystals, there are larger classes which naturally exist in that state. There 

 are certain planes, called planes of cleavage, in the directions of which natural 

 crystals are easily divided. These planes, in substances of the same kind, 

 always have the same relative position, but differ in different substances. The 

 surfaces of the planes of cleavage are quite invisible before the crystal is di- 

 vided ; but when the parts are separated, these surfaces exhibit a most intense 

 polish, which no effort of art can equal. 



We may conceive crystallized substances to be regular mechanical struc- 

 tures formed of atoms of a certain figure, on which the figure of the whole struc- 

 ture must depend. The planes of cleavage are parallel to the sides of the con- 

 stituent atoms, and their directions therefore form so many conditions for the 

 determination of its figure. The shape of the atoms being thus determined, it 

 is not difficult to assign all the various ways in which they may have been 

 arranged, so as to produce figures which are accordingly found to correspond 

 with the various forms of crystals of the same substance. 



When these phenomena are duly considered and compared, little doubt can 

 remain that all substances susceptible of crystallization consist of atoms of de- 

 terminate figure. This is the case with all solid bodies whatever which have 

 come under scientific observation, for they have been severally found in, or re- 

 duced to, a crystallized form. Liquids crystallize in freezing ; and if aeriform 

 fluids could by any means be reduced to the solid form, they would probably 

 also manifest the same effect. Hence it appears reasonable to presume that 

 all bodies are composed of atoms ; that the different qualities with which we 

 find different substances endued, depend on the magnitude and figure of these 

 atoms ; and these atoms are indestructible and immutable by any natural pro- 

 cess, for we find the qualities which depend on them unchangeably the same 

 under all the influences to which they have been submitted since their crea- 

 tion ; that these atoms are so minute in their magnitude, that they cannot be 

 observed by any means which human art has yet contrived, but still that there 

 are limits of magnitude which they do not exceed. 



It is proper, however, to observe here, that the various theories of mechani- 

 cal science do not rest upon any hypotheses concerning these atoms as a basis. 

 They are not inferred from this or any other supposition, and therefore their 



