150 DIMORPHISM. 



Is it probable that sulphur and carbonate of lime could be 

 made to appear in sets of crystals which are wholly unlike., 

 merely by a slight change of temperature, if form were the 

 consequence of an invariable atomic constitution ? Crystalline 

 form then may possibly depend upon some at present unknown 

 property of bodies, which may have a frequent and general, 

 but certainly not an invariable relation to their atomic con- 

 stitution. There may be nothing truly inconsistent with the 

 principles of isomorphism in one atom of a certain class of 

 elements having the same crystallographic value as two atoms 

 of another class, the relation which has been assumed to exist 

 between the sodium and chlorine classes and the others, parti- 

 cularly when the classes stand apart, and differ in their pro- 

 perties from all the others, as those of sodium and chlorine do. 



The atomic weight of hydrogen ought not to be divided or 

 reduced from 12.5 to 6.25, for that element certainly belongs 

 to the natural family of magnesium or the second class, of 

 which the equivalents must remain fixed, as they are practi- 

 cally the standards of comparison for all others. If hydrogen 

 be divided, so must magnesium, calcium, manganese and the 

 whole class. But it has at last been admitted that hydrogen 

 never combines in a less proportion than 12.5, and the indivi- 

 sibility of its equivalent therefore conceded;* although it is 

 proposed to make a distinction between the values of the equi- 

 valent and of the atom of a body which is altogether uncalled 

 for. 



DIMORPHISM. 



Many solid bodies admit of a variation of properties while 

 in that state of which gases and liquids are not susceptible. 

 The assumption of two incompatible crystalline forms by the 

 same body, in different circumstances, has already been 

 noticed as occurring with sulphur, carbon, carbonates of lime 

 and lead, bisulphate of potash and chromate of lead. It is also 

 observed, in the biphosphate of soda, and in a considerable 

 number of minerals. The dimorphous crystals may differ in 

 density, the densities of calc-spar and arragonite, the forms of 

 carbonate of lime, being 2.719 and 2.949, and indeed all resem- 

 blance in properties between the crystals may be lost, as in 



* By Berzelius. L'Institut, May 1838, page 160. 



