Crystals on Change of Temperature. 89 



molecules. If the cohesion has diminished to this extent in 

 any single direction in the seolotropic body we are considering, 

 the body has reached a condition at any rate analogous to the 

 fluid state ; and although, under the sole action of change of 

 temperature, it would for ever retain its symmetrical crystal- 

 line arrangement, still, if any attempt were made to determine 

 its hardness while in this state, the result must inevitably be 

 the total collapse of the parallelepipedal nature of the system. 

 If the crystal is within reach of the force of gravity, it will not 

 be necessary even to try the hardness ; the parallelepipedal 

 system on arriving at the above state will instantly be destroyed. 

 It would thus appear that, although by the action of change 

 of temperature the introduction of new planes of symmetry 

 common to the molecules and their grouping may be possible, 

 the reverse operation cannot be achieved until after a serious 

 interference with the parallelepipedal grouping, and not at all 

 in regions far away from disturbing forces. This seems to 

 point to the greater stability of the systems of higher sym- 

 metry, and to the fact that we may entertain the hope of being 

 able to transfer most crystals under favourable circumstances 

 into the cubic system — hexagonal crystals, however, being 

 denied this privilege by virtue of their possessing an equato- 

 rial plane of symmetry. The reverse operation might, how- 

 ever, be effected by the action of some force, such as magnetism, 

 which would not necessarily induce internal reactions symme- 

 trical to the common planes of symmetry. As remarkable 

 instances of this kind of transformation from a lower to a 

 higher type of symmetry, we may mention the transition of: — 



(1) amorphous bodies to the crystalline state, as in wrought 

 iron, barley sugar, and the fusible alloys of Rose and Newton; 



(2) an oblique to a rhombic crystal, as in the case of artificial 

 sulphur ; (3) a rhombic to tetragonal, as in that of iodide of 

 mercury ; (4) a rhombic to rhombohedral, as in the change of 

 arragonite to calcite. And although the red tetragonal iodide 

 of mercury does become yellow, and therefore presumably 

 rhombic, on heating, still the general rule seems to be that if a 

 crystal is to pass, under the action of the above kind of forces, 

 from a type of symmetry in which there is any number of 

 symmetry-planes common to the molecules and their grouping, 

 to a type in which any of these planes has disappeared, the 

 crystal must first pass through the fluid condition ; and in all 

 cases we may say that the parallelepipedal nature of the system 

 will first be destroyed. 



Neglecting cases where this destruction of the parallelepi- 

 pedal nature of the system takes place, we shall henceforth 

 assume the permanency for all temperatures of any of these 

 common symmetry-planes. 



