1891.] on Crystallisation. 385 



would arise; for instance, the electric field miglit produce a stress 

 in opposition to the aggregation of the molecules in the closest 

 possible way, and then the crystal would develop such faces as 

 would give rise to an equal and opposite stress. The presence of 

 molecules of some other material, difi'erent from those forming the 

 bulk of the crystal, might cause a similar eifect, so might inequalities 

 of temperature. 



In the case of an electric stress, or one due to inequalities of tem- 

 perature, when the electric stress, or the inequality of temperature 

 was removed, the crystal would be left with an internal strain, because 

 the stress due to the want of symmetry must be met by an equal 

 pressure. 



Crystals of this sort generally betray the internal strain, either 

 by developing electricity of opposite kinds at the two ends when they 

 are heated, or cooled ; or they affect polarised light, rotating the 

 plane of polarisation. That these effects are really due to the state 

 of internal strain is proved because tourmalines, and other crystals, 

 which are pyro-electric when unsymmetrical, show no such property 

 when symmetrically grown ; and sodium chlorate when in solution, 

 quartz when fused, and so on, lose their rotatory power. On the 

 other hand there are many substances which in solution show a 

 rotatory power, and as a rule such substances produce unsymmetri- 

 cally developed crystals. This is well seen in the tartrates. The 

 constitution of the molecules must naturally be such that they will not, 

 without some strain, form crystals, and equilibrium in the crystal is 

 attained by the opposing stress arising from want of symmetry in the 

 surface tensions. In all such crystals the rotatory power disappears 

 either in whole or in part when the substance crystallises. It is impos- 

 sible however, in biaxal crystals to tell whether there is rotation or not. 

 According to Des Cloizeaux the only crystal formed from a liquid 

 having rotatory power, which shows rotation in the solid state, is 

 strychnine sulphate. This substance forms crystals like prussiate of 

 potash, double square pyramids with the two apices truncated. Its 

 rotatory power in the crystalline form is much stronger than in the 

 liquid form. The crystals are not hemihedral, and the rotatory power 

 is not due to any stress arising from want of symmetry in surface 

 tensions. Effects more or less analogous to those due to the stress 

 arising from unsymmetric development may be produced in crystals by 

 external pressure. Thus a piece of rock salt, which in its natural 

 state has no action on polarised light, when compressed in a vice will 

 change the plane of polarisation. Also a cleavage slice of potassium 

 ferrocyanide which is uniaxal, may, by compression, be made to give 

 in convergent polarised light the two eyes and elliptic rings of a 

 biaxal crystal. 



