\ 
Bartow—A. Mechanical Cause of Homogeneity of Crystals. 665 
nitrate are always isotropic, the crystals of these substances which 
contain isomorphous admixtures are double refracting.’ 
Also the fact that a small addition of ammonia alum to potash 
alum develops optical properties in the mixture which are wanting 
in the pure substances ; in particular lamellar polarization investi- 
gated by Biot. 
It should also be noted that the nitrates, which are double- 
refracting and are mixed crystals, fly into small pieces when 
subjected to pressure, but that the isotropic pure crystals are not 
nearly so brittle.’ 
Finally, we have the important fact that the behaviour of 
mixed crystals when subjected to change of temperature resembles 
that of cooled glass,? and can also be closely imitated with gelatine 
models,* strong evidence being afforded in this way that the effect 
is one proceeding from strain.* 
There are cases in which bodies are mixed crystals and also 
contain water of crystallization,® and any optical anomalies which 
they display may, therefore, be connected with either or both of 
these properties according to the conditions prevailing when the 
observation is made. 
The intimate relation between external form and anomalous 
optical properties which is probably traceable to strain, comes out 
prominently in the garnets which, as a rule, are mixed crystals 
composed of several related compounds.’ 
1 Brauns, ‘‘ Optische Anomalien,’’ p. 50. Neues Jahrbuch, 1885, 1., pp. 96-118. 
Comp. also Brauns, Joc. cit., p. 206. 
2 Brauns, Joc. cit., p. 225. As to sudden fracturing during growth of mixed 
erystals, see [bid., 255. 
3 Brauns, Joc. cit., p. 224. 
*In particular such an imitation has been produced by Klocke, Klein and Ben 
Saude, see Jbid., p. 256. 
> Ibid., p. 257; and, in particular, see also Lehmann’s ‘‘ Molecular Physik,’’ 1., 
_p. 450. 
§ Thid., pp. 240, 340. Comp. p. 640 above. 
7 Tbid., pp. 245, 251, 253, also 357. 
