;i8 DAVlEbj ON POLARIZED LIGHT. 



of SO beautiful and uncommon a form, and so very different 

 from the usual productions of the same salt, that we believe 

 there is some mistake as to the substance. To reconcile 

 this anomaly, we have but to know that from using impure 

 water, a speck of dust, or many other accidental occun'ences, 

 these differences in form are constantly developed. The 

 size of the crystals is often varied by the degree of heat, or 

 in other word's, the quickness or slowness of evaporation, and 

 also by the strength of the solution. 



1 have said before, that " in most instances there is but a 

 seeming difference^' in the forms which the same salt takes 

 Avhen crystallized by different methods. In some salts there 

 is, however, a real change of shape ; indeed, it is said by 

 those who follow out this branch, that no salt ever assumes 

 the same shape crystallized by solution and sublimation ; the 

 two forms do not even possess any relation to each other. 

 Arsenic, corrosive sublimate, and sulphur, are common ex- 

 amples of this ; but a very striking illustration is"afforded^in 

 pyrogallic acid. Pyrogallic acid is produced by heating 

 common gallic acid to about 100°, which then sublimes in 

 brilliant Avhite plates. These plates are so large that they arc 

 easily examined by the unassisted eye. But if they arc dis- 

 solved in pure water, and then allowed to recrystallize, a 

 surprising change takes place, and a crystal is formed which 

 has a beautifuf effect when examined by polarized light. 

 Very often it assumes the appearance (from the arrangement 

 of tiie crystals) of the " eyes" of the peacock's tail, and this 

 is only one of many salts of a similar " behaviour." 



Again, I have before said that " the size of the crystals is 

 often influenced by the degree of heat used;" but in some 

 instances not only is the size varied, but the shape also 

 undergoes a change, and carbonate of lime has been crys- 

 tallized in six hundred different forms ; in this case, however, 

 all being traceable to the original crystal. 



Some of the wines give us beautiful objects for the polari- 

 scope, which differ as much as the countries from which we 

 are supplied. There is, however, one class of crystals which 

 are very similar in almost all, though their fitness for micro- 

 scopic purposes differs very greatly — the bitartrates of potash. 

 These are sparingly soluble in cold water, and therefore may 

 be washed before setting, and freed from almost all impurity. 

 They are well defined crystals, giving exquisite colours, and 

 are easily procurable, being naturally formed at the bottom 

 of the bottles. From the same wine we may also get a 

 totally different crystal. A drop is laid upon the slide, 

 and either by natural or artificial heat evaporated. For some 



