AND THE MICROSCOPE. 251 
These changes of form according to temperature during 
crystallization may be divided into three very distinct classes : 
Ist. Very hot; when the crystals run from the centre in 
sors expanding without any undulations, thus (see Photog. 
@. 1). 
Photograph No. 1. 
2nd. Medium heat; when the crystals show concentric 
waves of very decided form, thus (see Photog. No. 2). 
3rd. Cool; when the crystals are exceedingly minute, thus 
(see Photog. No. 3). 
The first-mentioned crystals are so formed because some 
powerfully acting cause has produced crystallization whilst 
the mass of salt was in a very soft state.. The growth of the 
crystal is then uninterrupted for a comparatively long period, 
and the surface unbroken. 
The second crystals are produced by the whole matter 
becoming so cool that the progress of formation is stayed at 
certain points by hardness, and immediately a fresh forma- 
tion started from these points. 
The third are simply the results of the same action as 
those of the second class of crystals, rendered more power- 
ful by a still greater degree of cold. 
To produce the most beautiful microscopic crystals for the 
polariscope, it is necessary that they should be formed at a 
temperature betwixt the second and third above mentioned, 
