224 APPENDIX, 



most part seen in all the crystals at once, to the exclusion 

 of any of the other forms. 



SECT. 5. These crystals generally undergo a sponta- 

 neous change in the course of one or two days after they 

 have been formed. Those (No. 4) resembling plumes 

 usually break up and resolve themselves into small 

 rhombs and other geometric forms. The elongated 

 crystals (Nos. 2, 3) undergo a remarkable change. They 



become traversed with innumerable fissures transverse 



^f 



to their length, and thus break up into thin plates, which 

 either cohere loosely or separate entirely. 



SECT. 6. All these forms are very pleasing objects for the 

 polarizing microscope. This arises from the very high de- 

 polarizing power of boracic acid, which enables its thinnest 

 plates to exhibit colours of great variety and brilliancy, 

 and causes even its dust or smallest particles to appear 

 luminous. The more energetically any substance acts 

 upon polarized light, the closer and more crowded are 

 the band and lines of colour which appear upon its 

 crystals. These isochromatic lines, of which there are 



frequently many alternations, denote lines of equal thick- 

 ness in the crystal. In the case of boracic acid, when 

 anhydrous or nearly so, these lines are more crowded 

 than in any other crystal that I have yet examined, inso- 

 much that to exhibit them distinctly is as fine a test of the 

 performance of a microscope as to resolve the more diffi- 



