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APPENDIX. 



acid, somewhat diluted, upon a plate of glass, and then 

 moderately heated until the crystal dissolves in the acid. 

 It is then set aside to crystallize. It is well to prepare 

 a number of these plates at once, varying the relative 

 proportion of the acid and salt, in order that the desired 

 kind of crystallization may be found in one or other of 

 them; for there is a considerable variety in the crystal- 

 line forms obtained by this method, some of which indeed 

 are very singular- But when that kind of crystallization 

 takes place which it is more particularly my intention to 

 speak of, the field of view of the microscope is seen 

 covered with minute circular spots, each of which is like 

 a tuft of silk radiating from a centre, and is composed 

 of a close assemblage of delicate acicular crystals forming 

 a star. But besides these, are seen interspersed among 

 them a number of circular transparent bodies, which are 

 evidently modifications of the former, being, in fact, tufts 

 or stars of acicular crystals, in such close assemblage as to 

 be in optical contact with each other, and to produce the 

 appearance of a single individual. Now let us suppose 

 a group of these circles to be under examination with the 

 polarizing microscope, and when the polarizers are 

 crossed we observe the following phenomenon : The 

 field of view being dark, the little circles become lumi- 

 nous, and we see upon each of them a well defined and 

 dark cross, dividing the crystal into four equal parts. 

 All these crosses are placed similarly, and are parallel to 

 each other, and their direction remains unaltered when 

 the crystals are turned round in their own plane by 



