480 



PROCEEDINGS OF THE AMERICAN ACADEMY 



which are easily seen with the lens, when the bead is not so highly 

 colored as to render them invisible, is in general an indication that the 

 proper point of saturation has been reached. 



In examinations with a compound microscope, I have found cork 

 slides, two and one fourth inches long, one inch broad, and one sixth 

 of an inch thick, very convenient. Two perforations are made through 

 the cork, somewhat larger than the beads, which are placed in the 

 centre, and easily retained in position by sinking the wire used for a 

 handle into slits in the cork prepared to receive them. (See diagram.) 



This adjustment holds the bead steadily, and, combined with the 

 flexibility of the wire, permits any part of the convex surface to 

 be brought into field, which is very important, as the crystals are 

 very frequently formed only close to the margin. 



In photographing the crystallizations, the beads being transparent, 

 and transmitted light employed, the solar rays were reflected from the 

 surface of a concave mirror, their power being diminished by the inter- 

 position of a piece of ground glass between the mirror and the bead ; 

 the curved surface of the bead rendering the illumination by the plane 

 mirror very unsatisfactory. 



In regard to the ensuing descriptions of the crystalline reactions of 

 the substances dealt with, although the phenomena differ very per- 

 ceptibly to the eye of the microscopic observer, yet I have found it 

 exceedingly diflScult to convey verbally any exact conception of their 

 differences ; and have, therefore, contented myself with roughly sketch- 

 ing the most prominent features, feeling assured that a few easily made 

 experiments will, to persons interested, be in every case far more 

 satisfactory than any written description. On the next page will be 

 found a list of the substances experimented upon. 



I cannot conclude these introductory remarks without expressing 

 my obligations to Professor Josiah P. Cooke, who has kindly furnished 

 me with many of the rarer substances experimented with, and to my 



