SPONTANEOUS CRYSTALLISATION OF MONOCHLORACETIC ACID, ETC. 341 



a-modification. The crystals most frequently obtained on a microscope slide, 

 whether from aqueous solution, from solution in alcohol, or from fusion by slow 

 cooling, are plates which appear to be identical with the crystals described by 

 SCHMELCHER. They have the form shown in fig. 2, with plane angles : 



PO = 137 (43), 

 OQ = 152 (28), 

 QP = 71 (109). 



From SOHMELCHER'S measurements the corresponding angles would be 



100 : 101 = 42 48', 



101 : 001 = 27 55', 

 001 : TOO = 109 17', 



and capital letters are assigned to the edges on fig. 2 corresponding to those of the 

 taces on fig. 1. 



The microscopic crystals are elongated plates extended along the edge P (= pp), 

 and generally terminated by the edge O (= 06) ; sometimes also by the edge Q (= qq). 

 The extinction is nearly parallel to the edge O, and 

 therefore makes an angle of 43 with the length of 

 the crystals (the edge P). The double refraction, 

 which is strong, is compensated by a quartz wedge 

 inserted with its axis perpendicular to the edge O. 



The crystals melt at about 61, and are therefore 

 the modification called a by PICKERING. 



They sometimes present the form of rhombs having 

 a plane angle of 71, when they are bounded by the 

 edges PQ alone ; they can then be identified by the 

 extinction angles 43 and 28 respectively, measured 

 with regard to the two edges of the rhomb. 



^-modification. These crystals can be obtained 

 best from solution in water ; the cooling solution 

 sometimes yields the a-modification and sometimes 

 the ft. They can be obtained direct in a drop of F I G . 3 . 



solution by scratching the slide under the cooling 



drop ; but the most convenient way of obtaining ft is to produce y by rapid cooling 

 and to transform it into ft by friction. They seem to be difficult to obtain directly 

 from fusion. 



They generally present the form shown in fig. 3 stout tables of rhombic outline 

 belonging to the monoclinic system. 



B = 010, C = 001, m = 110. 



