342 PRINCIPAL HENRY A. MIERS AND MISS FLORENCE ISAAC ON THE 



The measured angles are 



mm = 52 20', 



Cm = 73 40'. 



From these measurements the angle /8 = 100 : OOl, or the plane angle of the face B, 

 is calculated as 71 44-g-'; the plane angle on the face C is 54 43', and that on the 

 face m is 81 43'. There is a good cleavage parallel to B; the extinction on C is 

 straight, and that on B makes an angle of about 23|- with the edge C, and about 

 48|- with the edge m, while on m it makes an angle of about 36 with the edge mB. 

 The face B is perpendicular to the third mean line. The crystals possess strong 

 positive birefringence, and not a very large axial angle. The acute positive bisectrix 

 is inclined at 23|- to the edge BC and at 48| to the edge Bm. The refractive index 

 appears to be greater than that of the a-moclification. 



On a microscope slide crystals which appear to be identical with these are sometimes 

 obtained from the fusion of mixtures containing a little naphthalene ; usually also 

 from pure aqueous solutions. 



They are plates of rhombic outline, having a plane angle of about 72 ; the 

 extinction is about 23 with regard to one edge, and about 49 with regard to the 

 other. This serves to distinguish the crystals of this modification from the a-crystals, 

 which may also appear as rhombs of about 71. 



These crystals melt at about 55 and are, therefore, probably identical with the 

 /3- modification of PICKERING. 



y-modification. -If fused monochloracetic acid or the aqueous solution be cooled 

 suddenly, the substance crystallises in rhombs having a plane angle of about 59, and 

 the plane of the rhomb perpendicular to an acute positive bisectrix. The direction oi 

 extinction makes an angle of about 26 with one edge and about 33 with the other. 

 The plane of the optic axes is inclined at a small angle (3|) to the shorter diagonal 

 of the rhomb ; and the axial angle in air, as determined by an eyepiece micrometer, is 

 about 81^. 



These crystals melt at about 50, and are therefore possibly identical with the 

 y-modification of PICKERING. Under the microscope the rhombs of y-crystals are 

 easily distinguished from those of a-crystals by their plane angle and their extinction ; 

 but they may easily be confused with rhombs of y8-crystals lying on the face which 

 has a plane angle of about 55 and diagonal extinctions. 



B. Transformations of the Modifications of Monochloracetic Add. 



Transformations of y. When y-crystals which have formed upon a glass slide 

 are allowed to stand, they usually become transformed either into /B or into a. If 

 they are touched they at once become transformed into ft. In tubes of aqueous 

 solution which have been suddenly cooled y-crystals appear, and on shaking become 

 transformed into ft, the temperature of the solution rising at the same time to that of 



