( 519 ) 



The crystallographical investigation of these acids and their mixtures 

 kindly carried out by Dr. Jaeger leads to exactly the same result. 

 Dr. Jaeger reports as follows : 



Of the two isomeric compounds fi-Thiophenic acid crystallises the 

 most readily in sharply defined, small crystalline plates. 



Whether obtained by crystallisation from solvents or by fusion 

 and subsequent cooling, the compound exhibits the microscopical 

 appearance of the subjoined figure. The crystals are monoclino-pris- 



Fig. 2. Microscopcial aspect of %• and (3-Thiophenic acid. 



matic, and combinations of the form : { 001 }, very predominant, 

 {110} and {100}; the angle of inclination /3 deviates considerably 

 from 90°, so that the smaller individuals often exhibit rhomboidal form 

 owing to simultaneous development of {110} and {001}. Often the 

 plates are so thin that only a single parallelogrammatic circumference 

 can be observed with a very slight stunting of the sharp angle which 

 was determined at 42°— 43°, by { 100 }. 



In addition, small rectangular plates occur which, as the investi- 

 gation shows, are formed with { 100 } as predominant form, and 

 therefore show prolongation along the b-nxis. Although representing 

 apparently a second form they are, however, quite identical with 

 the parallelogrammatic phase. 



The optical axial plane is parallel { 010 } and falls along the longest 

 diagonal of the parallelograms or perpendicular to the longitudinal 

 direction of the needle-shaped individuals. In convergent light one 

 hyperbole with rings is visible at the border of the tield of vision. 



