( 622 ) 



flat needles which exhibit higher interference colours and like the 

 first named extinguisli normally on the longitudinal direction. Howe- 

 ver, the difference — if present at all — is so indistinct that taking 

 into consideration the inclination of these substances to alter their 

 crjstal-habiousit under var circumstances in so high a degree, I dare 

 not conclude to an already proved dimorphism. Experiments with 

 mixtures richer or poorer in Ch/oro-ester also exhibited the same 

 properties. It is, therefore, quite possible that we have two forms 

 for each of the three substances, but this has not yet been proved 

 and also it could not be ascertained whether in the given circum- 

 stances both eventually present forms stood to each other in the 

 position of monotropy or enantiotropy. 



A few short data as to the halogenised benzoic acids deserve mention. 



1. p-Cldorohcnzoic acid m.p. 243° has been measured by Fels 

 (Zeitschr. f. Kryst. 32, (1900) 389). It is monoelino-prismatic, with 

 a:h:c = 1,2738 : 1 : 3,3397, and /i = 78°24|'. The forms observed 

 have intricate symbols; besides a^\100\ and c = {OOlj, he finds 

 f7= i207|, = jlll!, e = {233!, u = \%2% v = jill!. Sp.gr. = 1,541. 



2. p-Fluoi'ohenzoic acid m.p. 182° synthesised by me is also 

 monoelino-prismatic. If to the forms occurring here we assign 

 the symbols: a = JlOOj, c = jOOlj, r = r203j, s = jlOSj, and q = \O^S\, 

 the indices being therefore analogous to those given above, the 

 parameters become : 



a:b:c = 1,1917 : 1 : 3,1825 

 ^ = 78°16'. 



Although there exists here an undeniable difference in habit, I 

 think I may still conclude that there is a direct isomorphism of 

 the Chloro- and i'Tworo-compound. A distinct plane of cleavage was 

 not found. The melting point of p-Fluorobemoic acid is also elevated 

 by addition of the 67-compound. 

 Angular values: 



c':r =69°56' 

 r:s =15°36' 

 s:a =16°14' 

 c:a =78°16' 

 c:q =76°28' 

 ?:=2x = 27°4' 



The habit is thin-tabled towards c,. with a rectangular circum- 

 ference. The crystals were obtained from alcohol -j- ether and were 

 generally badly formed. The extinction on c is orientated. 



