( 621 ) 



what changeable in the Br-estev (tig. 6 and 7), although no longei- 

 so strong as in tlie CZ-derivative. Witii an increasing atomic weight 

 of the halogens, the changeability of tlie crystal-habit, owing to a 

 change in conditions of crj-stallisation, decreases considerably and 

 gradually. 



The sp. gr. of the three compounds, their equivalent volume and 

 their topic parameters are : 



CT-ester : d,,o = j ,382 ; F=: 123,37 . ■/ : i^ "> = 5,1731 : 2,7774 : 9,5153. 

 5r-ester:fZ„o = 1,689; F=::127,29 .-/: i|': ü> = 6,661J :4,769J : 4,0070. 

 J-ester: d,„o = 2,020; r= 129,70.x : il': w = 6,8179: 4,8203 : 3,9464. 



It must be remarked here that the melting points of the three 

 esters increase regularly by 35'/,° notwithstanding the ditfercnce in 

 crystalform: 44°— 79'/,°- 114°. 



Tlie above admits of no other explanation than the assumption 

 that all three halogenised esters are dimorphous. The CAester must 

 still exist in a more labile rhombic form and the Br- and /-esters 

 in a more labile monodinic form. In one of Bruni's communications') 

 a "monoclinic'^ 2}-Bromobenzoic Methyl Ester is described by an 

 Italian investigator with the object of proving an "isomorphism" 

 with the analogous p-Nitrobenzoic ester. The given measurements 

 have, however, absolutely no connection with those applied to the 

 p-Chlorobenzoate, so that this monoclinic form can in no case be 

 the one intended. Moreover, none of the measured angular values 

 of the p-Bromo-derivative agree with those obtained by myself. It 

 appears to me doubtful whether the measurements mentioned in 

 Bruni's paper are really correct or it may be that the operator has 

 really not been working with p-Bromobemoic Methyl ester at all. 

 All etforts made by me to obtain from this substance a crystal form 

 diiferent to the rhombic one proved fruitless, whilst in the Italian 

 treatise, the supposed "monoclinic" form is represented as a perfectly 

 stable one which, therefore, occurs continuously. 



In order to prove an eventually existing dimorphism of these sub- 

 stances, I have made use of Lehmann's microscopical method with the 

 aid of the crystallisation microscope constructed by him. It appeared, 

 however, that in none of these cases a positive result could be 

 obtained. I think that in the case of each of these substances, lean 

 notice two ditferent ways of crystallisation under the microscope, 

 namely long, rather delicate needles and also parallelogram-limited 



>) Bruni and Padoa, Gazz. Chimic. Ital. (1904). 34a. 133—143; Rendic. Lincei 

 (1903). 5a. 12. 348. 



