( 615 ) 



This is the fu'st properly investigated series of halogen-substitution 

 products in organic chemistr}^ where CI, Br, and I replace each 

 other in a directly isomorphous manner. 



Notwithstanding this complete isomorphism there occurs here a 

 remarkable abnormality in the position of the melting points, just 

 as in the case of the isof/anorphous p-H(xlogen acetanllides. This 

 abnormality cannot, therefore, be explained in the manner described 

 above; in fact it is quite incom[)rehensible: 



a-compound: m. p. 112° C. Spec. gr. 2,111 Mol. Vol. 118,7 

 Br- „ m. p. 117^ C. „ „ 2,434 „ „ 121,1 



/- „ m. p. 1040 0. „ „ 2,645 „ „ 129,03 



In this case it is tlie /-compound which exhibits an abnormal 

 melting point. 



From all this it is evident that there is still something strange, 

 as regards the mutual morphotropous relations of the halogens, at 

 least, in the case of organic compounds. Some facts relating thereto 

 will therefore be communicated in what follows. 



I have, frequently, published papers on the Methyl esters of p- 

 Chloro-, and p-Bromohenzoic acid '). The Chloro- and i?/'omo-deriva- 

 tive each appeared to possess a different form, whereas the melting 

 point line of binary mixtures should lead to the conclusion that an 

 isoJi'morphism was present here, with a melting point line of the 

 rising type, although it seemed impossible then to define by physico- 

 chemical methods the limits of mixing for the two kinds of mixed 

 crystals. 



In order to treat the existing problem as fully as possible, I 

 prepared first of all the corresponding Fluoro- and /öc/o-compound. 



p- Fluor otoluene kindly presented to me by Prof. Holleman was 

 oxidised with KMnO, in alkaline solution, the p-Fluorobenzoic acid 

 was separated with HCi and then esterified by means of methyl 

 alcohol and hydrogen chloride. The ester, which has a strong odour 

 of uniseseed oil, is a liquid rendering measurements impossible, but 

 on the other hand the acid could be measured crystallographically. 



j).-Toluidine was diazotised and converted by means of KI into 

 p-Iodotoluene, this was distilled with steam, recrystallised and oxidised 

 as directed to p-Jodobemoic acid. In the same mannei-, p-Aminobenzoic 

 acid was converted by diazotation etc. into its acid and this was 



1) Jaeger, Neues Jahrb. f. Miner. Geo), und Palaeont. (1903). Bell. Bd. 1—28; 

 Zeits. f. Kryst. 38. (1903). 279-301. 



43* 



