( 521 ) 



On account of ilic evaporation of the two substances when melting, 

 one is obliged always to use a covering glass under the crystallisation- 

 microscope. 



On mixing the two isomers I have noticed the following on melting 



and subsequently cooling the mixtures. 



a. Mixture containing 61,6 °/ of £?-acid yields exclusively mixed 

 crystals of the /J-form ; formation of minute traces of «-crystals is 

 not improbable. 



h. Mixture containing 42 °/ °f £-acid yields chiefly mixed crystals of 

 the «-form ; at the edges of the fused mass, however, are found also 

 very small, slightly coloured parallelograms of the fJ-form. Negative 

 mixed crystals in the «-form (see d) were not noticed; only positive 

 ones with q <^v. 



c. Mixture containing 35,5°/ of £-acid behaves on solidification like b. 



d. Mixture containing 22,5 °/ of /J-acid, only yields mixed crystals 

 of the «-form both positive and negative doubly-refracting but with 

 q <^ r like the «-acid itself. 



e. Mixture containing 86 % of «-acid only gives mixed crystals 

 of the «-type with strong dispersion (> <^ v and a positive double 

 refraction. 



Dr. Jaeger comes to the following conclusion : 



"There exists here an isodimorphous mixing series with hiatus. This 

 extends from a /i-acid concentration ]> 22,5°/ to mixtures containing 

 61 — 62 °/o °* ,ue ^-compound. The mixed crystals of the «-type 

 become on addition of the negative ^-compound less strongly positive 

 optically and in the immediate vicinity of the hiatus even negative; 

 they, however still retain the strong dispersion with q <^ v, which 

 is so characteristic for the pure «-compound. 



On the other hand, the mixed crystals of the 0-type have at all 

 concentrations of 62 — 100 % a negative double refraction and a 

 very feeble inclined dispersion." 



V. Meyer states in his treatise that in the oxidation of mixtures 

 of the two thiotolenes, he has obtained various other mixtures of 

 «- and fMhiophenic acid, and that these showed no sign of separation 

 into their components when subjected to fractional crystallisation. 



Dr. Voerman, however, cannot confirm this observation. When lie 

 recrystallised a mixture of 85.3°/ «-acid and 14.7% of 0-acid (solidi- 

 fying point 120°. 3) from hot water, the solidifying point increased 

 to 121 .6 which corresponds with a mixture of 89" «- andll'/«of 

 /i-acid. As, however, V. Meyer does not state the temperature at 



