456 



below this point 1 iiave deterniinecl a series of heating and cooling 

 curves, the best examples of which are represented in the gra|)hical 

 representation (fig. 2). Descending we found 183°. 7, rising 184. °7. 

 At the same time it will appear from the graphical representation 

 that the curves exhibit resp. a minimum and a maximum, which 

 points to this that, the conversion at first proceeds only slowly, but 

 soon becomes constant so that the supplied resp. discharged heat 

 and the thermal effect of (he conversion compensate each other. 



A second couple of curves, for which mannite acts as catalyst, 

 presents only little more diverging values. Also the results of a few 

 more substances used are reported in table 4. 



TABLE 4. 



Catalyst 



Descending djc.:.,^ i.^:„\ 

 (max.) I '^•^'"g (""">• 



It is clear that Ihe catalysts counteract the retardation in the 

 conversion of the solid substance in a more or less degree, and that 

 this is particularly the case for glycerine and mannite, where the 

 limits for the point of transition from 13° (see §3) to 1°, resp. 1°.6 

 have contracted. Moreover it appears that the retardation in the 

 conversion without catalysts is much sn)aller in case of heating than 

 in ease of cooling. If, however, we imagine that also when catalysts 

 are used this difference in retardation continues relatively to exist, 

 then the point of transition would be calculated at 184.5° in the 

 experiment with glycerine, at 184.4° in that with mannite, in perfect 

 harmony with the experiments of § 5. 



7. Accordingly the experiments oi" § 5 and 6 yield the result 

 that the point of transition of ammonium chloride has been fixed 

 at 181.5° with a possible error of a few tenths of a degree. The 

 value given by Wallace (see §1) is therefore more than 25° too low. 



8. Demonstration of the al/oiropi/ of nmnumiam chloride. The 

 transition from the ji- into the «-form cannot be demonstrated by 

 crystallisation of an aqueous solution under atmospheric pressure, 



