RECENT ADVANCES IN SCIENCE 295 



of affairs may be due to a difference in molecular structure 

 between the two forms (tautomerism, dynamic isomerism, 

 polymerism) or to a different arrangement of identical mole- 

 cules within the crystals (physical dimorphism, polymorphism). 

 If the two forms are dynamic isomers they will retain their 

 identity in solution for a time at least, whereas if they are 

 physical polymorphs the differences between them will only 

 persist in the solid state, and will disappear in solution. To 

 determine to which class a given pair of substances belong 

 the author adds some of the variety B to a saturated solution 

 of the more soluble variety A. If the forms are polymorphic 

 and consequently both give the same molecules in solution, the 

 concentration of the solution will remain unchanged, since B 

 cannot dissolve owing to the solution being saturated, or the 

 solution might even become slightly weaker, owing to some of 

 A crystallising out on to B. If, however, the two forms are 

 tautomeric the solution will become stronger on adding B 

 since the latter will dissolve in the solution irrespective of the 

 presence of A, provided of course that the tautomeric change is 

 not too rapid. The method employed for observing the 

 changes in concentration is to determine the freezing-point of 

 the solution of one modification, and then to add some of the 

 second modification, and to redetermine the freezing-point. 

 If there is any appreciable lowering, the second substance is a 

 tautomer or isomer, and if not it is a polymorphic variety. 



Since the discovery of aldehyde ammonia by Liebig in 

 1835, it has been the generally accepted view that this sub- 

 stance was a simple additive compound formed from one 

 molecule of each of the two reacting substances. According 

 to Ossian Aschan, however (Berichte, 191 5, 48, 874), this view 

 is incorrect, and the constitution of this substance should be 

 represented by the formula : 



•OH 



CH 3 CH 

 CH 3 CH 



Nnh 2 oh 



NH 2 OH 



:h 3 ch 



\ni 



