xvi. Proceedings. \_Fehruary ^th, 1922. 



strengths of various amphoteric metallic hydroxides. In con- 

 tinuation of this work the authors investigated the behaviour 

 of stannic hydroxide under various conditions, and were thus 

 led to make a study of the different modifications of stannic 

 acid. Two lines of opinion regarding the nature of these 

 compounds have been held by chemists. On the one hand^ 

 Engel, Kleinschmidt, and others have regarded a- and /3-stannic 

 acids as compounds of definite composition, the latter compound 

 having a molecular weight five times that of the former. 

 Opposed to this view is that which has been held by Van 

 Bemmelen, Vignon, Biron, Mecklenburgh, and others to the 

 effect that the stannic acids form an uninterrupted series of 

 substances of gradually increasing complexity, the substances 

 becoming of more pronounced /3-character the greater their 

 degree of complexity. This difference in complexity and 

 behaviour was ascribed by Mecklenburgh to variations in the 

 size of the ultimate particles of the different modifications of 

 stannic acid, but no explanation has been given as to why,, 

 under certain conditions, the size of the particles, and therefore 

 the character of the acid, may suffer change. The authors, 

 while accepting most of the views of the latter school of chemists, 

 have endeavoured to explain the change of modification referred 

 to above. They advance the hypothesis that the change of an 

 a-modification of stannic acid into the jG-variety is to be traced 

 to the amphoteric nature of stannic hydroxide, molecules of this 

 substance, behaving as acid and base respectivel}^ combining 

 with each other to form products of graduall3^ increasing 

 complexity and partaking of the nature of salts. In the 

 presence of a strong acid or a strong base, these salt-like com- 

 plexes will suffer gradual decomposition, thus leading to a 

 reversal of the a — fS change. It was pointed out that this 

 hypothesis gives a satisfactory explanation of whj^ under certain 

 conditions of preparation, there should be a greater tendency 

 for the formation of /3- than of a modifications of stannic acid, 

 and it is also in harmony with the comparative behaviour of 

 the two modifications as recorded in the literature and with 

 that observed in the authors' own experiments. In conclusion, 

 attention was drawn to the fact that this hypothesis may be 

 applied to other substances besides stannic acid ; the existence 

 of different modifications of titanic acid, zirconic acid, etc., can 

 be explained in this manner, and the behaviour of these sub- 

 stances, as found in experiments at present in progress, is quite 

 in keeping with the requirements of the theoiy. 



