22 THE ATOMIC WEIGHTS OF 



involved, whereas the recombination of the base and the acid is 

 relatively slow. In other words, the re-formation of borax, under 

 such conditions, may lag behind, as it can not keep pace with the 

 evaporation of the solvent. Thus it would seem quite conceivable 

 that the original globules of borax solution, upon evaporation, 

 would leave minute residues containing some uncombined sodium 

 metaborate (or hydroxide) and boric acid. The latter combine 

 very slowly, on account of the weak nature of the acid, and the 

 union is complete only after prolonged fusion. 



According to this view the last traces of water expelled from 

 fused borax are not merely the last portions of the water of crystal- 

 lization proper, but are rather to be looked upon as water of 

 neutralization, resulting from the recombination of sodium meta- 

 borate (or hydroxide) with boric acid, both of which were pro- 

 duced by a transient hydrolysis during the earlier stages in the 

 dehydration; and this view explains why the last traces of water 

 should be removed with greater difficulty than the bulk. Hence, 

 the final loss of water in the dehydration of borax may, in all 

 probability, be due to the completion of such reactions as the 

 following : 



2 NaB0 2 +2HB0 2 = Na 2 B 4 7 +H 2 O 



2 NaOH 



The preceding remarks seem to justify the general conclusion 

 that many other salts which are extensively hydrolyzed in water 

 solution should also show an anomalous behavior when deprived 

 of the last trace of their "water of crystallization." Such, indeed, 

 appears to be the case. Every chemist is familiar with the diffi- 

 culties encountered in drying sodium carbonate without decom- 

 posing some of the carbonate itself. Undoubtedly the anhydrous 

 substance itself begins to dissociate in this case and thereby suffers 

 a loss of carbon dioxide; moisture, however, seems to aggravate 

 this loss and to cause some carbon dioxide to be given off at a 

 lower temperature. Again, in the work upon the atomic weight 

 of columbium, 16 it was found quite difficult to dehydrate sodium 

 metacolumbate (Na 2 Cb2O6.7H 2 0) completely. This salt, which 

 is also strongly hydrolyzed by water, had to be ignited for 10 

 hours or more before constant weight was reached. As this salt is 



J. Am. Chem. Soc. 37, 1793 (1915). 



