I9I4.] HYDROUS CALCIUM VANADATES. 47 



however, unknown, and it may be that the original water content 

 was higher. 



The same statements apply to hewettite, but here the excess of 

 0.6 molecule of water above nine molecules seems to be real, since 

 it is by no means accounted for by the fraction of one per cent, 

 represented by the nearly horizontal upper part of the curve, which 

 fraction might be considered hygroscopic or absorbed water. ^"^ It is 

 conceivable that this excess is connected with molybdenum, but much 

 more likely that it has to do with a vanadyl-vanadic compound as 

 already suggested (p. 41). 



For the moment we will assume that the formula of both min- 

 erals, when holding the maximum amount of water, is CaO, 3V2O5, 

 9H2O. Of what acid, then, are they salts? 



The ratio of CaO to VgOg shows that they cannot be salts of 

 orthovanadic acid. Moreover, the known orthovanadates are very 

 few in number and exhibit little stability, but pass readily into hexa- 

 vanadates. From the fact that six of the nine molecules of water are 

 quickly removable at ordinary temperatures in dry air and the 

 others are much more firmly held, it might seem justifiable to assume 

 six molecules of water of crystallization and three of constitution. 

 Such disposition of them necessitates derivation of the minerals, as 

 quarter-saturated salts, from the hypothetical acid HgYgOig, an 

 octobasic hexavanadic acid, a possible derivative of orthovanadic 

 acid. We are confronted, however, with the fact that neither such 

 an acid nor salts of it are known. Hexavanadates, however, de- 

 rived from the tetrabasic acid H^VgOi-, also a derivative of ortho- 

 vanadic acid, have been described and they resemble in general the 

 two minerals in question, so far as can be determined from the 

 meager data available. 



Tetrabasic hexavanadic acid offers the possibility of two isomers 

 of a salt of a bivalent metal. It seems then necessary to consider 

 these minerals as acid salts of this acid, and the name metahewettite 

 for one of them is not only justified but appropriate. If this refer- 

 ence is proper, only one molecule of water of constitution is possible 



1*^ Not all of the hygroscopic or absorbed water was necessarily removed 

 in the first part of the dehydration over sulphuric acid. 



