450 Lindgren and Ilillebrand — Minerals from Arizona. 



in which H ff = Pb" or Mn". This is to be regarded as a saturated 

 salt of one of the numerous possible derivatives of ortho-man- 

 ganous acid that may be derived from it by removal of water r 

 in the present case as follows : 



3H 4 Mn0 4 - 5H 2 = H 2 Mn 3 7 



An acid of the same empirical formula would result by 

 removal of two molecules of water from three of metamanga- 

 nous acid, H 2 Mn0 3 . 



It is probably best to rest for the present content with the 

 above relatively simple formula and to regard the water found 

 as due to incipient alteration. But if the water is to be con- 

 sidered as wholly or in part essential, and furthermore constitu- 

 tional — and this may very well be the proper view to take- 

 then the formula becomes much more complex, namely R/H 2 

 (Mn ]2 29 ), when none of the water is allotted to the foreign 

 matter. This formula is still referable graphically to a more 

 highly condensed manganous acid, and a number of isomers 

 would be possible. 



Such intricate formulas as this should not cause the least 

 surprise, however unlikely they may at first appear to be. The 

 great number of manganites, in varying degrees of saturation 

 and hydration, observed in nature and prepared artificially, some 

 of them of even greater complexity that the above, are cer- 

 tainly not all mixtures of only a few simply constituted mole- 

 cules. A very short study of the graphic formula correspond- 

 ing to the above empirical formula R" 4 H a (Mn 12 29 ) will show 

 what a vast number of closely related bodies are theoretically 

 possible by hydrating the molecule step by step, or by adding to 

 or reducing the number of divalent atoms, or substituting for 

 them those of another valence. Similar varieties in great number 

 would be derivable from other condensed manganous acids of 

 both higher and lower orders, and it is plain that because of 

 the very slight differences in percentage composition between 

 many of them, it is almost as hopeless to expect analysis to 

 reveal the exact empirical formula in the majority of cases as it 

 is for the enormously complex albuminous bodies of organic 

 chemistry. This is especially true because in so many cases the 

 mineral manganites described are far from being homogeneous 

 species. They are either mixtures of two or more of these 

 closely related complex molecules, or else are contaminated by 

 foreign bodies. It is not surprising then that so many com- 

 pounds of uncertain formula that may be regarded as salts of 

 manganous acid have been prepared in the laboratory or are 

 found in nature. From the known tendency of these bodies 

 to form under laboratory conditions which may very well be 

 repeated in their general character in nature, it is to be expected 



