17 
' This table shows that the solubility .of manganese in water is much 
greater in the heated soils/being most soluble in the ignited samples. 
The surprising feature of this table is the fact that several of the soils 
in the series show manganese in- a much more soluble form than the 
manganiferous soil containing 9.21 per cent total Mn 3 4 . A possible 
explanation is found in the results upon the cultivated and unculti- 
vated soils (Nos. 416 and 417, respectively), namely, that cultivation 
and the accompanying aeration has the effect of producing a lower 
state of oxidation or other changes which render the manganese less 
soluble in water. An observation of Nos. 416 and 417 (cultivated and 
uncultivated) shows a large decrease in solubility as a result of culti- 
vation, and the highly manganiferous soil (No. 9) has been in culti- 
vation for some time. - 
The table showing the solubility of manganese in dilute nitric acid 
presents a remarkably consistent series of results, as is shown by the 
increase in solubility as a result of the action of heat up to 250° C, 
followed by a large decrease as effected by ignition. This is true with 
only two exceptions in the entire series. 
It is difficult to explain the effects of heat upon the solubility of 
manganese. This element occurs in some Hawaiian soils as concre- 
tions, especially in the highly manganiferous soils, and is present, at 
least partially, as manganese dioxid. But in the normal types con- 
cretions are absent, and here the manganese probably exists largely 
in a lower state of oxidation, and hence in a more soluble form. In 
any case manganites and salts may occur to a limited extent. As 
already noted, the soils heated to 250° C. and ignition gave the more 
concentrated water extract, an average indicating the maximum con- 
centration from the ignited soils. The effect of heat upon the phys- 
ical properties is probably the prime factor which influences the sol- 
ubility in water. With one exception the oxids of manganese are 
quite insoluble in nitric acid, this oxid being manganous oxid (MnO). 
Therefore the higher oxids, such as manganomanganic oxid (Mn 3 4 ) 
and manganic oxid (Mn 2 3 ) , which are both essentially combinations 
of manganese dioxid and manganous oxid, 1 , are soluble in. nitric acid 
to the extent of their MnO content, while their Mn0 2 content remains 
insoluble. Consequently the solubility of manganese oxids increases 
with increase in heat owing to the above-mentioned decrease in state 
of oxidation, as high temperatures convert Mn0 2 into Mn 3 4 and 
Mn 2 3 , each of which is partially soluble in nitric acid. Therefore 
heat would tend to increase the solubility in nitric acid of that portion 
occurring as Mn0 2 . 
In addition, it is known that the action of heat upon organic com- 
pounds of manganese and also certain of its salts converts them into 
i MnjO<— 2 Mno+MnOj; Mn I 8 =MnO + MnOj. Hence it may be observed that MnjO« contains the 
more soluble manganese. 
14000°— Bull. 30—13 3 
