19 
in such form as to be more soluble in weak solvents, No. 164, a 
mineral soil, being the least soluble. Attention is also called to 
the effect of cultivation or aeration upon the solubility of lime and 
magnesia, namely, that the cultivated soils contain these elements 
in far more soluble form. 
From a study of the table of magnesia determinations it is evi- 
dent that the action of the solvents upon this element is quite sim- 
ilar to their action on lime as regards the effects of heat, but that 
the lime is very much the more soluble both in water and in dilute 
nitric acid. The results of the extractions with water show a max- 
imum solubility in the samples heated to 250° C, the least soluble 
magnesia in the air-dry samples. This exactly correlates with the 
results of the lime determination. The solubility in dilute nitric acid 
does not correspond so closely, but the general tendency is similar 
in that the air-dry samples and those dried at 100° C. contain this 
element in the highest state of solubility, while in the ignited soils 
it is least soluble. An important fact to which attention is called 
is that, although most of the soils in this series show from digestion 
with hydrochloric acid (1.115 specific gravity) a higher magnesia con- 
tent than lime, one of them four times as much, yet the lime, with 
very few exceptions, is considerably more soluble. One exception 
is to be found in sample No. 290, which represents a soil having a 
characteristic soapy property indicating the presence of hydrous 
magnesium silicate. 
The effect of heat on the solubility of lime and magnesia is more 
striking than in case of the other elements. It is highly probable that 
the increased concentration of the water extract of the soil heated to 
100° C. over the air-dried sample is produced through physical causes, 
namely, destruction of the soil film and delrydration accompanied by 
a slight decomposition of organic matter. On the other hand, the soil 
when heated to 250° C. undergoes more completely all the above trans- 
formations as well as decomposition of organic matter. Since calcium 
and magnesium are two of the elements universally combined with 
organic matter, there necessarily follows an increase in solubility as 
a result of the more complete decomposition. The soils showing the 
greatest solubility of these elements in water were those containing 
the highest organic matter. 
The decrease in solubility of lime and magnesia in water and in 
nitric acid at 250° C. and ignition is hard to explain. It is undoubt- 
edly partly due to chemical changes in the soluble forms resulting 
from the decomposition of the organic matter and is also influenced 
by the decrease in exposed surfaces as a result of an aggregation of the 
soil particles and probably other physical factors. It is suggested 
that one of the chemical changes taking place as a result of heat is 
that of a replacement of the potash and soda in the silicates by 
