HARD PAN 125 



to be brought by water, while a limestone hardpan might 

 form, under favorable conditions, it seems more probable 

 that the cementing material would be one of the other 

 substances mentioned, or a mixture of them. 



"The mineral constituents of the soil are for the most 

 part salts, but with a few exceptions salts with a very 

 limited solubility. Nevertheless, to some extent at least 

 they are soluble, as are other salts, and their solubility 

 may be increased or diminished by the presence of another 

 salt solute, as has been indicated in a former part of this 

 paper. These salts carbonates, silicates, aluminates, 

 ferrates, etc. - - are without exception salts of weak acids 

 and may be expected to be much hydrolized in as far as 

 they are soluble at all. This has been very beautifully 

 illustrated in recent experiments by Clark, who has treated 

 a large number of minerals carefully pulverized with pure 

 water. On the addition of a few drops of dilute alcoholic 

 phenolphthalein a marked alkaline reaction could be ob- 

 served in the great majority of the cases investigated. 

 The reaction may be indicated thus, assuming a very 

 simple example to exist: 



RSi0 3 + HOH^ROH + H 2 Si0 3 . 



"All these other substances are very slightly ionized 

 in comparison with ROH. If R be a well-marked base, 

 such as sodium or calcium, the solution will therefore be 

 alkaline, as has been shown to be the case with calcium 

 carbonates, sodium silicates, etc. The fact that the 

 silicate is complex will not alter this general property. 

 Precisely similar conduct is to be expected of aluminates 

 and ferrates. This means that there will actually exist 

 in the solution some of the hydrates of alumina, silica, or 

 iron, as the case may be, which will remain as such on 

 evaporation, though the absolute amount may be very 



