THE; MINERAL CONSTITUENTS OF THE son, SOLUTION 55 



been designated as humic, ulmic, crenic, apocrenic, azohumic 

 acids, etc. Their existence has been predicated upon two facts: 

 First, humus is soluble in alkaline solutions but is more or less 

 completely reprecipitated on the addition of an excess of a strong 

 mineral acid, a phenomenon also characteristic of many organic 

 acids. But many other organic substances than acids are also 

 soluble in the presence of alkalis and insoluble in the presence 

 of an excess of strong mineral acids. .Second, organic-copper 

 complexes have been obtained from humus constituents, and 

 supposed to be copper salts of various humus acids. The de- 

 scriptions of these complexes so far given do not show that they 

 met the usual criteria for definite compounds, but indicate on 

 the contrary that they were the results of absorption or possibly 

 adsorption phenomena. Consequently the existence of "humic" 

 acids is purely hypothetical and without experimental or other 

 scientific verification, and calls for no further consideration here. 

 It is a widespread and popular notion that substances with 

 a slight solubility also dissolve slowly, and that consequently 

 the solubility of the minerals in the soil water must necessarily 

 be a very slow process. This is, however, a misapprehension. 

 It has been shown with a number of the common rock-forming 

 minerals, that if they be powdered and then stirred into a rela- 

 tively small volume of water, they dissolve very rapidly at first, 

 and in a very short time, generally a few minutes, the solution 

 is nearly saturated with respect to the mineral. Complete sat- 

 uration, however, may require many days. The general shape 

 of curve expressing the rate of solubility is shown in the ac- 

 companying figure. 1 For soils, this fact has been verified re- 

 peatedly, in the following way : A cell fitted with parallel elec- 

 trodes is placed in circuit with a slide-wire 2 or Wheatstone 

 bridge in such a manner that the resistance of the cell contents 

 can be quickly determined. Distilled water is then placed in 

 the cell and its resistance found. Generally this will be up- 



1 See, for example, Umwandlung des Feldspats in Sericit (Kali- 

 glimmer) von Carl Benedick, Bull. Geol. Inst. Upsala, 7, 278-286 (1904). 



2 See Electrical instruments for determining the moisture, tempera- 

 ture and soluble salt content of soils, by L. J. Briggs, Bull. No. 15, and 

 The electric bridge for the determination of soluble salts in soils, by 

 R. O. E. Davis and H. Bryan, Bull. Noo. 61, Bureau of Soils, U. S. 

 Dept. Agriculture. 



