578 STEVENSON— FORMATION OF COAL BEDS. [November 3. 



formula C04H1SO9, while Ditmer thought them the same thing, the 

 latter being produced by drying the former. 



Humic acid is a colloid and is not absorbed by plants, though its 

 oxidized product, crenic acid, seems to be taken up. Humic acid is 

 very slightly soluble in water at 6° C. ; if dried at 120° C. it is much 

 less soluble and if completely dried at a high temperature it is insol- 

 uble. Its alkaline salts are readily soluble but those with alkaline 

 earths and metallic oxides are insoluble or nearly so in water, though 

 readily in aqueous alkaline solutions. Calcium humate dissolves in 

 3,125 parts of water and ferric humate in 5,000 parts, but these form 

 soluble double salts with ammonia. Ulmic and humic acids are 

 rarely free except in bogs. A noteworthy property of humic acid 

 is that, as a colloid, it renders sand impermeable to water. 



These feeble acids yield others upon oxidation. Humic gives 

 crenic, which is present in all waters, in rotten wood, in peat and in 

 cultivated soil. Julien has found it, as well as its oxidized product, 

 apocrenic, in American peat. Crenic acid, pale yellow and trans- 

 parent, is readily soluble in water ; in drying, it becomes opaque and 

 blackens when exposed to the light. Alkaline crenates are very 

 soluble but the calcareous salts are only slightly soluble. Those of 

 iron and aluminium are insoluble, but, according to Bischoff, the 

 iron salt is soluble in ammonia, so that it may be dissolved in the 

 presence of decaying nitrogenous substances. The apocrenates have 

 same distribution as the crenates but they are less soluble. These 

 organic acids bleach clays and have solvent effect on silica ; the most 

 efficient being the brown or ulmic constituents. 



Liebig,^" writing soon after Mulder published the results of his 

 investigations, stated that a solution of caustic potash blackens in 

 contact with vegetable mould. Dilute sulphuric acid precipitates 

 from the solution a light, flocculent brown or black substance which 

 absorbs oxygen rapidly. After drying it is not soluble in water. 

 Cold water dissolves only one ten-thousandth of its weight from 

 vegetable mould and the dissolved material is chiefly salts ; but boil- 

 ing water extracts several substances, yellow or yellow-brown. On 



*"'J. Liebig, "Chemistry in its Application to Agriculture and Physiology,'' 

 Philadelphia, 1843, pp. 112, 113. 



176 



