Organic Acids in the Examination of Minerals. 33 



Humboldtine, hydrous ferrous oxalate, forming an incrustation on brown 

 coal. 



Succinite, or amber, containing 2+ to 6 per cent, succinic acid. 



Melllte, or hydrous mellitate of alumina, containing over 40 per cent, 

 of the organic acid. 



Pigotite, a salt of alumina and mndescous acid (Johnston). Formed on 

 granite by the action of wet vegetation. 



To these belong also the peculiar minerals grouped by Dana 

 under the name Acid Hydrocarbons, found in peat-bogs and 

 in brown coal, and containing the ill-defined bodies, geoceric, 

 georetinic, and butyro-limnodic acids. 



25. The manner in which silicates are decomposed and 

 silica rendered soluble for the use of the vegetable world, has 

 been a subject of much investigation. Friedel and Crafts,* in 

 their remarkable researches on the ethers of silicic acid, and 

 Friedel and Ladenburg,t in a paper on silico-propionic acid, 

 have given a new insight into the functions of silica and its 

 transformations in the organic kingdom. The reading of the 

 latter paper before the French Academy of Sciences, June 27, 

 1870, excited a lively interest ; the authors describe silico-pro- 

 pionic acid, Si C^HsO^jH, as a white amorphous body closely 

 resembling silica, insoluble in water, and soluble in hot con- 

 centrated potassium hydrate. This communication drew from 

 M.Paul Thenardf a remarkable announcement with respect 

 to the solvent power of nitro-humic acid § on silica j he stated 

 in substance that the dark-colored acids of the soil consist of 

 a mixture of acids of the humic and nitro-humic series, which 

 contain a notable amount of silica, the amount varying from 

 7.5 to 24 per cent.; and he conjectures that acids of the 

 "nitro-humic series form spontaneously in the soil at the 

 expense of the humic acid, the ammonia of rain-water, nitro- 

 gen of the air, and of the silica pre-existing in the soil." 



Dr. J. S. Newberry, in a private communication to the 

 writer, describes the peculiar manner in which quartz pebbles 



* Bull. soc. cMm., V, 174, 238 (1863). 



t Comptes Eendus, LXX, 1407. 



| Comptes Rendus, LXX, 1412. 



§ In translating " acide azhumique " by the term "nitro-humic acid," we follow 

 custom, but we are of the opinion that " azo-humic acid " would bemore correct, since the 

 word " nitro-humio " implies the existence of a "nitro'' radical, N0 2 . 



