SUBSTANCES SOLUBLE IN DILUTE SODA. 



of soda) is sometimes a decomposition product of metaralic acid or 

 allied mucilaginous substances. The action of caustic soda on the 

 latter often results in the formation of products that are not pre- 

 cipitable by alcohol. But this body that thus remains in solution 

 on adding alcohol will be more often found to belong to the 

 albtfminoids. (See 235.) 



108. Phlobaphene. A brown residue insoluble in water would 

 frequently consist of phlobaphene. (See also 48.) It should be 

 collected on a tared filter, washed, dried, weighed, and deducted 

 from the evaporation-residue in 106 before the weight of the 

 substances derived from mucilage, caseine, etc., can be arrived at. 

 (See also 246.) 



The polyporic acid, isolated by Stahlschmidt, 1 may also be 

 mentioned here. It is insoluble in water, ether, benzene, bisul- 

 phide of carbon, and glacial acetic acid, sparingly soluble in warm 

 chloroform, alcohol, and amylic alcohol, but dissolved by dilute 

 ammonia, forming a violet liquid, from which it is precipitated 

 by hydrochloric acid. It crystallizes in rhombic plates, and melts 

 at about 300. 



'Humus.' I am convinced that the 'humus ' mentioned in old 

 plant-analyses was in reality partly phlobaphene and its decom- 

 position-products. In the majority of vegetable substances 

 humus is not to be found, unless they are already in a state of 

 decomposition. Perhaps some thick barks and lignified fungi 

 might yield substances with characters resembling those possessed 

 by humus. To solvents such substances would, it is true, shoAv 

 behaviour similar to that of the phlobaphenes ; but in distinguis 

 ing them we may take advantage of the fact that the majority of 

 the so-called humic substances contain hydrogen and oxygen 

 the proportion in which they exist in water, and that humus does 

 not yield the decomposition-products mentioned in 42 when 

 acted upon by fused caustic potash. 



1 Annal. d. Chem. und Pharm. clxxxvii. 177 (1877) (Journ. Chem. Soc. 

 xxxii. 620). 



