90 SUBSTANCES SOLUBLE IN DILUTE SODA. 



ot soda) is sometitaes a decomposition product of metaroMc add or of 

 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 

 albuminoids. (See § 235.) 



§ 108. Bhlohaphene — ^A brovra. residue insoluble in water would 

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

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

 from the evaporation-iesidue in § 106 before the weight of the 

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

 (See also § 246.) 



The polyporic add, isolated by Stahlschmidt,i j^^y also be 

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

 phide of carbon, and glacial acetic a<;id, sparingly soluble in warm 

 chloroform, alcoholy ajid 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°. , 



'Hunms'.' — 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 woidd, it is true, show a 

 behaviour similar to that of the phlobaphenes ; but in distinguish- 

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

 . the so-called humic substances contain hydrogen and oxygen in 

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

 not yield the decompositiourproducts mentioned in § 42 when 

 acted upon by fused caustic potash. 



' Anna], d. Chem, und Pharm. olxxxvii. 177 (1877) (Journ. Chem. Soc. 

 xxxii. 620). 



