72 THE SPIRIT OF THE SOIL 



of the nature of humus, it may be well to set the 

 position out diagrammatically : 



Extract humus, with alkali. 



| 



Alkaline solution, Acidify. Insoluble 



| residue. 



Acid filtrate. Precipitate, boil with alcohol. 



Crenicacid. Humic acid, soluble. Humic acid, insoluble. Humin. 



From the Crenic acid group and from the soluble 

 Humic acid group it cannot be too often insisted 

 that we are dealing with groups of substances and 

 not with pure bodies as many as twenty distinct 

 compounds have been isolated, while so far the con- 

 stituents of the two other groups (insoluble) have 

 defied chemical analysis. About the compounds so 

 isolated the curious fact has been recognized that 

 only one of them is a carbohydrate. A carbohydrate 

 is a compound that consists of a certain amount of 

 Carbon combined with Hydrogen and Oxygen in 

 such a proportion as would result in the formation of 

 water. It can be expressed generally in the formula, 

 C m H 2n O n , where m and n are any whole numbers. In 

 view of the fact that a very large percentage of plant 

 substance consists of carbohydrates, it is curious that 

 they should not be represented largely in the decom- 

 position products of organic material. 



In the course of the researches on Nitrogen-fixing 

 bacteria at the Botanical Laboratory in the Uni- 

 versity of London, King's College, it became desir- 

 able to try and determine what relation existed 

 between carbohydrates and the unknown Humic 



