CHEMISTEY OF SOILS 



109 



ture of products marking a transition stage in the gradual 

 decay of organic matter. Owing, however, to the fact that 

 its origin is practically always the same, it exhibits more 

 or less specific properties, and the variation in composi- 

 tion does not exceed definable limits. Its more important 

 physical properties are dark colour, low density, great 

 porosity, and high specific heat. It therefore affects the 

 properties of soils in the manner already described. 



It gives rise to colloidal bodies of an acid nature, often 

 referred to collectively as humic acid or geic acid. It is 

 generally accepted, however, that this is not one but several 

 distinct substances. Names and formulae have been 

 ascribed to those which, it is claimed, have been isolated 

 and examined, but the opinions of various authorities on 

 the subject are widely divergent, and their results are so 

 inconsistent that it is difficult to repose confidence in any 

 of them. According to Mulder, humus is mainly composed 

 of six different compounds, known respectively as humin, 

 humic acid, ulmin, ulmic acid, crenic acid, and apocrenic 

 acid. Stockbridge gives the same formula for humin and 

 humic acid, and also for ulmin and ulmic acid. Another 

 authority declares that humic acid and ulmic acid are one 

 and the same substance, and proposes a formula which 

 does not correspond with those of the others for either sub- 

 stance. The formulae proposed respectively by Mulder and 

 Stockbridge for the various substances are as follows : 



