HUMUS 



73 



acid and Humin groups. Exact definitions became 

 necessary at the outset of the research, and the 

 groups were therefore defined as follows. The term 

 Humic acid was given to substances thrown down 

 as brown colloid precipitates by mineral acids 

 from the water or alkaline extracts of humus, and 

 the term Humin to substances insoluble in water 

 and alkalies, but rendered soluble by fusing with 

 caustic soda or potash, from the solution of which 

 Humic acid can again be precipitated. 



Three different kinds of sugars were taken as con- 

 venient carbohydrates, Laevulose, Sucrose, and Dex- 

 trose, and boiled under a reflux condenser with a 

 3 per cent, solution of Hydrochloric acid for varying 

 periods of time. A series of colour changes take 

 place, and the colourless solutions turn successively 

 yellow, red, and brown, and then begin to precipitate. 

 It was obvious at once that there were conspicuous 

 differences with the different carbohydrates, Lsevu- 

 lose and Sucrose changing rapidly, but Dextrose 

 going very much more slowly. This is well shown 

 by the following table:* 



* A simple experiment illustrating this can be done by pouring 

 concentrated sulphuric acid on to a thick syrup of cane-sugar. 

 By this violent method these changes can be more or less well 

 observed, but once the reaction starts, it is completed almost 

 immediately. 



