114 NATURE AND PROPERTIES OF SOILS 



ture is boiled, practically all of the carbon is thus driven off. 

 This gas is drawn through a train of absorption bulbs, caught 

 in a solution of potassium hydroxide, and thus weighed. 



A second determination is now made on a new sample of 

 soil, leaving out the chromic acid. The carbon dioxide given 

 off under such conditions is that of an inorganic nature. The 

 weight of this gas substracted from the total carbon dioxide 

 leaves the organic carbon dioxide. 



The data from the use of the chromic acid method may be 

 expressed as organic carbon or as organic matter. Multiply- 

 ing the carbon dioxide by .471 or the carbon by 1.724 is con- 

 sidered as giving an approximate figure for the organic mat- 

 ter. 



The results obtained with the chromic acid method are usu- 

 ally lower than those from ignition or combustion, due par- 

 tially to the oxidation resistance of the carbonized matter, 

 already discussed. This material, while it succumbs to igni- 

 tion, resists the action of the sulfuric and chromic acids to 

 a very large degree. The water of hydration is, of course, not 

 a factor in the chromic acid method. 



Bomb Combustion} — Two grams of soil, .75 gram of mag- 

 nesium powder, and 10 grams of sodium peroxide (Na 2 2 ) 

 are thoroughly mixed in a closed dry calorimeter bomb. The 

 mixture is then exploded by heating, all of the carbon of the 

 soil being changed to the carbonate form by the reaction. 

 The fused charge is now removed to a flask and by treating 

 with acid, the carbon in the form of carbon dioxide may be 

 driven off into a Parr apparatus and measured under stand- 

 ard conditions of temperature and pressure. 



The amount of inorganic carbonate carbon in the soil must 



1 Wiley, H. W., Official and Provisional Methods of Analysis; U. S. 

 Dept. Agr., Bur. Chem., Bui. 107, p. 234, 1908. 



There are a number of other methods of complete combustion. Very 

 often the combustion is carried on in a current of oxygen over hot 

 cuprous oxide. The organic carbon may thus be very accurately 

 determined. 



