CROP CONSTITUENTS AND DETERMINATION 109 



94. Ash. — By burning the material and continuing to 



heat it until all the organic matter has been destroyed, the 

 weighed residue will constitute the ash or the mineral con- 

 stituents of the plant substance. These mineral constituents 

 will not occur in the ash in the same form as in the plant, 

 for the bases which were originally united with organic acids 

 will be present in the ash as sulphates, phosphates, silicates, 

 and carbonates. Sulphur and phosphorus in proteins will 

 occur here as sulphates and phosphates of the bases. Silica, 

 which may or may not have been present in the plant in 

 organic combination, will occur in the ash as silicates. 

 The excess of inorganic basic elements over inorganic acid 

 elements will be present in the ash as carbonates. The ash 

 residue from the crude fiber determination will not serve 

 in this determination, since much of the inorganic material 

 has been removed bv the treatment with acid and alkali. 



95. Crude Protein. — By using a well known method for the 

 determination of total nitrogen (Section 84) and multiplying 

 the result by 6.25 the percentage of crude protein is obtained. 

 This is based on the assumption that proteins contain on the 

 average 16 per cent, of nitrogen. The assumption is only 

 approximately correct. This method, however, does not 

 take into consideration the presence of nitrogen in other 

 than protein form. In some cases there is a considerable 

 amount of this non-protein nitrogen which is of importance 

 in feeding. Consequently it is customary in most cases to 

 determine the so-called "Albuminoid Nitrogen" or protein 

 nitrogen and calculate this to proteins. The method is 

 based on the fact that proteins are precipitated by copper 

 hydroxide, whereas amides, ammonia, and nitrates are not. 

 After washing the precipitate of proteins and copper 

 hydroxide the total nitrogen is determined and the result 

 multiplied by 6.25 as before. The difference between the 

 total nitrogen and protein nitrogen, is multiplied by 4.7 

 and called amides, on the assumption that amides are 

 asparagine and contain 21.2 per cent, of nitrogen — a very 

 broad generalization. 



96. Nitrogen-free Extract, or Digestible Carbohydrates.— 

 This is merely the difference between 100 per cent, and the 



