ORGANIC MATTER AND NITROGEN 93 



with the same sample immediately after the determina- 

 tion of carbonates. 



A small reflux condenser and a tube of heated copper 

 oxide should be interposed between the combustion 

 flask and the first absorption tube. The CuO tube is 

 used to complete the combustion of some of the products 

 formed ; 5-10 grams of powdered potassium bichromate 

 are added to the combustion flask A (Fig. 16) ; a 

 current of air is drawn through the apparatus by means 

 of the aspirator for a few minutes, and concentrated 

 sulphuric acid then added by means of the dropping 

 funnel. The contents of the flask are then heated 

 until the acid begins to give off fumes, and 

 the temperature maintained at this stage for about 

 ten minutes, or until the combustion of the organic 

 matter is complete. 



In calculating the percentage of organic matter it 

 is assumed that its carbon content is 55 per cent, so 

 that the weight of COg formed multiplied by the factor 

 0-496 gives the organic matter. 



73. Nitrogen. 



The nitrogen is estimated by Kjeldahl's method 

 (see 7). Ten to 20 grams of the powdered soil 

 are used, and no special precautions need be taken for 

 the nitrates existing in the soil, since their quantity is 

 so small as to be almost negligible beside the total 

 nitrogen present. 



In the subsequent estimation of the ammonia formed 

 it is desirable to employ steam distillation, owing to the 

 amount of insoluble matter present, which prevents 

 regular ebullition. Instead of steam distilling, the 

 sulphuric acid digest may be diluted with water, filtered 

 through a large Buchner funnel, and an aliquot portion 

 of the filtrate distilled with NaOH in the usual manner. 



