218 § 113. FERTILIZERS. 



when this instrument is used, all foam must bo carefully 

 removed from the surface of the liquid, by filter paper. 



A difference of 4° C. in the temperature of the liquid 

 usually makes a difference of about 1° in the reading of 

 the urometer. 



The specific gravity of urine ranges between 1.01 and 

 1.04. 



h. Total amount of dry substance in solution. — De- 

 termine this by evaporating a weighed quantity in a cur- 

 rent of dry hydrogen in such a manner as to estimate the 

 ammonia that is expelled at the same time (§ 90, d). Take 

 4-6 c.c. of the urine, accurately weighed. The evapora- 

 tion to dryness is completed in 4-5 hours. 



In human urine, that has an acid reaction due to acid 

 sodic phosphate, the ammonia may be assumed to have 

 been derived from urea, and by multiplying the amount 

 of it by 1.765, the corresponding amount of urea will be 

 obtained. But in the urine of herbivorous animals the 

 ammonia resulting from this decomposition must be esti- 

 mated by the difference between the ammonia set free on 

 evaporation to dryness, and that found in the urine by di- 

 rect determination. Generally, however, these quantities 

 of ammonia are very small, and can be left out of consid- 

 eration. 



The non-TOlatile matter in this residue left on evapora- 

 tion is determined by evaporating a fresh quantity of 

 100 c.c. of the urine in a platinum dish, and igniting the 

 residue (§ 91, 1) ; determine carbonic acid in the ash in 

 the usual manner. 



c. Carbonic acid (free and combined). — Determine this 

 in two portions of 100 c.c. of the fresh urine. To one 

 portion add baric chloride containing ammonia in excess, 

 and to the other baric chloride alone ; heat both mixtures 

 nearly to boiling, collect the precipitates on dried and 

 weighed filters, wash them, and dry them at 100°, weigh, 



