136 THE SECRETIONS: 



exhausted with alcohol of O83 ; the alcoholic solution is evapo- 

 rated in the water-bath to a state of dryness, and the residue 

 extracted with anhydrous alcohol. The anhydrous alcohol is 

 evaporated at a very gentle temperature ; the residue is dissolved 

 in a little water, and cold nitric acid (perfectly free from nitrous 

 acid) added to it. The basin is then placed for some hours in 

 snow, or in an artificial freezing mixture. The moist nitrate of 

 urea is collected upon a filter, which is enveloped in the folds 

 of thick blotting-paper, and pressed, as long as fresh blotting- 

 paper continues to absorb moisture. The filter, which, with its 

 contents, is now nearly dry, is exposed to a temperature of from 

 104 to 122, and then quickly weighed. The known weight 

 of the filter is deducted from the weight of the filter and its 

 contents, or, which is better, the nitrate of urea is separated 

 very carefully from the filter, again dried, (since it readily ab- 

 sorbs moisture,) and weighed. From the nitrate we estimate 

 the quantity of urea. 1 (See Vol. I, p. 52.) 



4. Determination of the uric acid. 



Three or four ounces of urine must be treated with three or 

 four drachms of hydrochloric or nitric acid, and allowed to rest 



1 [Marchand has recently examined the combinations of urea and nitric acid, and 

 has arrived at conclusions very different from those of other observers. He used 

 nitrate of urea, which was precipitated from its solution by the addition of nitric acid. 

 It was pressed between folds of bibulous paper, and dried at 240. Its aqueous solu- 

 tion was digested with carbonate of baryta, and the nitrate of baryta decomposed by 

 sulphuric acid. The nitric acid amounted to 60-66 per cent. ; hence the compound 

 consisted of C 2 H 4 N 2 O 2 +2NO 5 -f-HO. A portion was heated to 284; it then yielded 

 a quantity of sulphate of baryta corresponding to 65-72 per cent, of nitric acid. It 

 had probably become anhydrous, for in that state it would contain, by calculation, 

 64-3 per cent. As the neutral nitrate of urea was not obtained in this manner, 

 Marchand dissolved the acid compound in water, and added urea. The dried crys- 

 talline compound now obtained contained 55 per cent, of nitric acid, and was there- 

 fore composed of 2C 2 H 4 N 2 O 2 -I- 3N0 5 -f HO. 



When the liquid from which the above-mentioned salt crystallized was treated 

 with urea and the solution again crystallized, he obtained a compound which lost no 

 weight at 230, and after being exposed for some time to this temperature, con- 

 tained 44-1 per cent, of nitric acid. He concludes by observing that the compound 

 usually separated in analyses does not contain the amount of urea stated in the text, 

 but only 33'89 per cent. Marchand further states that crystallized oxalate of urea 

 contains three atoms of water, two of which (14-6 per cent.) escape at 248, while 

 the third is retained till decomposition ensues. (Journ. fiirprak. Chem. Feb. 1845.] 



