ACIDS AND HYDROXYAC1DS. 



615 



To demonstrate the presence of oxalic acid, and to estimate its amount, a 

 litre of urine should be treated with calcium chloride and ammonia, and 

 afterwards made acid with acetic acid. After twenty-four hours' standing, 

 the crystalline precipitate, which contains uric acid crystals mixed with 

 calcium oxalate, is filtered off and treated with dilute hydrochloric acid. The 

 oxalate dissolves, and is reprecipitated, after filtering off the uric acid, by the 

 addition of ammonia. At a dull red heat it is converted into calcium 

 carbonate, and may be weighed as such. 



FIG. 56. Calcium oxalate. 



Acids and hydroxyacids of the fatty series, with derived 

 substances. Normal urine contains minute quantities of the volatile 

 fatty acids, especially acetic, but also formic, propionic, and butyric 

 acids. 1 They do not, as a rule, amount collectively to more than some 

 50 mgrms. in the day's excretion, and they arise doubtless from the 

 bacterial decomposition of carbohydrates and proteids in the lower 

 bowel. Fatty acids of low atomic weight, such as the above, are less 

 easily oxidised in the organism than are those of greater complexity. 2 



The amount is considerable in the urine of herbivora, and in man it is 

 increased by many diseases, especially by such as lead to increased decomposi- 

 tion in the bowel, or to prolonged constipation. 



When a specimen of urine undergoes ammoniacal fermentation, the 

 volatile acids are increased at the expense of the carbohydrates it contains. 3 



These acids are obtained from the urine by distillation with phosphoric 



^Cf. v. Jaksch, Ztschr. f. physiol. Chem., Strassburg, 1886, Bd. x. S. 536. The 

 earlier literature is here summarised. 



2 C. Schotten, ibid., 1883, Bd. vii. S. 375. 

 3 Salkowski, ibid., 1889, Bd. xiii. S. 264. 



