THE ORGANIC CONSTITUENTS OF THE URINE. 241 



amounts sufficient for purposes of study is a very complicated process, 

 and requires facilities which are not generally found in university 

 laboratories, it will suffice at this place to describe a method by 

 which they can be collectively estimated. For a consideration of 

 the chemical properties of the more important members of the group 

 and their isolation from other sources, as also of their relation to 

 uric acid and the nucleinic bases, the reader is referred to other 

 sections. 



Quantitative Estimation. The xanthin bases are best isolated 

 from the urine according to Salkowski's method, which is based 

 upon their precipitation as silver compounds, together with uric acid, 

 the separation of the latter, and the determination of the amount of 

 silver in combination with the bases. To this end, 600 c.c. of 

 urine are first treated, as described in the quantitative estimation 

 of uric acid, according to Ludwig-Salkowski. The final filtrate 

 after removal of the uric acid, together with the washings, is then 

 treated with ammoniacal silver solution and the xanthin bases thus 

 reprecipitated. The precipitate is collected on a small filter, washed 

 with water, dried, and incinerated. The ash is dissolved in dilute 

 nitric acid, and the silver estimated by titrating with a solution of 

 potassium sulphocyanide of known strength, using ammonio-ferric 

 alum as an indicator. As it has been ascertained that in an equal 

 mixture of the silver compounds of xanthin, hypoxanthin, guanin, 

 etc., one atom of silver represents 0.277 gramme of nitrogen or 

 0.7381 gramme of the bases, 1 c.c. of the sulphocyanide solution, 

 that is commonly used in the estimation of the chlorides of the 

 urine (page 219), will correspond to 0.002 gramme of nitrogen or 

 0.00542 gramme of the bases. 



The method of Kriiger and Wulif, which was greatly in vogue a 

 few years ago, has been abandoned, as the values thus obtained were 

 too high. According to this method, the alloxuric bodies, viz., uric 

 acid and xanthin bases, were first estimated by precipitating with 

 copper sulphate and sodium bisulphite and determining the amount 

 of nitrogen in the precipitate. In a second portion of the urine the 

 uric acid was then estimated and the corresponding amount of 

 nitrogen deducted from the first result. The difference was referred 

 to the xanthin bases. 



Oxalic Acid and Oxaluric Acid. 



Of the origin of oxalic acid and oxaluric acid, both of which may 

 oe regarded as normal constituents of the urine, but little is known. 

 The former is supposedly present as a calcium salt, which is held 

 in solution owing to the presence of diacid sodium phosphate, but 

 readily separates out on standing and is then frequently encountered 

 in urinary sediments. Here it generally occurs in the very charac- 

 teristic envelope or dumb-bell forms, and can be readily distin- 

 guished from other constituents by its insolubility in acetic acid, 



16 



