URINE. 131 



is then precipitated. Or, after the sulphuric acid has been pre- 

 cipitated with a baryta- salt, ammonia may be added to the fil- 

 tered fluid, upon which phosphate of baryta will be precipitated. 



12. Silicic acid. The only method of detecting the existence 

 of this acid is by evaporating the urine, incinerating the residue, 

 dissolving it in water, treating the insoluble portion with hydro- 

 chloric acid, and incinerating the residue that still remains. 

 In this way we obtain the silicic acid. 



13. Hydrofluoric acid, or fluoride of calcium, occurs in very 

 minute traces, and can only be recognized by operating on a 

 very large quantity of urine. The precipitate thrown down by 

 ammonia must be collected, washed, placed in a platinum or 

 porcelain crucible, treated with sulphuric acid, and its action 

 on glass observed. 



14. Soda. This base is contained in large quantity in the 

 urine, both as chloride of sodium and in combination with acids. 

 Chloride of sodium and the lactates can be removed from the ash 

 of the residue of the urine by spirit ; the solution must then 

 be evaporated, and on submitting the salt to the action of the 

 blowpipe, the intensely yellow flame which indicates the presence 

 of soda is perceptible. The presence of soda may be also shown 

 in other ways. Upon treating urine evaporated to the thick- 

 ness of a syrup with alcohol, the chloride of sodium will dis- 

 solve, and by spontaneous evaporation will in part crystallize 

 in the form of octohedra, which are partially perceptible even to 

 the naked eye. These consist of a combination of urea and 

 common salt. Fig. 24 exhibits such octohedra, obtained from 

 evaporated and filtered urine. When the evaporation is con- 

 ducted rapidly, these forms are replaced by a series of crystals 

 shaped like crosslets and daggers, and usually crenate at the 

 margin. See fig. 24.*. If urine is allowed to stand in a 

 a shallow vessel, until it has become decomposed, and a portion 

 of the urine has evaporated, a crystallized salt will be found, 

 in which prisms and octohedra can be recognized both with the 

 naked eye and with the microscope. The rectangular prisms, 

 fig. 25, exhibit the combination of phosphate of soda with phos- 

 phate of ammonia (sal microcosmicum) . 



