ITS QUANTITATIVE ANALYSIS. 443 



the urea and the other constituents of the urine may be quantita- 

 tively determined. 



I regard the following as the only correct method of ascertain- 

 ing the quantity of the solid residue : small quantities of the fluid 

 (see p. 2) should be placed in a vacuum with sulphuric acid, 

 care being taken in exhausting the air that the urine does not boil 

 and is not allowed to bubble ; from ten to fifteen grammes may in 

 this manner be very readily evaporated in a shallow basin. The 

 application of heat, as, for instance, of the air-bath, is, however, even 

 more objectionable for drying the residue than for evaporating the 

 urine ; the urinary residue commonly forms a tough, extract-like, 

 and very hygroscopic mass, and hence several precautions are here 

 required, besides those which were noticed in vol. i., p. 340, for 

 drying animal substances. In the first place, the urinary residue 

 ought only to be dried in a vacuum at a mean temperature, because 

 it invariably becomes decomposed on the application of heat, 

 although in some cases more than others. When the urine is 

 heated on an air-bath, as, for instance, at about 90 or 100 C., it 

 always becomes enveloped in an atmosphere which contains am- 

 monia, but which regains its ordinary condition when the air has 

 been frequently changed, and a corresponding loss of weight may be 

 observed on each repeated weighing. The process of weighing is 

 here attended with the greatest difficulties, since the urinary residue 

 is almost more hygroscopical than that of the bile ; and on this 

 account the precautions there indicated, or some other means, must 

 be employed to hinder the increase of weight which may be induced 

 by the attraction of water during weighing. It is of little use to 

 place sulphuric acid or chloride of calcium within the case of 

 the balance ; but, instead of the shallow evaporating basin, a wide 

 vessel may be employed, having a ground-glass stopper or glass 

 plate, which, immediately after the drying and before the weighing, 

 should be attached to the evaporating vessel. We certainly cannot 

 hope to effect a perfect drying of the urinary residue without tha 

 application of artificial heat; but we may, at all events, obtain- 

 results by this method which admit of being compared with 

 one another, and which would be unattainable if we employed 

 heat. 



Alkaline urine that is to say, urine containing carbonate of 

 ammonia is very ill adapted for quantitative analysis. If, there- 

 fore, it is deemed necessary to analyse it, it must be neutralised 

 before it is evaporated, or, what is still better, acidified, by means 



