336 PHYSIOLOGICAL CHEMISTRY. 



To determine the specific gravity of urine, if necessary filter 

 the urine, or if it contains a urate sediment, first dissolve it by gentle 

 heat, then pour the clear urine into a dry cylinder, avoiding the 

 formation of froth. Air-bubbles or froth, when present, must be 

 removed with a glass rod or filter-paper. The cylinder, which must 

 be about full, must be wide enough to allow the urinometer to 

 swim freely in the liquid without touching the sides. The cylinder 

 and urinometer should both be dry or previously washed with the 

 urine. On reading, the eye is brought on a level with the lower 

 meniscus which occurs when the surface ,of the liquid and the 

 lower limb of the meniscus coincide ; the* 'reading is then made 

 from the point where this curved line cuts the scale of the 

 urinometer. If the eye is not in the same horizontal plane with the 

 convex line of the meniscus, but is too high or too low, the surface 

 of the liquid assumes the shape of an ellipse, and the reading in 

 this position is incorrect. Before reading press the urinometer 

 gently down into the liquid and then allow it to rise, and wait until 

 it is at rest. 



If the quantity of urine at disposal is not sufficient to fill the 

 cylinder to the proper height it may be diluted, according to circum- 

 stances, with an equal volume or several volumes of water. This does 

 not give quite accurate results, and with small quantities of urine 

 it is best to determine the specific gravity by means of the pyk- 

 nometer. 



Each urinometer is graduated for a certain temperature, which 

 is marked on the instrument, or at least on the best. If the urine 

 is not at the proper temperature, the following corrections must be 

 made : For every three degrees above the normal temperature one 

 unit of the last order is added to the reading, and for every three 

 degrees below the normal temperature one unit (as above) is sub- 

 tracted from the specific gravity observed. For example, when a 

 urinometer graduated for -f 15 C. shows a specific gravity of 1.017 

 at -f 24 C., then the specific gravity at -f 15 0. = 1.017 + 0.003 

 = 1.020. 



II. Organic Physiological Constituents of the Urine. 



t 

 Urea, Ur, which is ordinarily considered as carbamid, CO(NH 2 ) 2 , 



may be synthetically prepared in several different ways, namely, 

 from carbonyl-chloride, or carbonic-acid ethyl-ether and ammonia, 

 COC1 2 + 2NH 3 = CO(NH 2 ) 2 + 2HC1, or (C 2 H 5 ) 2 .0 2 .CO + 2NH 3 = 

 2(C 2 H 6 .OH) -f CO(NH 2 ) 2 ; by the metameric decomposition of 

 ammonium cyanate, (NH,).O.CN = CO(NH 2 ) 2 (WOHLER, 1828); 

 and in many other ways. It is also formed by the decomposition 



