644 URINE. 



The specific gravity of urine, which is dependent upon the relation 

 existing between the quantity of water secreted and the solid urinary 

 constituents, especially the urea and sodium chloride, may vary con- 

 siderably, but is generally 1.017-1.020. After drinking large quantities 

 of water it may fall to 1.002, while after profuse perspiration or alter 

 drinking very little water it may rise to 1.035-1.040. In new-born 

 infants the specific gravity is low, 1.007-1.005. The determination 

 of the specific gravity is an important means of learning the average 

 amount of solids eliminated from the organism in the urine, and on this 

 account the determination becomes of true value only when at the same 

 time the quantity of urine voided in a given time is determined. The dif- 

 ferent portions of urine voided in the course of the twenty -four hours 

 are collected, mixed together, the total quantity measured, and then the 

 specific gravity taken. 



The determination of the specific gravity is most accurately obtained 

 with the pycnometer. For ordinary cases the specific gravity may be 

 determined with sufficient accuracy by means of areometers. The 

 areometers found in the trade, or urinometers, are graduated from 1.000 

 to 1.040; for exact observations it is better to use two urinometers, one 

 graduated from 1.000 to 1.020, and the other from 1 .020 to 1 .040. 



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 should be about four-fifths 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 sur- 

 face of the liquid and the lower limb of the meniscus coincide ; the read- 

 ing is then made from the point where this curved line coincides with 

 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.- 



Each urinometer is graduated for a certain temperature, which, 

 at least in the case of the better ones, is marked on the instrument. 

 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 subtracted 

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

 graduated for 15 C. shows a specific gravity of 1.017 at 24 C., then the 

 specific gravity at 15 C.= 1.017 + 0.003- 1.020. 



When great exactitude is required, as, for instance, a determina- 

 tion to the fourth decimal point, we make use of a urinometer constructed 



