SPECIFIC GRAVITY. 149 



ish color, which changes to blood-red on adding an alkali. 

 Santonin gives a yellow; carbolic acid and salol a dark 

 green to black; antipyrin and quinine often darken it. 



The specific gravity of urine varies with the amount 

 of water and dissolved solids. With an increase of the 

 water it approaches 1.000, and becomes greater as the 

 solids increase. Hence it is easy to ascertain the amount 

 of the solids which are present. If the second and third 

 decimal figures of the specific gravity are multiplied by 

 2.33 it will give very nearly the weight of dissolved sub- 

 stances in one thousand parts of urine (grammes per liter). 

 Thus, urine of sp. gr. of 1.021 contains about 49 grammes 

 in a liter. 



The specific gravity varies under normal conditions 

 from 1.002 to 1.030. It is usually between 1.015 and 

 1.025. If sugar is not present the variation in specific 

 gravity is due almost entirely to that of the urea. Clin- 

 ically the specific gravity of urine is determined by an 

 hydrometer, called a urinometer, which consists of a spin- 

 dle weighted so as to float in pure water at the line marked 

 1.000. The specific gravity is indicated by the figures on 

 the spindle at the surface of the liquid. Urinometers 

 should always be tested in pure water and if they are not 

 correct the reading in the urine must be changed to corre- 

 spond with the error. Since the specific gravity varies 

 with the temperature some standard temperature must be 

 adopted. Most instruments are graduated at 60 F. (15.6 

 C.). The urine must be brought to this temperature be- 

 fore testing or, if accuracy is desired, the reading corrected 

 by adding 1 in the fourth decimal place for every degree 

 Fahrenheit above 60 or subtracting 1 for each degree 

 below 60. In order to obtain accurate results the degrees 

 should not be too close together on the spindle. 



