542 THE SECRETIONS. 



In these cases, cages are employed, whose walls are made 

 partly of sheet iron or zinc, and partly of wire netting. The 

 floor of the cage should be made of thick glass rods (about 

 four-tenths of an inch in diameter), placed very closely together. 

 These rods are so arranged that the spaces between them will 

 allow urine to trickle away, whilst the solid excreta are re- 

 tained. 



The glass rods are firmly inserted into the wooden base of 

 the cage ; this is furnished with a drawer, into which is accu- 

 rately fitted a flat glass or porcelain dish, such as is used by 

 photographers in washing photographs. The dish is perforated 

 by a hole, in which a tube (preferably of glass) is accurately 

 fitted, and leads to the collecting vessels outside. 



If can: be taken to wash the glass-rod bottom of the cage 

 and the collecting-glass dish placed beneath it, the urine may 

 be collected in a state of great purity. 



** 191. Determination of the specific gravity of 

 Urine. This may be effected in either of the two ways de- 

 scribed in A pp. 216, for the determination of the specific 

 gravity of thuds, viz., by means of a hydrometer or with the 

 specific gravity bottle. 



The hydrometer employed for taking the specific gravity of 

 urine is called a urinometer; in this country its stem is usually 

 divided so as to indicate densities ranging from 1000 to 1060 

 (water being 1000); it is preferable to use two urinometers : 

 one indicating densities from 1000 to 1030, the other from 

 1030 to 1060. The length of the stem being the same as that 

 of the ordinary instruments, the accuracy of the reading will 

 be much increased. Before using a urinometer, its accuracy 

 should be checked by immersing it in fluids of known specific 

 gravity. If the specific gravity of three samples of urine be 

 Mcciu ately taken with the bottle, data are obtained for checking 

 the accuracy of the urinometer. 



Although, under certain circumstances, important informa- 

 tion may be obtained by a determination of the specific gravity 

 of an isolated sample of urine, generally it is only when the 

 specific gravity of a sample of the mixed and measured urine 

 of the twenty-four hours is ascertained, that we learn much 

 from the experiment. 



A knowledge of the specific gravity enables one to form a 

 near approximation to the total quantity of solid matter ex- 

 creted by the kidneys in a given time. 



It has been empirically determined that the specific gravity 

 of urine generally bears a close relation to the solid matters 



1 which ii contains in solution. ^Sir Robert Christison pointed 

 out, many years ago, that if the whole numbers which express 

 the difference between the density of a sample of urine and the 

 density of water (expressed as 1000) be multiplied by the factor 



