262 PHYSIOLOGICAL CHEMISTRY. 



Up to the present time the determination of the electrical conduc- 

 tivity of any of the fluids of the body has been put to very slight 

 clinical use. Experience may show the conductivity value to be a 

 more important aid to diagnosis than it is now considered, particu- 

 larly if it is taken in connection with the determination of the 

 freezing-point. By a combination of these two methods the por- 

 tion of the osmotic pressure due respectively to electrolytes and 

 non-electrolytes may be determined. For a discussion of electrical 

 conductivity, the method by which it is determined and the princi- 

 ples involved consult standard works on physical or electrochemis- 

 try. 



Collection of the Urine Sample. If any dependable data are 

 desired regarding the quantitative composition of the urine the ex- 

 amination of the mixed excretion for twenty-four hours is abso- 

 lutely necessary. In collecting the urine the bladder may be emp- 

 tied at a given hour, say 8 A. M., the urine discarded and all the 

 urine from that hour up to and including that passed the next 

 day at 8 A. M., saved, thoroughly mixed and a sample taken for 

 analysis. Powdered thymol, 



CH 3 



'OH 

 CH 3 CH-CH,, 



is a very satisfactory preservative since the excess may be re- 

 moved by filtration, if desired, and any small amount which may 

 go into solution will have no appreciable influence upon the de- 

 termination of any of the urinary constituents. It has no reducing 

 j>ower and so may safely be used to preserve diabetic urines. To 

 insure the preservation of the mixed urine of the twenty-four hour 

 period it is advisable to place a small amount of the thymol powder 

 in the urine receptacle before the first fraction of urine is voided. 

 In order to further insure the preservation of the urine the cleaned 

 and dried urine receptacle may be rinsed with an alcoholic solution 

 of thymol and subsequently thoroughly dried before introducing the 

 urine. 



Toluene is also used for the preservation of urine. 



In certain pathological conditions it is desirable to collect the 

 urine passed during the day separately from that passed during the 



