384 PHYSIOLOGICAL CHEMISTRY 



termined as K = 0.012. Up to the present time the determination of 

 the electrical conductivity 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, 

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

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

 of the osmotic pressure due respectively to electrolytes and non- 

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

 ductivity, the method by which it is determined, and the principles 

 involved consult standard works on physical or electro-chemistry. 



Collection and Preservation of the Urine Sample. If any depend- 

 able data are desired regarding the quantitative composition of the urine 

 the examination of the mixed excretion for twenty-four hours is ab- 

 solutely necessary. In collecting the urine the bladder may be emptied 

 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. Until 

 recently it was believed that powdered thymol (para-isopropylmetacresol) 



CH 3 



'OH 



CH 3 CH CH 3 , 



was a very satisfactory preservative since the excess might be removed 

 by nitration, if desired, and it was believed that the small amount which 

 went into solution would have no appreciable influence upon the deter- 

 mination of any of the urinary constituents. It appears however that 

 thymol is not such a satisfactory urinary preservative as was believed. 

 Evidence has been presented showing it to be unsatisfactory for the 

 preservation of urines which contain sugar, acetone or diacetic acid, 

 and in which it is desired to estimate the quantitative content of these 

 constituents. Claim has also been made that thymol is not a satis- 

 factory preservative for urines that are to be examined quantitatively for 

 phosphates or magnesium. Thymol being a phenol will cause an in- 

 accuracy when phenols are being determined quantitatively. Urines 

 preserved by thymol will also give a confusing white ring when sub- 

 jected to the nitric acid test for albumin (see Chapter XXIV). 



Toluene is a very satisfactory preservative for urine. In using this 

 preservative simply overlay the urine with the toluene. Rosenbloom 1 



1 Rosenbloom: New York Medical Journal, 99, 735, 1914. 



