THE NITROGENOUS EXCRETION. URINE. 379 



in the one instance, but not in the other, as it is practically a 

 non-conductor. This being the case it is evident that valuable 

 information may be obtained by a combination of the two 

 methods, as it is possible to determine the fraction of the 

 osmotic pressure of the urine due to electrolytes and non-elec- 

 trolytes. Such applications are frequently made. 



While the osmotic pressure of the blood is nearly constant, 

 that of the urine is extremely variable. Ordinarily the limits 

 are between A^ 1.3 and 2.0, but after great water 

 consumption on the one hand, or consumption of much nitro- 

 genous food, or salt, without sufficient liquid, on the other, 

 the freezing point of the urine may vary from A = o. i 

 to 3.0. That is, the urine concentration may range from 

 one-fifth that of the blood to over five times the blood concen- 

 tration, expressed in active molecules. It will be remembered 

 that a freezing point depression of i C. corresponds to an 

 osmotic pressure of 12.1 atmospheres. 



The applications of this cryoscopic method to urine are 

 mainly in the direction of diagnosis. Since it is possible to 

 collect the urine from each kidney separately, by ureter cath- 

 eterization or equivalent means, a test of the two portions will 

 disclose any difference in the performance of the two organs. 

 Normally, the secretions from the two kidneys, for a given 

 time, should be the same. A freezing point determination is 

 easily made and will show if one kidney is doing more work 

 than the other. By including a conductivity test it may be 

 found that the difficulty in excretion is more pronounced for 

 one class of substances than for another. 



