380 PHYSIOLOGY CHAP. 



According to Hober (1903) the physico-chemical concept must 

 be distinguished from the purely chemical concept of acidity. 

 While physico-chemical methods only measure the concentration 

 of the hydrogen ions actually present, i.e. dissociated from the 

 molecules of the acid, the titrimetric (chemical) method measures 

 not only these, but also the hydrogen ions which at the outset of 

 the experiment are still bound up in the acid molecules, and only 

 become dissociated later on ; that is, it measures both the actual 

 and the potential hydrogen ions (Ostwald). In the case of urine 

 also we must distinguish between these two acidities, the ion- 

 acidity and the titration-acidity, which are, not only theoretically, 

 but also (as appears from certain experiments of Hober) practically, 

 two distinct magnitudes, varying independently of each other, and 

 each having its special significance, which must not be neglected 

 in judging of the secretory state of the kidneys, or of general 

 metabolism. 



After a longer or shorter time (two or three days) the urine 

 undergoes ammoniacal fermentation, and the reaction becomes 

 alkaline. This is due to the action of organised ferments (usually 

 Micrococcus ureae and Bacterium ureae) which are able to convert 

 urea into ammonium carbonate. This fermentation constantly 

 occurs in urine exposed to non-sterilised air, but it may also take 

 place in the urine within the bladder, if non-sterilised surgical 

 instruments are introduced The urine undergoing alkaline 

 ammoniacal fermentation gives off a foetid ammoniacal odour and 

 becomes turbid, owing to the formation of a sediment which 

 consists principally of crystals of ammonium-magnesium phosphate 

 and ammonium urate. According to Musculus the urea micro- 

 organisms secrete a diastatic enzyme which is the direct agent of the 

 cleavage, and which can be isolated by precipitation with alcohol. 



The molecular concentration of human urine, as determined 

 by the cryoscopic method, i.e. by the lowering of the freezing-point, 

 differs considerably from that of blood serum. While in the 

 latter A = 0*55 C., in the urine it may reach the value of A = 1-85 C. 

 (Winter), or even of A = 2'3 C. (Dreser). This higher molecular 

 concentration of urine as compared with blood plasma is not, 

 however, constant ; even in man it occasionally falls below that of 

 the blood, and may reach a minimum of A = 0*4 C. (Dreser). We 

 shall discuss the value of these data for the theory of the mechanism 

 of urinary secretion at a later point. 



H. Frenkel and J. Cluzet (1901) also drew attention to another 

 physico-chemical property of urine : its surface tension. Since 

 the surface tension of a solution depends not only (as in molecular 

 concentration) on the number but also on the chemical nature of 

 the dissolved molecules, it is clear that, under given conditions, 

 the changes in surface tension may afford indications as to the 

 chemical constitution of a solution. The surface tension of urine, 



