OSMOTIC PRESSURE OF URINE. 651 
of osmotic pressures by Pfeffer's method a determination of the freezing- 
point of the solution. As van b' Hoff has shown, if a is the depression of 
freezing-point and T the absolute freezing-point of the solvent {i.e., for 
water, 273 . and w the latent heat of fusion of ice = 79 caL), then the 
work A can be reckoned from the following formula: — 
dA= ' xdv. 
Thus for 1 per cent, solution of cane-sugar (a = *055) 
7 . *055*79 , 
dA= xdv. 
Eld 
To reduce this result to gravitation units we must multiply by 424, 
and we thus find that to separate the volume dv of pure water as ice 
from 1 per cent, cane-sugar solution, a force is necessary equal to the 
'055 x 79 x 424 
pressure of a column of water of t^-> — ~ metres in height. 
A depression of a = — 1 " corresponds therefore to an osmotic 
79 x 4.04 
pressure of " .,-._,"" - ; that is to say, to 122*7 metres of water. "We 
have therefore to multiply a by 1227, in order to obtain the osmotic 
pressure in metres of water of any solution. 
Xow it is evident that, according to Ludwig's hypothesis, the osmotic 
pressure of the urine might attain to but could never exceed that of 
the blood plasma. On estimating the osmotic pressures of these two 
fluids, we find that, under normal circumstances, the osmotic pressure of 
the urine is considerably greater than that of the blood, so that work 
must have been done in the separation of this concentrated fluid from 
the more dilute blood plasma. Dreser l has estimated this work in a 
case in which, during one night, 200 c.c, of urine were secreted with 
a = 2'. -> .. This was separated by the kidneys from the blood with 
a = -5G. In the production of this fluid Dreser finds that the work 
done by the kidney amounts to 37*037 kilogramme metres. This figure 
by no means represents the maximum force which can be exerted by the 
kidney. From a cat which had been deprived of water for three days, 
Dreser drew off some urine with a = 472 C. The blood at the same 
time had an osmotic pressure corresponding to A = 0'6b' C. These 
differences in freezing point denote an osmotic difference of 498 metres 
water, i.e. a pressure of 49,800 grrns. per square centimetre. If this 
work of concentration were carried out by the cells of the tubules, 
these results would imply that these cells can exert a force six times 
greater than the absolute force of human muscle (8000 grms. per square 
centimetre). 
Assuming that the whole work of the tubules is confined to the act 
of concentration, Dreser seeks, moreover, to demonstrate that the 
glomerular secretion also involves the activity of living cells. Since 
the blood pressure of 200 mm. Hg = 2*72 metres water, and a 1 o- C. = 
122 - 7 metres water, the highest possible difference between dilute urine 
and blood, assuming that no concentration had taken place, could only 
be a = 0-~"022 C. Dreser finds, however, that after beer drinking, ai id 
1 Arch. f. exper. Path. u. PharmakoL, Leipzig, 1892, Bd. xxix. S. 307. 
