A STUDY OF THE PROBLEM OF WATER ABSORPTION. 29 
effect of a given solution on the turgor of living cells cannot 
be predicted from a knowledge of the osmotic pressure of that 
solution. The nature of the dissolved substance is quite im- 
portant when we are dealing with living cells. 
Many facts at variance with the osmotic theory of water 
absorption have been explained by saying that the living 
membrane is not quite semi-permeable and that its per- 
meability is different at different times and under different 
circumstances, but there are certain phenomena that are not 
to be explained in this simple manner. When the small in- 
testine of a cat was partially filled with a certain watery 
solution, Cohnheim found that there was a passage of water 
out of the gut. He explained irreciprocal permeability by 
saying that two forces are here concerned; one the force of 
osmotic pressure and the other a force independent of osmotic 
pressure and located in the living cells. 
The secretion from the kidneys contains some substances 
in higher and some in lower concentration than the blood; it 
contains less chlorides and more sulphates and urea than does 
the blood (4). We may explain the passage of urea from 
the blood to the urine, until its concentration in the latter is 
equal to that in the former, as due to diffusion, but it is not 
possible to explain how it becomes more concentrated in the 
urine than in the blood in this way. Blood corpuscles that 
take their nourishment from the serum in which they are 
immersed are rich in potassium phosphate, while the serum 
is poor in this substance, but rich in sodium chloride (15). 
Selective absorption has been explained by stating that the 
substances absorbed are those to which the membrane is per- 
meable, while those that are not absorbed are unable to pass 
through the living septum. This explanation is very satis- 
factory until we come to those cases where the final concentra- 
tion of the diffusing substance becomes greater inside than 
outside of the cell. This has been explained by supposing 
that some of the substance, after passing into the cell, enters 
into combination with some other substance and is thus ren- 
dered incapable of diffusion. But there is little evidence to 
show that this actually takes place. Besides, it has been 
