A Method of Teaching Diffusion and Osmosis. 91 
is supported in a vertical position with the smaller end dipping into 
mercury. As evaporation removes water from the wet membrane, water 
from the tube takes its place, and compensation is made for the de- 
creased pressure by a rise of mercury into the lower end of the tube. 
But this is really a demonstration of osmosis. Evaporation in this case 
is merely the diffusion of water and air, and the process takes place 
through a membrane which allows water to pass more readily than air. 
It will be noted that the major flow is from water to air rather than 
from a less dense medium to a more dense. 
It is true that when a solution and the ptre solvent are considered, 
density may sometimes act, both qualitatively and quantitatively, as an 
indicator within certain limits; but we are by no means sure that it 
will work in all cases. It is probably worth mentioning that most of the 
experimental work that has been done with solutions and pure solvents 
have dealt with solutions whose density is greater than that of the pure 
solvent; but some combinations are possible in which the opposite is the 
case, and some interesting results might come from experiments with 
some of these. In the cases where the comparative density rule does 
work in determining the direction of the major flow and the ultimate 
pressure produced, color would probably serve as well for an indicator 
if a colored solute were selected and a sufficiently sensitive method of 
measuring intensity of color were devised; yet no one would think of 
connecting color with the fundamentals of the process. Density has 
about the same relation to the process as has color; chemical affinity is 
the driving force and the only consistent indicator of the qualitative and 
quantitative features of the process. 
It will be seen that much depends upon the nature of the membrane 
through which the diffusion takes place, and to the physical chemist or 
the research student of physiology this is a very important thing. But 
to the student of the elementary aspects of biology, whose welfare is now 
being considered, the mechanism of the membrane is less important if 
he knows that for some reason it tends to be semipermeable. Whether 
the permeability of living membranes can be explained on a purely 
physico-chemical basis, or whether we must still have recourse to a 
vitalistic explanation until physics and chemistry have made sufficient 
progress to include these phenomena, is still an interesting problem of 
research. 
It must be emphasized that, from the biological point of view, the 
effort expended in explaining diffusion and osmosis is lost if we fail to 
make clear their definite application to problems of plant and animal 
life, and many of our text-books fail to do this satisfactorily. Many 
of the texts examined make the assertion or leave the impression that 
' the cell wall is the osmotic membrane concerned, and many leave with 
