A Method of Teaching Diffusion and Osmosis. 89 
some characteristic, presumably size, to make their way through even 
against a greater pressure than they themselves are exerting by their 
motion, while certain others fail to penetrate the membrane, even when 
aided by a difference of pressure. The most obvious objection to any 
consideration on this basis is found in the fact that certain liquids having 
large molecules—as some of the alcohols—are able to pass through cer- 
tain membranes more readily than water. 
Kahlenberg reports numerous experiments, both qualitative and quan- 
titative, to show the fallacy of many of these theories. He makes no 
attempt to deal with the subject on a biological basis, but his results 
bring us near a working explanation for biological purposes. He at- 
tributes osmotic pressure to the relative affinities of two fluids for each 
other and for the separating membrane. 
The report of the recent symposium of the Faraday Society on the 
subject of osmotic pressure was consulted in the hope that it would be 
of material aid; but it was found to contain little that is tangible or 
serviceable from our standpoint. Like Pfeffer’s classic works, it was 
found to contain much about the mathematics of osmotic pressure and 
little about the process of osmosis. 
The text-book definitions and discussions of osmosis and diffusion 
have been based upon one or a mixture of the theories here outlined. The 
prevailing influence of Pfeffer’s work is evident in most of them, and, 
consequently, we see in them much about water and aqueous solutions 
of various densities. Osmotic pressure is too often emphasized at the 
expense of osmosis, and students of biology, who should be trying to 
understand the nature of the process and its relation to the plant, are 
still bored by having to read books and listen to lectures which empha- 
size the stupendous pressures exerted in cells; many a student finishes 
his course with a firmly fixed idea that relative density is the thing that 
makes the gases of the air and the water of the soil enter the plant 
body, and that density alone prevents all the sap of a plant from leaking 
cut throvgh the root hairs. 
By means of a condensation and organization of what is known of 
the processes involved, there has been worked out a set of definitions 
and a method of presenting the subject which is believed to be superior 
to that given in most text-books of botany and plant physiology. 
The first step in the teaching process is the well-known experiment 
of placing a crystal of some colored soluble salt, such as copper sulphate 
er sodium bichromate, in the bottom of a tall glass jar of water and 
watching the color ascend for a few days. The process is named diffu- 
sion, and the student is encouraged to work out his own definition. Dif- 
fusion is seen to consist of the dispersal of the particles of one substance 
among the particles of another substance, without aid from external 
