88 Proceedings of Indiana Academy of Science. 
A METHOD OF TEACHING DIFFUSION AND OSMOSIS IN CONNEC- 
TION WITH BIOLOGICAL WORK. 
PAUL WEATHERWAX, Indiana University. 
Osmosis and diffusion are processes met with in many lines of bio- 
logical science, and the problem involved in an attempt to make the 
phenomena clear, especially to an elementary class, is often a difficult 
one. It is not proposed to add here anything new from the physical or 
chemical standpoint to the array of facts already clustered around these 
subjects; it is intended, rather, to give clear, concise definitions of the 
terms and to present a method of teaching the subjects which has been 
found successful in connection with elementary botany. The need of 
such presentation has been felt after the perusal of twenty-five or thirty 
text-books on general botany and plant physiology, most of which are 
noncommittal or inconsistent with facts when discussing these phe- 
nomena. 
When the name osmosis was coined, the process was little understood 
and many irrelevant considerations were connected with it. Since then 
the process has been found to be of much more general occurrence than 
was at first supposed, and our definitions and explanations must be 
generalized to meet our better understanding of it. A brief history of 
the explanations of diffusion and osmosis that have been before biologists 
of the last few years will help to clear up the situation. 
Pfeffer looked for the secret of osmosis in the behavior of solutions 
of cane sugar, potassium nitrate, etc., when separated from the pure 
solvent, usually water, by membranes of various kinds. He did much 
to bring the process to the attention of biologists, but he necessarily saw 
only a limited portion of the field to be covered. 
Van’t Hof attempted to generalize the problem and asserted that in 
dilute solutions the dissolved substance behaved approximately as it 
would in the gaseous form, the temperature and volume being the same 
as that of the solution, and osmotic pressure being substituted for gas 
pressure. But this hypothesis has been found to attempt to explain too 
much even for dilute solutions and is of no avail at all in connéction 
with more concentrated solutions, which are also capable of demonstrat- 
ing osmotic pressure. It also has the defect of not making sufficient 
allowance for membranes that are not perfectly semipermeable. 
The kinetic theory offers an explanation based upon the assumption 
that certain molecules bombarding a membrane are able, because of 
