90 Proceedings of Indiana Academy of Science. 
sources. It is also pointed out that an energy transformation has taken 
place in the migration of the particles of the salt upward through the 
water; the source of this energy is in the chemical affinity between the 
salt and the water. 
The next step is to demonstrate the existence of this energy in its 
static form. The ordinary osmosis experiment, in which a parchment 
diffusion shell filled with a thick sugar syrup is immersed in a jar of 
water, is set up. When the difference in level has been established, the 
process that has taken place is named osmosis, and a definition of osmosis 
is in order. It is seen that the syrup and water have tended to diffuse 
into each other through the membrane, but the water has been more 
successful than the syrup in getting through; in other words, the mem- 
brane is more permeable to the water than to the syrup. Osmosis may 
be defined, then, as the diffusion of two fluids through a membrane that 
tends to be semipermeable. 
It is necessary to speak of two fluids, rather than two liquids, as 
many texts do, because the process is characteristic of gases also under 
proper conditions, and this phase of the process is a very important one 
in a biological connection. It is not deemed wise to complicate the defi- 
nition or the explanation with reference to the few cases in which 
osmosis has been shown to take place between a solid and a liquid. 
It seldom, if ever, happens in practical work that the membrane is 
erfectly semipermeable. If we were defining the ideal process, it might 
be well to speak of an ideally semipermeable membrane; but, after all, 
our aim is to make the situation clear to a student of biology, and he 
seldom has to deal with questions of complete semipermeability. To 
define osmosis as merely diffusion through a membrane, as some texts 
do, is insufficient, for a membrane equally permeable to both fluids would 
not demonstrate osmosis. 
It will be noted that the student is not confused by the introduction 
of relative density into the definition here proposed. The density idea 
is a remnant of the day when the full application of the process was 
not understood—when combinations of solution and pure solvent, sepa- 
rated by a suitable membrane, constituted practically the only system 
that had been thoroughly investigated. Now osmosis is known to take 
place between numerous combinations of pure substances, and numerous 
examples are afforded where the old rule of density works the wrong 
way. 
The reference to density is especially deceptive in certain cases where 
one of the diffusing substances is a gas. An interesting illustration of 
this is afforded by an experiment often made to show the “lifting power 
of evaporation.” A thistle tube filled with water has a piece of wet 
bladder tied over the larger end in contact with the water, and the tube 
