place in part
according to a general principle, known as _osmosis_, or dialysis.

[Fig. 31]

Fig. 31--*Vessel* with an upright membranous partition for illustrating
osmosis.

*Osmosis.*--The term "osmosis" is used to designate the passage of liquids
through some partition which separates them. Thus, if a vessel with an
upright membranous partition be filled on the one side with pure water and
on the other with water containing salt, an exchange of materials will
take place through the membrane until the same proportion of salt exists
on the two sides (Fig. 31). The cause of osmosis is the motion of the
molecules, or minute particles, that make up the liquid substance. If the
partition were not present, this motion would simply cause a mixing of the
liquids.

*Conditions under which Osmosis occurs.*--Osmosis may be shown by suitable
experiments (see Practical Work) to take place under the following
conditions:

1. The liquids on the two sides of the partition must be _unlike_ either
in density or in composition. Since the effect of the movement is to
reduce the liquids to the same condition, _a difference in density causes
the flow to be greater from the less dense toward the denser liquid_, than
in the opposite direction; while _a difference in composition causes the
substances in solution to move from the place of greater abundance toward
places of less abundance_.

2. The liquids must be capable of wetting, or penetrating, the partition.
If but one of the liquids penetrates the partition, the flow will be in
but one direction.

3. The liquids on the two sides of the partition must readily mix with
each other.

*Osmosis at the Cells.*--In the body osmosis takes place between the blood
and the lymph and between the lymph and the cells, the movements being
through the capillary walls and the membranes inclosing the cells (Fig.
27). Oxygen and food materials, which are found in great abundance in the
blood, are less abundant in the lymph and still less abundant in the
cells. According to the principle of osmosis, the main flow of oxygen and
food is from the capillaries toward the cells. On the other hand, the
wastes are most abundant in the cells where they are formed, less abundant
in the lymph, and least abundant in the blood. Hence the wastes flow from
the cells toward the capillaries.

*Solutions.*--Neither the blood plasma nor the lymp