[Sidenote: Relation of solution to assimilation]
Solution is very important in the study of foods and human nutrition.
Only substances which can be dissolved can be assimilated. Many
substances which will not dissolve in pure water will dissolve in water
which contains something else in solution. The blood is water containing
many things in solution. The salts of the blood keep the other food
elements in solution, many of which would not dissolve if the blood
did not contain these salts. The chief work of the digestive juices
is to reduce foods to a soluble form so that they can be taken into
the circulation by absorption; otherwise they would pass through the
alimentary canal practically unchanged.
[Sidenote: Milk as an example of both "Solution" and "Mixture"]
We must learn to distinguish carefully between chemical solution and
merely mixing things with water. A good example is milk. In addition to
water, milk contains principally fat, sugar, and casein. The sugar is
truly dissolved in the water. The fat and the casein are fine particles
held in suspension. If the milk stands for a while, the fat particles
rise to the top as cream. If it stands long enough, the casein particles
adhere to each other and settle to the bottom, leaving the water with
the dissolved sugar or whey in the middle.