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. IMPORTANCE OF WATER IN THE HUMAN BODY [Sidenote: Proportion of water and solids in the human body] Water, which forms about sixty-six per cent of the human body, is by far the most important substance therein. It comprises the major part of the blood serum and every tissue and organ. If a normal human body weighing 150 pounds were put into an oven and thoroughly dried, there would be left only about 50 pounds of solid matter, all the rest being water. The proportion of water in animal and vegetable substances is also very great. As water is also a conspicuous factor in all foods, either in chemical combination, or in solution with other elements mechanically mixed, it is obvious that water is an important factor in food science. USES OF WATER IN THE BODY The uses of water in the body may be roughly grouped into three divisions, as follows: 1 Water in small quantities enters into the actual chemical composition of the body. As we will notice in the discussion of carbohydrates, water combines chemically with cane-sugar when it is digested and transformed into glucose. (See Lesson IV, "Cane-sugar," page 112.) 2 Water forms a portion of the tissues and acts as a solvent in the body-fluids. [Sidenote: What blood carries in solution] In this function the water is not changed chemically, but is only mixed with other substances; thus the blood is in reality water with glucose, peptone, etc., in solution, and carrying along with them red blood-corpuscles and fatty globules. 3 Water is a most important factor in the digestion, and the assimilation of food, and the elimination of waste. [Sidenote: Drinking with meals]