The action produced by the tendency of fluids to mix, or become equally diffused in contact with each other, is known as _osmosis_, a form of molecular attraction allied to that of adhesion. The various physical processes by which the products of digestion are transferred from the digestive canal to the blood may be illustrated in a general way by the following simple experiments. The student must, however, understand that the necessarily crude experiments of the classroom may not conform in certain essentials to these great processes conducted in the living body, which they are intended to illustrate and explain. Illustration: Fig. 62. Experiment 62. _Simple Apparatus for Illustrating Endosmotic Action._ “Remove carefully a circular portion, about an inch in diameter, of the shell from one end of an egg, which may be done without injuring the membranes, by cracking the shell in small pieces, which are picked off with forceps. A small glass tube is then introduced through an opening in the shell and membranes of the other end of the egg, and is secured in a vertical position by wax or plaster of Paris, the tube penetrating the yelk. The egg is then placed in a wine-glass partly filled with water. In the course of a few minutes, the water will have penetrated the exposed membrane, and the yelk will rise in the tube.”—Flint’s _Human Physiology_, page 293. Experiment 63. Stretch a piece of moist bladder across a glass tube,—a common lamp-chimney will do. Into this put a strong saline solution. Now suspend the tube in a wide mouthed vessel of water. After a short time it will be found that a part of the salt solution has passed through into the water, while a larger amount of water has passed into the tube and raised the height of the liquid within it. 161. The Quantity of Food as Affected by Age. The quantity of food required to keep the body in proper condition is modified to a great extent by circumstances. Age, occupation, place of residence, climate, and season, as well as individual conditions of health and disease, are always important factors in the problem. In youth the body is not only growing, but the tissue changes are active. The restless energy and necessary growth at this time of life cannot be maintained without an abundance of wholesome food. This food supply for young people should be ample enough to answer the demands of their keen appetite and vigorous digestion. In adult life, when the processes of digestion and assimilation are active, the amount of food may without harm, be in excess of the actual needs of the body. This is true, however, only so long as active muscular exercise is taken.