is a most important physiological factor. Our schools are as apt in frightening it away as our churches are in inviting it. Sleep is the opportunity for repair. During its hours of quiet rest, when muscular and nervous effort are stilled, millions of microscopic cells are busy in the penetralia of the organism, like coral insects in the depths of the sea, repairing the waste which the day's study and work have caused. Dr. B.W. Richardson of London, one of the most ingenious and accomplished physiologists of the present day, describes the labor of sleep in the following language: "During this period of natural sleep, the most important changes of nutrition are in progress: the body is renovating, and, if young, is actually growing. If the body be properly covered, the animal heat is being conserved, and laid up for expenditure during the waking hours that are to follow; the respiration is reduced, the inspirations being lessened in the proportion of six to seven, as compared with the number made when the body is awake; the action of the heart is reduced; the voluntary muscles, relieved of all fatigue, and with the extensors more relaxed than the flexors, are undergoing repair of structure, and recruiting their excitability; and the voluntary nervous system, dead for the time to the external vibration, or, as the older men called it, 'stimulus' from without, is also undergoing rest and repair, so that, when it comes again into work, it may receive better the impressions it may have to gather up, and influence more effectively the muscles it may be called upon to animate, direct, control."[11] An American observer and physiologist, Dr. William A. Hammond, confirms the views of his English colleague. He tells us that "the state of general repose which accompanies sleep is of especial value to the organism, in allowing the nutrition of the nervous tissue to go on at a greater rate than its destructive metamorphosis." In another place he adds, "For the brain, there is no rest except during sleep." And, again, he says, "The more active the mind, the greater the necessity for sleep; just as with a steamer, the greater the number of revolutions its engine makes, the more imperative is the demand for fuel."[12] These statements justify and explain the instinctive demand for sleep. They also show why it is that infants require more sleep than children, and children than middle-age folk, and middle-age folk than old people. Infants must have sleep for repair and rapid growth; children, for repair and moderate growth; middle-age folk, for repair without growth; and old people, only for the minimum of repair. Girls, between the ages of fourteen and eighteen, must have sleep, not only for repair and growth, like boys, but for the additional task of constructing, or, more properly speaking, of developing and perfecting then, a reproductive system,--the engine within an engine. The bearing of this physiological fact upon education is obvious. Work of the