3. How does potential energy differ from kinetic energy? 4. What kind of energy is possessed by a bent bow? By a revolving wheel? By a coiled spring? By the wind? By gunpowder? 5. How does decomposing water with electricity store energy? 6. Account for the energy possessed by the oxygen of the air and food substances. 7. Trace the energy supply of the body back to the sun. 8. Why must both oxygen and food be introduced into the body in order to supply it with energy? 9. How may overwork and overexercise diminish the energy supply of the body? 10. How may one increase the amount of his energy? PRACTICAL WORK *Suggested Experiments.*—1. The change of kinetic into potential energy may be shown by stretching a piece of rubber, by lifting a weight, and by separating the armature from a magnet. 2. The change of potential into kinetic energy may be shown by letting weights fall to the ground, by releasing the end of a piece of stretched rubber, and by burning substances. 3. The change of one form of kinetic energy to another may be illustrated by rubbing together two pieces of wood until they are heated, by ringing a bell, and by causing motion in air or in water by heating them. If suitable apparatus is at hand, the transformation of electrical energy into heat, light, sound, and mechanical motion can easily be shown. 4. A weight connected by a cord with some small machine and made to run it, will help the pupil to grasp the general principles in the storage of energy through gravity. A vessel of water on a high support from which the water is siphoned on to a small water wheel will serve the same purpose. 5. The storing of energy by chemical means may be illustrated by decomposing potassium chlorate with heat or by decomposing water by means of a current of electricity. 6. Study the transfer of energy from the body to surrounding objects, as in moving substances and lifting weights. Fill a half gallon jar two thirds full of water and carefully take the temperature with a chemical thermometer. Hold the hand in the water for four or five minutes and take the temperature again. Inference. CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION In our study so far we have been concerned mainly with the introduction of materials into the body. We are now to consider the removal of materials from the body. The structures most directly concerned in this work are known as *Glands.*—As generally understood, glands are organs that prepare special liquids in the body and pour them out upon free surfaces. These liquids, known as _secretions_, are used for protecting exposed parts, lubricating surfaces that rub against each other, digesting food, and for other purposes. They differ widely in properties as well as in function, but are all alike in being composed chiefly of water. The water, in addition to being necessary to the work of particular fluids, serves in all cases as a carrier of solid substances which are dissolved in it.