But it is impressed upon us by our experiences that sunlight is even more potent than moisture to produce chemical change. Photographic processes are dependent upon the power of light to produce chemical changes. The fading of our tapestries and our garments, the tanning of our skins, the development of green material in the leaves of plants, all are evidently the direct result of sunlight. A picture hung on the wall prevents the wall paper behind it from being faded by the light, or it prevents the wood behind it from being turned yellow by the light. Folds in our garments prevent them from being faded all alike. Very many substances to be found in a chemical laboratory, in a drug store, or in a kitchen must be kept in the dark if they are to be guarded against chemical change. No experienced housewife would let a barrel of flour or potatoes sit in the sun, and every housewife knows that the sun is the best agent for bringing about those chemical changes which she desires. Hence she puts her bedding, her milk pans, her bread box, her butter jar, etc., "out to sun." She has open plumbing, that the sun may enter those dark and dirty corners. If you would guard a substance against chemical change, keep it in a dry, dark place. We have come to associate the sun and the weather as disintegrating forces. Hence the south and east sides of the building need most frequent repairs. Every one who has made time exposures in photography knows that the sunlight from the east is, as a rule, two or three times as powerful as that from the west. There is less moisture and dust in the air to screen us from the early morning sun than from the late afternoon sun. When there is enough moisture in the air to make the sun look red, those rays from it which would produce chemical action, called actinic rays, are cut off. Photographic processes are then exceedingly slow. It is like exposing a plate in a dark room behind the ruby glass. But our daily experiences teach us that not only moisture and light but also heat stimulates chemical action. We restrain chemical action by cold when we put things in the ice box. We hasten chemical action by heat when we put things on the stove. Winter restrains all the chemical activities of nature, and summer quickens all the vegetable and mineral kingdoms into chemical activity. If we would preserve a substance from chemical change we must keep it in a _cool, dark, dry_ place. Now those conditions which will favour the chemical activity of a battery cell will enable it to produce electricity, and those conditions which will restrain chemical action will enable us to preserve the cell from running down. But we have lately learned that other forms of radiation besides light and heat exist and aid in chemical action. We may produce radiographs--pictures on photographic plates--without light but with invisible rays, which are akin to light and to electricity. XVI ELECTROCUTION AT MILLVILLE