It is significant that the "dry" battery cell must be moist in order that chemical action may go on in it. Compare with that fact several others that we may learn from observation, for example: Baking powders must be kept dry to retain their strength. That is, if they get moist chemical action will begin in them, and the gas which is one of the products of this chemical action will pass off. Now it is the sole function of baking powders to produce gas within the dough, and if the gas has wholly or partially escaped they will fail to make the bread stuff "light." The same reasons obtain for keeping seidlitz powders and other effervescing salts, such as vichy and kissingen, dry. It is to prevent the chemical action which is provoked by the presence of water. The same thing is true of the rusting of iron, and the various kinds of corrosion of metals. We may prevent such action indefinitely by keeping them dry. Berries, fruits, meats, milk, eggs, grain--all kinds of foods--are preserved from spoiling--from chemical changes--by drying them and keeping them dry. The same thing is true of wood, paper, cloth, etc. A wooden fence post may last from five to ten years. A fence rail, being less exposed to moisture, may last two or three times as long. The interior wood of a house may last a century or two, while the exterior wood, being exposed to the weather, may require repairs very frequently. Shingles on the roof do not last as long as shingles on the side of the house. Those on a steep roof last longer than those on a flatter one. A pitch of at least forty-five degrees in a roof is desirable to keep it dry. The north and west sides of a house being least exposed to storm in this climate last the longer. Precious books, records, deeds, wills, etc., on paper must be preserved in dry air. A sail will keep strong and white if kept dry.