Carbon dioxid turns limewater milky as it combines with the lime in the limewater to make tiny particles (a precipitate) of limestone. If you pour seltzer water or soda pop into limewater, you get the same milkiness, for the bubbles of carbon dioxid in the charged water act as the carbon dioxid in your breath did. If you pumped enough air through the limewater you would produce some milkiness in it, for there is always some carbon dioxid in the air. The purpose of these experiments is only to give you a general notion of how a chemist analyzes things,--by putting an unknown substance through a series of tests he can tell just what that substance contains; and by accurately weighing and measuring everything he puts in and everything he gets out, he can determine how much of each thing is present in the compound or mixture. To learn to do this accurately takes years of training. But the men who go through this training and analyze substances for us are among the most useful members of the human race. INFERENCE EXERCISE Explain the following: 551. A little soda used in canning an acid fruit will save sugar. 552. The fats you eat are mostly digested in the small intestine, where there is a large excess of alkali. 553. The dissolved food in the liquid part of the blood gets out of the blood vessels and in among the cells of the body, and it is finally taken into the cells through their walls. 554. Ammonia takes the color out of delicate fabrics. 555. Dishes in which cheese has been cooked can be cleaned quickly by boiling vinegar in them. 556. Prepared pancake flour contains baking powder. It keeps indefinitely when dry, but if the box gets wet, it spoils. 557. When water or milk is added to prepared pancake flour to make a batter, bubbles appear all through it. 558. When a roof leaks a _little_, a _large_ spot appears on the ceiling. 559. Gasoline burns quietly enough in a stove, but if a spark gets into a can containing gasoline vapor, there is a violent explosion. 560. Turpentine will remove fresh paint. GENERAL REVIEW INFERENCE EXERCISE Explain the following: 561. We can remove fresh stains by pouring boiling water through them. 562. A ship can be more heavily laden in salt water than in fresh water. 563. Water flies off a wet dog when he shakes himself. 564. In cooking molasses candy, baking soda is often added to make it lighter. 565. An egg will not stand on end. 566. Women who carry bundles on their heads stand up very straight. 567. To get all crayon marks off a blackboard, the janitor uses _vinegar_ in water. 568. Sunlight makes your skin darker. 569. Water puts out a fire. 570. You get a much worse shock from a live wire when your hands are wet than when they are dry. 571. Stone or brick buildings are cool in summer but warm in winter.