35 Common Science 



is made of, and the setting free of the carbon dioxid 

 (CO2) that is in one of them. Try this experiment : 



Experiment 104. Put half a teaspoonful of baking powder 

 in the bottom of a cup and add a little water. What hap- 

 pens? 



The chemical action which takes place in the -baking 

 powder and releases the gas in bubbles the gas is 

 carbon dioxid (CC^) will not take place while the 

 baking powder is dry; but when it is dissolved, the 

 chemical change takes place in the solution. 



If you ate your food entirely dry, you would have a 

 hard time digesting it ; and this would be for the same 

 reason that baking powder will not work without water. 

 Perhaps you can drink too much water with a meal and 

 dilute the digestive juices too much; certainly you 

 should not use water to wash down your food and take 

 the place of the saliva, for the saliva is important in 

 the digestion of starch. But you need also partly to 

 dissolve the food to have it digest well. Crackers and 

 milk are usually more easily digested than are plain 

 crackers, for the milk partly dissolves the crackers, and 

 drinking one or two glasses of water with a meal hastens 

 the digestion of the food. 



Application 78. Explain why paint preserves wood : why 

 iron will rust more quickly in a wet place than it will either 

 under water or in a dry place ; why silver salts must be dis- 

 solved in order to plate a spoon by electricity. 



Inference Exercise 

 Explain the following : 



501. There is dew on the grass early in the morning. 



502. Cold cream makes your hands and face soft. 



