Fire and Heat 133 



the water is driven, off ; then burn and capture its product in a bottle. 

 Add lime water and shake. Does the test show the presence of carbon 

 dioxid? What does this show as to the composition of sugar? Try 

 the same experiment with other familiar substances, as starch, dried 

 vegetables, and fruits. Different substances may be assigned to dif- 

 ferent members of the class for testing and report of results. 

 The following materials may be tested : 



1. Mix a small quantity of baking powder in water, and heat 

 gently. 



Capture and test the gas evolved with limewater. 



2. Mix one ounce of baking soda with one half ounce of cream of 

 tartar ; dissolve the mixture in water, heat gently, and test the gas 

 produced. 



3. Dissolve half a teaspoonful of baking soda in warm water and 

 mix with sour milk ; warm slightly if necessary ; test the gas produced. 

 Other substances, as lemon juice, cider, grape juice, may be similarly 

 mixed with soda and the product tested. 



What do these tests show as to the use of carbon dioxid in the home ? 

 Bread made with baking powder or with yeast in fermentation (page 

 562) owes its lightness and consequent digestibility in large part to 

 carbon dioxid. Fermentation of fruits and fruit juices (pages 565-6) 

 produces carbon dioxid. What use is made of this in preparing 



Amount of carbon dioxid in expired air. Fats, oils, sugars, 

 and starches when consumed as food are changed in the body 

 by union with oxygen into water and carbon dioxid. As long 

 as we live, whether at work or at rest, we are oxidizing carbon 

 in p some form and eliminating carbon dioxid. This means that 

 respiration is necessary both to supply oxygen and to eliminate 

 carbon dioxid. Each individual sets free on the average about 

 two pounds of carbon dioxid a day. The expired air contains 

 approximately 16 per cent oxygen, 4.38 per cent carbon dioxid, 

 and is saturated with water vapor. 



Exercise : Laboratory production of carbon dioxid. A quantity of 

 the substance is needed for study of its behavior and nature. For 

 its production you need apparatus similar to that used in producing 

 hydrogen (page 119). Into the flask put a small quantity of well 

 broken marble, cover with water, close with the stopper, and then pour 

 some hydrochloric acid into the funnel. The gas is quickly gener- 



