124 Introduction to the Study of Science 



other products remain invisible in spite of the cooling and may 

 be made visible by some other condition. Now scientists have 

 discovered that lime water renders visible another of the prod- 

 ucts of combustion. Pour a small quantity of lime water into 

 the bottle and mix the contents by shaking. The limewater 

 turns a milky white color. This proves that some product of 

 combustion is present which is not made visible by cooling. 

 That it is not the water vapor which has this effect, may be 

 proved by mixing some clean water with limewater. 



Exercise. The problem may be simplified considerably by burn- 

 ing some pure carbon or charcoal in oxygen. Into a bottle full of 

 oxygen first pour about two ounces of limewater, close the bottle with 

 a stopper, and shake vigorously. Oxygen produces no change in the 

 color of the water. Now introduce a lump of charcoal which has 

 been heated to a bright glow. When the oxygen is consumed by the 

 burning, close the bottle and shake. What change occurs in the color 

 of the limewater? As neither oxygen nor the water vapor produced 

 by hydrogen burning in oxygen has a similar effect, you are justified 

 in concluding that it must be the product of the burning of carbon, 

 which turns limewater milky white. 



You have learned that pure carbon and oxygen combine in 

 combustion to form a new compound which is an oxid com- 

 pound. Now one of the best established facts of chemistry is 

 that in ordinary conditions carbon completely burned unites in the 

 ratio of one part to two parts of oxygen. This is what occurs in 

 the present instance, and the product must therefore be carbon 

 dioxid. Carbon dioxid is the only odorless, invisible gas which 

 has this effect upon limewater, and this may, therefore, be 

 taken as an infallible test for its presence. 



Carbon in the candle. Recall now the facts observed when 

 you mixed limewater with the products of the burning candle. 

 Evidently carbon must have been burned to produce the -carbon 

 dioxid, the presence of which was shown by the limewater test. 

 The carbon can have in this case but one source, namely, the 

 candle. It is clear, therefore, that the substance of the candle 

 contains both hydrogen and carbon. 



