CARBON AND SOME OF ITS COMPOUNDS 167 



This is another important fact. It is clear that carbon dioxide escapes 

 from our mouths in breathing ; and so it does from every animal. 

 Not only, then, do all cases of ordinarj' burning result in the addition 

 of carbon dioxide to the air, but also every act of breathing. It does 

 not matter how small an animal is, all the time it is alive it is continually 

 adding to the atmosphere a certain amount of the colourless, odourless 

 gas which puts out flames and turns lime-water milky. 



Purifying action of plants. That there is alwaj^s a certain amount 

 of carbon dioxide in the air can be proved by exposing fresh lime- water 

 in a shallow vessel. Very soon the lime-water becomes covered with 

 a thin white layer of chalk, which is formed by the combination of the 

 carbon dioxide in the air with the lime in the lime- water. One reason 

 why there is never very much carbon dioxide in the air out of doors 

 is because there is an agency continuously at work getting rid of this 

 gas. This agency is the green parts of plants which occur everywhere. 



When fresh watercress is put into a bottle completely full of water 

 containing carbon dioxide in solution, and the bottle is inverted in 

 a basin of water without allowing air to get into the bottle, it is found 

 that, when the bottle and its contents are exposed to bright sunlight, 

 bubbles of gas collect at the top of the bottle. These bubbles, when 

 tested, are found to be pure oxygen. If, however, the bottle with the 

 cress in it is kept in the dark no bubbles of oxj-gen collect. Or if a 

 bottle of water in which carbon dioxide is dissolved be put in sunlight, 

 without any watercress, no oxygen collects in the top of the bottle. 



In other words, two things are necessary for the formation of the 

 bubbles of oxygen collected from the green plant as described. They 

 are (1) the vegetation, (2) the sunlight. The same conditions have been 

 found to hold true always, thus proving that green plants in the presence 

 of bright sunlight have the power of turning oxygen out of carbon 

 dioxide. They keep the carbon for themselves, and it helps them to 

 grow. If the experiment were carefully performed it would be found 

 that the watercress had increased in weight after being exposed in these 

 circumstances to bright sunlight for some time. 



81. PRODUCTION OF CARBON DIOXIDE FROM CHALK 

 BY THE ACTION OF AN ACID. 



i. Preparation of carbon dioxide. Into a flask or bottle, fitted like that 

 in Fig. 114, place some chalk, or small pieces of marble. Place the delivery 

 tube in a glass cylinder or a jar with a wide mouth. A disc of cardboard, 

 tlii'tiiL'li which the delivery tube passes, should rest on the top of the jar. 

 Pour dilute hydrochloric acid down the funnel. During the effervescence 



