40 EXPERIMENTAL PLANT PHYSIOLOGY 



lime-water to form a white powder named calcium- 

 carbonate. 



Experiment 21 



Aim. — To find out whether the amount of carbon- 

 dioxide in the air is affected by the breathing of living 

 creatures in it. 



Method. — Take two glass dishes each containing a 

 little lime-water. Leave one on the table. Breathe 

 vigorously into the other through a glass-tube. 



Observations. — The lime-water in the second case 

 very quickly turns milky, while the other becomes so 

 only gradually. 



Inference. — The amount of carbon-dioxide in the 

 air is considerably increased by the breathing of living 

 things. 



Experiment 22 



Aim. — To determine whether combustion affects the 

 amount of carbon-dioxide in the air. 



Method. — Two glass plates are taken. On one is 

 placed a very short piece of lighted candle and on both 

 a small dish of lime-water. Bell-jars are then placed 

 over the plates. 



Observation. — The lime-water under the bell-jar 

 containing the lighted candle becomes milky much 

 more quickly than the other. 



Inference. — The amount of carbon-dioxide in the air 

 is increased by the burning of substances. 



(It must be noted here that it is only those sub- 

 stances that contain carbon which produce carbon- 

 dioxide on burning.) 



Carbon-dioxide is thus formed in the atmosphere by 

 the breathing of all living things; it is also produced 

 when certain substances burn ; and it may be shown too 

 that it is formed when animal and vegetable matter 

 decay. 



Some of this gas must now be prepared in order that 

 its properties may be tested more fully. 



