THE FOOD OF THE PLANT 39 



more rapidly and brightly than it did in air. A glow- 

 ing splint when put into a jar bursts into flame. 



5. It is slightly heavier than air. 



6. It does not turn lime-water milky. 

 Inferences. — Oxygen is an invisible, colourless and 



non-smelling gas. If soluble in water it must be only 

 slightly so. It is neutral to litmus. Substances burn 

 in it much more readily than they do in air. 



Other substances in the air.— Thus it is seen that 

 the air is made up almost entirely of oxygen and 

 nitrogen. But- there are also other gases present in 

 small quantities. 



The leaves of a plant are continually transpiring ; 

 it follows, therefore, that the air must ahvays contain 

 a certain proportion of water-vapour. It will now be 

 shown that it also contains another gas which is most 

 important in the life of a plant. 



Experiment 20 



Aim. — To find out the effect produced by air on 

 lime-water. 



Method. — Place some lime-water in a shallow dish 

 and leave it exposed to the air. 



Observation. — The lime-water gradually becomes 

 milky. 



Inference. — Since lime-water in an open dish be- 

 comes milky while that in a corked bottle remains 

 clear, it seems probable that the change in the lime- 

 water in the first case is brought about by contact with 

 the surrounding air. 



Now neither oxygen nor nitrogen can produce milki- 

 ness in lime-water. It is natural, therefore, to infer 

 that there must be some other constituent in the air to 

 which the milkiness is due. This constituent is the 

 gas carbon-dioxide. The milkiness is caused by the 

 combination of the carbon-dioxide with the lime of the 



