54 FIEST STUDIES IN PLANT LIFE 



4. What kind of gas does man breathe out ? 



First of all, you must know that when carbon -dioxide 

 touches lime-water it makes it milky. Put some clear 

 lime-water* into a clean tumbler. Breathe through a 

 straw or reed into the water, and you will see it 

 become milk-white. This proves that the air you 

 breathe out is carbon-dioxide. And now I have to 

 prove that every fire and light that is lit makes this 

 poisonous gas. 



5. What kind of gas is nnade by lights ? Take a 

 glass bottle with a narrow neck — a clean pickle bottle 

 will do — and thrust into it the burning end of a taper. 

 It will burn for a little, just as the soldiers in the 

 cellar could live for a little ; and then it will 

 go out just as the soldiers' lives went out. The 

 bottle has become for the taper what the Black Hole 

 of Calcutta was for these men. To prove that the 

 bottle is now full of the gas which you breathe out, 

 pour into the bottle some clear lime-water, and shake 

 it up. It becomes milky-white as before. In the 

 same way, carbon-dioxide is made by every fire and 

 light. 



6. Is there much carbon-dioxide in the air ? 

 You will see, then, that a great deal of this gas must 

 be made every day in houses and workshops and 

 wherever there are men or animals or lights or fires ; 

 but the ocean of air that surrounds the globe is a vast 

 one, and so the amount of carbon-dioxide near a plant 

 is usually small. You can see this for yourself if you 

 leave a glass jar of lime-water for a little in the air of 



* To make lime-water, take a lump of lime twice the size of a hen's egg, 

 and put it in a quart-bottle of water. Allow it to settle for a day or two, 

 and then pour off the clear liquid into a clean glass bottle. Keep corked 

 till it is needed. 



