HOW PLANTS OBTAIN FOOD FROM AIR 



41 



We noticed before, on heating some dried leaves, 

 that they were largely composed of charcoal. 



If we heat the cress seed strongly we can also 

 change it into charcoal. 



The best way to do this is to place half a 

 gramme of the cress seed in a test-tube, and lead 

 into the test-tube a stream of gas obtained by 

 pouring an acid on chalk. This gas passing 

 through the test-tube will prevent the charcoal 

 formed from burning away and so leaving nothing 

 but ash when the test-tube is strongly heated. 



In order to obtain this 

 gas, take a wide-mouthed 

 bottle fitted with an india- 

 rubber cork in which two 

 holes have been bored. 

 Fit a bent glass tube into 

 one hole, and a thistle 

 funnel into the other. 

 Place in the bottle some FIG. 10. 



pieces of broken marble 



and a little water. Fit into the bottle the cork. 

 Take another piece of glass tubing about nine inches 

 long and connect it by a piece of india-rubber tubing 

 to the bent tube through the cork. 



Now pour some strong hydrochloric acid down 

 the thistle funnel. The acid at once begins to attack 

 the marble, and bubbles of gas rise from it, which 

 pass out of the bottle along the glass tube. 



Now push the glass tube into the test-tube so as 



