22 THE WORK OF THE LEAF [chap. 



Thus, though the carbon in the soil has only lost 

 7000 lb. during the period under examination, no 

 less than 25,200 lb. per acre has been taken away in 

 the crops, and the air is the only source from which it 

 could have been derived. The air, we know, contains 

 from 3 to 4 volumes of carbon dioxide in every 10,000, 

 and small as the proportion may seem, it yet represents 

 an enormous quantity of carbon dioxide upon which the 

 plant can draw. 



We will now take some experiments showing that 

 plants have the power to split up this carbon dioxide in 

 the air and take the carbon from it. As we are dealing 

 with gases we shall have to put our experimental plants 

 in water in order to see and collect the gases, and it 

 will, therefore, be necessary to take well or river water 

 that has had an opportunity of dissolving a little air, and 

 especially carbon dioxide, which is rather more soluble 

 than the rest of the air. By boiling some of the water 

 in a preliminary experiment it is easy to show that such 

 water contains dissolved carbon dioxide, which is expelled 

 in heating. Take a small bunch of young green active 

 shoots of mint or watercress and put them in a jar of 

 water, covering them with an inverted funnel the shank 

 of which leads into a test-tube full of water. Stand the 

 jar in the brightest light available, and repeat the 

 experiment with another jar placed in the dark. To 

 complete the demonstration a third jar should be used, 

 containing water that had been boiled to expel all gases 

 and then cooled down ; this also should be placed in the 

 light. As the light falls on the mint in the unboiled 

 water little bubbles of gas will begin to appear on the 

 tips of the leaves ; from time to time they will break 

 away and ascend into the test-tube above. No such 

 bubbles appear from the mint in the dark, or from that 

 which is in the light but immersed in boiled water. 



