30 THE FOOD OF PLANTS CHAP. 



about one-fifth part of what they weighed before ; 

 all the water having been driven off in vapour. 



We thus see that four-fifths, by weight, of 

 the leaf is water. 



Take a piece of platinum foil about four inches by 

 three inches, and bend up the edges so as to make 

 a little platinum dish. 



Weigh it and note the weight. 



Place the dried leaves in it and weigh again. 

 By subtracting the first weighing from the second 

 we find the weight of the leaves. Place the 

 platinum tray on a pipeclay triangle over a Bunsen 

 burner or petroleum blowpipe, and light the gas. 



The leaves will burst into flame and burn for a 

 short time. When the flame has died away remove 

 the burner and notice the charred bits of leaf in the 

 tray. This charred leaf is charcoal. We have now 

 reduced the leaf to charcoal. 



Replace the lamp, and arrange it under the dish 

 so that the platinum becomes red hot. 



The pieces of leaf glow, the charcoal burns 

 away, and a pinkish ash is left behind. 



This ash is the unburnable or mineral part of the 

 leaf. 



Allow the platinum dish to cool and weigh again, 

 and subtract the weight of the empty dish. This 

 gives us the weight of the ash, which is a very small 

 fraction of the weight of the dried leaves. 



We see from these experiments that the 

 leaf consists of 



