50 THE FOOD OF PLANTS CHAP. 



the air coming from the charcoal, place on the ground 

 glass plate a little beaker containing clear lime- 

 water, with the tube from the funnel dipping into 

 it, and draw the air through as before. The lime- 

 water quickly becomes milky, showing that the 

 burning charcoal is producing the same 

 vitiated air as the burning candle and the 

 breathing animal, 



The Charcoal is combining with the Air to 

 form a Gas 



Now let us watch the charcoal burning, and try 

 and understand what is happening. 



We notice that the charcoal disappears, leaving a 

 little ash behind, and that the air passing through it 

 is at the same time altered. We see no smoke rising 

 from the charcoal, and must suppose that the charcoal 

 passes away with the air as a gas of some kind. 

 We know the air is an invisible gas or mixture of 

 gases. We know that by heating coal in a retort, 

 another invisible gas coal-gas is formed. Here 

 also the charcoal and the air seem to form an in- 

 visible gas of such a kind that it puts out a burning 

 flame and turns lime-water milky in appearance. 



This gas, formed of charcoal and air, seems 

 to act as a food to the plant, the plant taking 

 the charcoal to itself and restoring the air to 

 its former condition. 



