THE LEAF 121 



significant, and therefore scarcely worth our attention. 

 If plants extracted their carbon exclusively or even chiefly 

 from the organic substance of the soil, then soil covered 

 with vegetation, the products of which are periodically 

 removed in one way or another, must in course of time 

 become poorer in humus ; on the contrary, everyday 

 experience shows that soil becomes richer in humus 

 when it is under cultivation, as in corn-land, pasture, 

 or wood. In cultivating our fields we extract from 

 them every year in the form of crops more organic 

 substances than we introduce into them in the form 

 of manures ; and yet soil carefully manured becomes 

 richer in humus. It is clear that ultimately plants 

 do not reduce the amount of organic substances present 

 in the soil, but even increase it ; and therefore they can- 

 not find at all events their principal source of carbon 

 in the soil. But if not in the soil it must be in the air ; 

 and if so, it is probably absorbed by an organ pre- 

 eminently aerial by the leaf. Let us see what is the 

 source of the carbonaceous food in the air and how it 

 is obtained by the plant. 



Together with nitrogen and oxygen, atmospheric air 

 also contains a very small quantity, about three ten- 

 thousandth parts, of carbonic acid. This gas, although 

 colourless and in appearance indistinguishable from 

 air, is a compound of carbon and oxygen. No one will, 

 I am sure, doubt the accuracy of this statement, yet 

 we must see the proof of it, as of every other state- 

 ment, as far as possible with our own eyes ; and it is 

 possible in the present case. In order to detect the 

 presence of carbon in carbonic acid, we must remove 

 the oxygen. This can be done by causing the oxygen 

 to combine with a body having still stronger affinity for 

 it. Such is, for instance, the metal magnesium, the 

 wire of which burns with a dazzling light. I light this 

 piece of wire and sink it into a glass jar containing 

 common air ; the wire burns down and a perfectly 



