Be patient with him.

If he cannot tell you what carbon is, he can tell you what is carbon, which is well worth knowing.

He will tell you, for instance, that every time you breathe or speak, what comes out of your mouth is carbonic acid; and that, if your breath comes on a bit of slacked lime, it will begin to turn it back into the chalk from which it was made; and that, if your breath comes on the leaves of a growing plant, that leaf will take the carbon out of it, and turn it into wood.

And surely that is worth knowing,--that you may be helping to make chalk, or to make wood, every time you breathe.
Well; that is very curious.
But now, ask him, What is carbon?
And he will tell you, that many things are carbon.

A diamond is carbon; and so is blacklead; and so is charcoal and coke, and coal in part, and wood in part.
What?
Does Analysis say that a diamond and charcoal are the same thing?
Yes.
Then his way of taking things to pieces must be a very clumsy one, if he can find out no difference between diamond and charcoal.
Well, perhaps it is: but you must remember that, though he is very old--as old as the first man who ever lived--he has only been at school for the last three hundred years or so.