A term for autumn leaves when all their lovely tints are fled

A term for autumn leaves when all their lovely tints are fled;

A mountain in Arabia, lifting high its rocky head ;
What witches and astrologers pretend they truly are ;
A state from which I greatly hope your conscience still is far:

Those four are all alike, you'll see, in mere pronunciation,
But diverse in orthography and in signification.
Transpose the second, you will gain the title of a king,
And what you would be sure to do if he should enter in ;
Transpose the fourth, you'll see at once how ancient warriors treated
The cities of the enemy, with passion overheated ;
Transpose the third, and lo ! the first will straightway be revealed.
Now, reader, I shall like to see this mystery unsealed.