ATD passim, quaternions

[Anville Azote]

Quaternions, explained by John Baez (yes, the cousin of Richard
Farina's sister-in-law):

http://pass.maths.org.uk/issue32/features/baez/

(also http://plus.maths.org/issue33/features/baez/index.html on the octonions)

At a higher level, we have this by his colleague David Corfield:

http://www.dcorfield.pwp.blueyonder.co.uk/2006/03/history-of-quaternions.html

-A. A.