most-positive-float, most-negative-float coming up nil

[Burton Samograd]

Version: "GNU Emacs 24.0.90.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.20.1)
 of 2011-10-07 on bsamograd-ubuntu"

Just playing around with emacs lisp and I thought I would try the
most-positive/most-negative variables to see numeric range. 

most-positive-fixnum
2305843009213693951
most-negative-fixnum
-2305843009213693952
most-positive-float
nil
most-negative-float
nil

Any reason why the float values are coming up nil?

--
Burton Samograd