LaTeX.org

In a book:
\documentclass[11pt]{book}

I defined:
\newtheorem{Ex}{Example}[chapter]

to set Examples in a font different from the rest of the book. By default, they come up in italics, I think because I am using
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

How can I make the ...