Where in the book including Alice does it have word contingency?

The word "contingency" doesn't appear in Lewis Carroll's "Alice's Adventures in Wonderland" or "Through the Looking-Glass".

It's likely a word that wouldn't have been used in the Victorian era, when the books were written, as it carries a more modern, formal, and technical connotation.

If you're looking for a specific passage that relates to the concept of contingency, you might want to consider words like "chance," "accident," "unforeseen," "unexpected," or "uncertain," which are all used in the books.