E-books can be found in the library search by starting with a overall topic search before narrowing the results by format. If you are starting from the libraries website enter your topic...
...then, you can narrow your results to only e-books with the Narrow My Results option for Format in the left panel and clicking More options.
Then place a check in the Include column for Ebooks format option and Continue.
PRO TIP: If you are using the advanced library search, make sure you do not limit your initial search by material type (right panel) as this doesn't not include e-books.
All e-books including historical sources such as letters, diaries, and pamphlets include records in the catalog with the phrase, electronic books, as a genre heading. This allows you to limit your search to e-books.
You can combine "electronic books" (use the quotes) with a topic or author when running a simple search on the library catalog.
Since the electronic books heading is applied to sources covering a wide span of years, you may want to consider limiting the publication date. See above link "Set Other Search Limits".
Determine a date range and then click on "Set Limits". Run your keyword search, e.g. "electronic books" AND chemistry.