The full text of most Rice dissertations and theses is available online in the Digital Scholarship Archive at
To locate theses and dissertations in the library catalog you can:
Search by author or by title keyword. If you are looking for dissertations from a particular department, do a keyword search: type rice thesis and department name. Example: rice thesis and chemistry.
Searching by subject will seem to work but it is not the best choice because current dissertations do not have Subject Headings assigned.