Gallica is the full-text archive of the French National Library (Bibliothèque nationale de France, BnF). It is enormous and can be very hard to search. If you use the advanced search (Recherche avancée), there are some tools available to help you find what you're looking for.
Search by year of publication will help you only get results from the time period in which you're interested :
If you use limits like this, you're unlikely to run into documents that are unavailable in full-text.
One very tempting option is to use the check box by
This is can be deceiving. It indicates that you'll be searching ONLY for that content in Gallica with OCR (optical character recognition), rather than for everything available in Gallica full-text. If you want to limit to only material available without restrictions, use
at the very bottom of the page.