For Google Books you will often want to use date-restricted searches (for some reason these don't work so well after the early 1900s, at least for me), but don't restrict to a single publication year because (especially in the case of 1800s literature) these can vary by $2$ or $3$ years according to how the year was determined (by conference or presentation meeting date, by journal issue date, by journal volume date, etc.). Also, phrase searches for the paper's title are usually best, but sometimes you'll have to remove the quotes used for a phrase search because the .pdf-to-text conversion for searches can mess up some words. Indeed, sometimes it's best to leave out a few words in the title, especially when the original publication is written in old-style English or gothic German style (something you usually don't know in advance, so you do this later on when more obvious searches are not working). Sometimes it's best to include the author as an additional search word (but use only LAST/FAMILY name), and sometimes it's best to not include the author. Moreover, Google searches (regular and Book) are sometimes random-like and inconsistent with results given, especially for obscure stuff (no hits or only 2 or 3 hits), such as using MORE search words/phrases can sometimes produce ADDITIONAL hits, so you'll want to be very persistent in searches and not try to use logical deduction when trying different searches (e.g. thinking that adding a word cannot increase search results; substituting what should be an equivalent form, such as the German word über/uber/ueber, will not change the search results).
Besides Google Books $\ldots$
Numdam -- Has.pdf files of individual papers can be downloaded, pretty much everything from French 1800s math journals. Some of the links I give in the following answer are to Numdam URLs: Reference Request - Functions of the form $\frac{ax^2+bx+c}{dx^2+ex+f}$. Also, although I think all (or nearly all) of the papers cited in the following answers are to Google Books, probably all of the French journals mentioned are at Numdam: Evaluate $\lim_{x\to 0}\frac{x-\sin x}{x\sin x}$ without to use L'Hopital AND Conditionally convergent series AND What set of criteria led Hamilton to discover the quaternions? (in this answer the journals are where the reviews are published) AND differential equation of conics.
HathiTrust -- A huge number of old books and old journals can be found here, most of which I can download page-by-page, but entire document download (by journal article, or by book/journal volume) requires affiliation with a member institution, which I don't have.
Biodiversity Heritage Library -- Has a large number of old scientific society journals, which are where a lot of math papers before the last few decades of the 1800s were published. From here I've downloaded complete sequences up to the mid 1920s of many journal/periodical volumes, such as Report of the British Association for the Advancement of Science and similar U.S. scientific associations. See my comments to Around 1904, did the scientific community take the atomic hypothesis seriously? AND this answer to What is the history of linear vs logarithmic scales?
Royal Society of London Catalogue of Scientific Papers 1800-1900. Volume I, Pure Mathematics (1908) -- This is an essentially complete list of papers, categorized by topic/subject, fitting the restrictions of its title. English translations of the paper titles are used (often shortened when the original title is long), which makes searching for the papers difficult (can't use phrase or word searches in online resources) but there is the advantage that the translated titles somewhat indicate what the paper deals with. I've found this so useful that I obtained a print-on-demand hardcopy around 12-14 years ago.
Best way to find papers in Mathematics by a specific author? -- My answer to this Mathematics Stack Exchange question lists many useful sites for math literature searching.
Educational Times and Mathematical Questions and Solutions from the Educational Times -- I'm listing this periodical individually only because I discussed which volumes appear to exist online in my answer to What is "educ times"? A journal? in Academia Stack Exchange, and thus it was probably not seen by many participants in this Stack Exchange who might be interested in this information.
Google search tips for information about authors -- See my answers to Who is Donald L. Webb AND Who was N.M. Stephens who refuted the Stronger Feit-Thompson Conjecture?