According to http://www.uspto.gov/web/offices/ac/ido/oeip/taf/issuyear.htm:
The current patent numbering system began with a patent issued on July 13, 1836. Prior to that date, 9,957 patents had been issued.
These 9,957 patents before patent US1 include the first U.S. patent, which was numbered X1 or X000001. It was granted on July 31, 1790 to Samuel Hopkins.
Still from http://www.uspto.gov/web/offices/ac/ido/oeip/taf/issuyear.htm:
Some numbers within a series may be unused. Therefore, the number of patents granted during a year cannot be determined by simply subtracting the number of the first patent issued in one year from the number of the first patent issued in the next year.
Therefore I guess there is no patent #100 in the current numbering system. There doesn't seem to be one either in the previous numbering system. Patents 100 and X100 cannot be found from USPTO full-page images database.
Experience shows that you can get a 404 from Google Patents while an application already exists and can be found elsewhere (USPTO Public Pair, Espacenet...), probably because of database updates.