Gloria, Bates' rosters of the Pennsylvania troops, unfortunately, does not include Naval personnel (or direct entries into the Union Navy), or Marines. Dave Sullivan's four volumed set of the UNITED STATES MARINE CORPS IN THE CIVIL WAR also does not include the name of Jacob Mentzer in the indices. So it seems that you will have to obtain the military and pension records from the National Archives. I assume you have already checked his name in the census records of other years. He is shown as a grocer, residing with his wife, Annie, and four daughters, at Lancaster, in the 1900 census records. Born July, 1838, he is shown to have married Annie in about 1868.