The wonderful Proceedings of the Athanasius Kircher Society blog summarizes the longest-running scientific experiments. For example, the Pitch Drop Experiment at the University of Queensland has been running since 1927. From the description of that experiment (seen here):
The first Professor of Physics at the University of Queensland, Professor Thomas Parnell, began an experiment in 1927 to illustrate that everyday materials can exhibit quite surprising properties. The experiment demonstrates the fluidity and high viscosity of pitch, a derivative of tar once used for waterproofing boats. At room temperature pitch feels solid - even brittle - and can easily be shattered with a blow from a hammer (see the video clip below). It's quite amazing then, to see that pitch at room temperature is actually fluid!The Athanasius Kircher post and reader comments point to several other unusual experiments that are even older. Link
In 1927 Professor Parnell heated a sample of pitch and poured it into a glass funnel with a sealed stem. Three years were allowed for the pitch to settle, and in 1930 the sealed stem was cut. From that date on the pitch has slowly dripped out of the funnel - so slowly that now, 72 years later, the eighth drop is only just about to fall.