Siméon-Denis Poisson (1781-1849) was forced into the study of medicine by his family but he abandoned it for mathematics. In Paris at the ecole Polytechnique, Laplace and Lagrange were his instructors, and later his lifelong friends. Poisson's most important work concerned the application of mathematics to electricity and magnetism, and other areas of physics. In 1812 he published a paper which contained many of the most useful laws of electrostatics and his theory that electricity is made up of two fluids, in which like and unlike repel and attract each other.