His principal work, Tentamen Theoriae Electricitatis et Magnetismi, published at St Petersburg in 1759, was the first systematic attempt to apply mathematical reasoning to these subjects. He also published a treatise, in 1761, De Distributione Caloris per Tellurem, and he was the author of memoirs on different subjects in astronomy, mechanics, optics and pure mathematics, contained in the journals of the learned societies of St Petersburg and Berlin. His discussion of the effects of parallax in the transit of a planet over the sun's disc excited great interest, having appeared between the dates of the two transits of Venus that took place in the 18th century.
Electrical theories
Aepinus and Henry Cavendish devised theories of electricity which were essentially the same, yet had been framed without any communication between these two philosophers. Aepinus however published his theory about ten years before that of Cavendish. These are essentially modern theories which eventually put to rest the idea of two fluids. , Tartu, Estonia Their theories said that
... the electric fluid is a substance, the particles of which repel each other, and attract the particles of all other matter, with a force inversely as the square of distance.
The particles of all other matter also repel each other, and attract those of the electric fluid with a force varying according to the same law. Or, if we consider the electric fluid as matter different from all other matter, the particles of all matter, both those of the electric fluid, and of other matter, repel particles of the same kind, and attract those of a contrary kind, with a force inversely as the square of the distance.
Cavendish and Aepinus believed that the electric fluid was
another sort of matter.... Its weight in any body probably bears a very small proportion to the weight of the matter in the body, but yet the force with which the electric fluid in any body attracts any particle of matter in that body, must be equal to the force with which the matter of the body repels that particle, otherwise the body would appear electrical, as will afterwards appear.
In effect, they are imagining the 'electric fluid' as consisting of electrons with almost no mass, but still with substantial electrical attractive and repulsive power. Static electricity then becomes a matter of a superabundance of electrons in one body, and a shortage of electrons in another.
