The mass of a body can be considered as inertial or as gravitational. The inertial mass is that a property of a body with which it opposites an acceleration; the gravitational mass, instead, is the property which affect the gravitational interaction between bodies.
The possibility of assimilating the two masses had already been theorized by Newton in his “Principia” and also Kant in the “First metaphysical principles of natural science” in 1786 had written that the “determination of the mass of a body can be expressed at the same time as inertial and or as original attraction”; however just in the 20th centuryit was possible to evaluate with faithful experiments that the inertial mass and the gravitational one coincide.
In fact it was between 1889 and 1908 that Ronald von Eötvos; f rom Hungary done accurate experiments about it using a slight balance rail long 40cm (with two masses at the extremes) hung to a thread of platinum-iridium. This tool let him ebserve that the masses reacted in the same way to the force of gravity, which attracts everything to the centre of Earth, and to the centrifugal force (which is inertial) wich affects all the bodies on the surface of the Earth because of its rotation.
The baron could then conclude the equality of the value of the two types of mass which was after explained by Einstein which affirmed in his “Principle of Equivalence” that the effects of a gravitational field can be completely equalized to those observed in an uniformely accelerated system.