In nuclear reactions of cracking heavy nuclei of radioactive chemical elements, the resulting masses were measured and found that their sum is smaller than before cracking the nucleus and some scientists were quick to conclude that the lack of mass turned into energy and matter is lost.
So, the law of conservation of mass of Lavoisier, considered the father of modern chemistry and executed on the scaffold during the French Revolution, would not be valid.
In reality, the mass is found in the mass of neutrino and alpha particles (helium nuclei) of high speed that result from cracking. Weight increases as a result of the increase of speed and decreases as a result of decrease of speed. Weight is a measure of inertia, momentum is a measure of the amount of motion and strength is a measure of the interaction. Do not forget this!