<P> Once understood, the conservation of mass was of great importance in progressing from alchemy to modern chemistry . Once early chemists realized that chemical substances never disappeared but were only transformed into other substances with the same weight, these scientists could for the first time embark on quantitative studies of the transformations of substances . The idea of mass conservation plus a surmise that certain "elemental substances" also could not be transformed into others by chemical reactions, in turn led to an understanding of chemical elements, as well as the idea that all chemical processes and transformations (such as burning and metabolic reactions) are reactions between invariant amounts or weights of these chemical elements . </P> <P> Following the pioneering work of Lavoisier the prolonged and exhaustive experiments of Jean Stas supported the strict accuracy of this law in chemical reactions, even though they were carried out with other intentions . His research indicated that in certain reactions the loss or gain could not have been more than from 2 to 4 parts in 100,000 . The difference in the accuracy aimed at and attained by Lavoisier on the one hand, and by Morley and Stas on the other, is enormous . </P> <P> In special relativity, the conservation of mass does not apply if the system is open and energy escapes . However, it does continue to apply to totally closed (isolated) systems . If energy cannot escape a system, its mass cannot decrease . In relativity theory, so long as any type of energy is retained within a system, this energy exhibits mass . </P> <P> Also, mass must be differentiated from matter (see below), since matter may not be perfectly conserved in isolated systems, even though mass is always conserved in such systems . However, matter is so nearly conserved in chemistry that violations of matter conservation were not measured until the nuclear age, and the assumption of matter conservation remains an important practical concept in most systems in chemistry and other studies that do not involve the high energies typical of radioactivity and nuclear reactions . </P>

Who said 'atoms are neither created nor destroyed—just rearranged'