<P> The conservation of mass was obscure for millennia because of the buoyancy effect of the Earth's atmosphere on the weight of gases . For example, a piece of wood weighs less after burning; this seemed to suggest that some of its mass disappears, or is transformed or lost . This was not disproved until careful experiments were performed in which chemical reactions such as rusting were allowed to take place in sealed glass ampoules; it was found that the chemical reaction did not change the weight of the sealed container and its contents . Weighing of gases using scales was not possible until the invention of the vacuum pump in 17th century . </P> <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> The law of conservation of mass was challenged with the advent of special relativity . In one of the Annus Mirabilis papers of Albert Einstein in 1905, he suggested an equivalence between mass and energy . This theory implied several assertions, like the idea that internal energy of a system could contribute to the mass of whole the system, or that mass could be converted into electromagnetic radiation . However, as Max Planck pointed out, a change in mass as a result of extraction or addition of chemical energy, as predicted by Einstein's theory, is so small that it could not be measured with the available instruments and could not be presented as a test to the special relativity . Einstein speculated that the energies associated with newly discovered radioactivity were significant enough, compared with the mass of systems producing them, to enable their mass - change to be measured, once the energy of the reaction had been removed from the system . This later indeed proved to be possible, although it was eventually to be the first artificial nuclear transmutation reaction in 1932, demonstrated by Cockcroft and Walton, that proved the first successful test of Einstein's theory regarding mass - loss with energy - loss . </P>

What are the laws of conservation of matter