<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> <P> The change in mass of certain kinds of open systems where atoms or massive particles are not allowed to escape, but other types of energy (such as light or heat) are allowed to enter or escape, went unnoticed during the 19th century, because the change in mass associated with addition or loss of small quantities of thermal or radiant energy in chemical reactions is very small . (In theory, mass would not change at all for experiments conducted in isolated systems where heat and work were not allowed in or out .) </P> <P> The conservation of relativistic mass implies the viewpoint of a single observer (or the view from a single inertial frame) since changing inertial frames may result in a change of the total energy (relativistic energy) for systems, and this quantity determines the relativistic mass . </P>

Who performed experiments leading to the law of constant composition