The law of conservation of matter, also known as the conservation of mass, states that the amount of matter in a closed system never changes. This appears on the surface to be wrong when someone looks at the simple issue of what happens to matter when burned on Earth. It would seem that the matter was destroyed to some degree in the process, and this was believed to be the case until the 18th century. If Earth were an entirely sealed system, the heat, light, sound energy, and escaping gasses generated in the burning process would still be detectable. Matter, therefore, can change form in a closed system, as can energy, but it can never be created or destroyed.
In the 1700s, French chemist Antoine-Laurent Lavoisier began to clearly define the principle of the law of conservation of matter, though the actual idea can be traced back to an ancient Greek belief that nothing can come from nothing. Lavoisier performed experiments where he created reactions in completely sealed chambers, a novel idea at the time. In his experiments, he measured the weight of all of the byproducts of a reaction afterwards and determined that it had not changed. He is credited with the discovery and naming of oxygen as a byproduct of burning materials, which led him to an understanding of the conservation law of matter. His research earned him the title as the Father of Modern Chemistry.
It wasn't until over a century later that Albert Einstein discovered that matter and energy are interchangeable. Einstein proved that the law of conservation of matter and energy were just two ways of looking at the same process. Chemistry now has established methods for calculating the molecular weight of substances, so that it is clear that when compounds are combined or separated, matter has not been lost or destroyed.
Unfortunately, though Lavoisier pioneered research into the conservation of mass, he was never able to complete his experiments. As a member of an unpopular French group of tax collectors known as Ferme Generale, he exploited his position to make a fortune, and was sentenced to death during the French Reign of Terror. He requested a 15-day extension on his execution to complete his scientific experiments, which he thought would be valuable to posterity. The judge in the case replied, however, that, “...the Republic has no need of scientists,” and he was guillotined on 8 May 1794.