<P> Since atoms were found to be divisible, physicists later invented the term "elementary particles" to describe the "uncuttable", though not indestructible, parts of an atom . The field of science which studies subatomic particles is particle physics, and it is in this field that physicists hope to discover the true fundamental nature of matter . </P> <P> The idea that matter is made up of discrete units is a very old one, appearing in many ancient cultures such as Greece and India . The word "atom" was coined by the ancient Greek philosophers Leucippus and his pupil Democritus . However, these ideas were founded in philosophical and theological reasoning rather than evidence and experimentation . Because of this, they could not convince everybody, so atomism was but one of a number of competing theories on the nature of matter . It was not until the 19th century that the idea was embraced and refined by scientists, as the blossoming science of chemistry produced discoveries that could easily be explained using the concept of atoms . </P> <P> Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory . The first was the law of conservation of mass, formulated by Antoine Lavoisier in 1789, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products). The second was the law of definite proportions . First proven by the French chemist Joseph Louis Proust in 1799, this law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance . </P> <P> John Dalton studied and expanded upon this previous work and developed the law of multiple proportions: if two elements can be combined to form a number of possible compounds, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers . For example: Proust had studied tin oxides and found that their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3% oxygen (these were tin (II) oxide and tin dioxide respectively). Dalton noted from these percentages that 100g of tin will combine either with 13.5 g or 27g of oxygen; 13.5 and 27 form a ratio of 1: 2 . Dalton found that an atomic theory of matter could elegantly explain this common pattern in chemistry . In the case of Proust's tin oxides, one tin atom will combine with either one or two oxygen atoms . </P>

Who proposed that all matter is composed of atoms
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