In 1808, the noted chemist John Dalton published the New System of Chemical Philosophy, which was a collection of his many experiments and subsequent theories. His ideas on the nature of matter can be summarized in four distinct hypotheses. First, elements are composed of small, solid particles called atoms that cannot be split. Second, all atoms of a particular element are the same, that is, their size, shape, mass, and chemical properties. Third, compounds are made of atoms of more than one element, and the ratios of the atoms of these elements are expressed as an integer or a fraction. Fourth, a chemical reaction only involves the separation, combination, or rearrangement of atoms, not their creation or destruction.
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These hypotheses served as independent validation for three well-known scientific ideas, including Joseph Proust's law of definite proportions, which states that "different samples of the same compound always contain its constituent elements in the same proportion by mass." They also support the law of multiple proportions, which says "if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers." The fourth hypothesis is another way of stating the law of conservation of mass, that "matter can be neither created nor destroyed."