John Dalton (1766-1844) was the first chemist who, in explaining different phenomena, used the theory of the study of an atom. He did researches on gases. He discovered the law of partial pressures. Dalton's law states that pressure of nonreactive gaseous mixture is equal to a sum of pressures of each separate element of the mixture having the same volume as the mixture. Another Dalton's discovery, which he made in 1804, was showing that if two elements have more than one combination, then weight amounts of one of them belonging to the unchanging amount of the second one are staying in relations of small integers (the law of stoichiometry). For example, for chlorine oxides (Cl2O, Cl2O6, Cl2O7) masses of oxygen belonging to chlorine unit are staying in proportions 1:6:7. Dalton noticed that the results he got could be simply explained using the conception of an atom. Expanding the study of atom ideas he assumed that chemical combinations were created by combining atoms of different elements. He was of the opinion that atoms of different elements had different masses, as a mass unit he took the mass of one atom of hydrogen. He laid the foundation of modern study of atom and described world's microstructure explaining most of occurrences known those days. After over two thousand years, finally there were so many proofs of atom's existence, that hypothetical till then, atoms became real (although Dalton's particles were not the same as Democritus ones because it appeared they were not final components of matter).