ATOM

(Gr., ‘cannot be cut,’ ‘indivisible’), signifies the smallest po ible particle which is believed to enter into the constitution of matter. This forms the basis of Dalton’s atomic theory of matter, as opposed to the older hypothesis that matter is continuous and infinitely divisible. The real nature of ultimate atoms has not been determined. Chemical theory and pipeiioe are still based on the atomic hypothesis. The molecule is defined to be the smallest particle of matter theoretically obtainable by mechanical subdivision. It is considered to be made up of two or more atoms separable only by chemical means. When two substances chemically react on one another, the action is looked upon as taking place between the molecules of the substances, resulting in an exchange of atoms with the formation of molecules with new atomic compositions—(i.e.) two new substances are formed. Atomic Weight.—Although the atom is ’ a theoretical conception, definite weights have been ed to the atoms of elements, the weight of the hydrogen atom being taken as the unit. The atomic weight of an element is decided from consideration of the laws of chemical combination, based on experimental results, together with the hypothesis of Avogadro re equal volumes of gases con the same number of molecules,