§ =76.= With the acceptance of Dalton’s Atomic Theory, it became necessary to determine the atomic weights of the various elements, _i.e._, not the absolute atomic weights, but the relative weights of the various atoms with reference to one of them as unit.[92] We cannot in this place enter upon a discussion of the various difficulties, both of an experimental and theoretical nature, which were involved in this problem, save to remark that the correct atomic weights could be arrived at only with the acceptance of Avogadro’s Hypothesis. This hypothesis, which is to the effect that equal volumes of different gases measured at the same temperature and pressure contain an equal number of gaseous molecules, was put forward in explanation of a number of facts connected with the physical behaviour of gases; but its importance was for some time unrecognised, owing to the fact that the distinction between atoms and molecules was not yet clearly drawn. A list of those chemical substances at present recognised as “elements,” together with their atomic weights, will be found on pp. 106, 107. [92] Since hydrogen is the lightest of all known substances, the unit, Hydrogen = 1, was at one time usually employed. However, it was seen to be more convenient to express the atomic weights in terms of the weight of the oxygen-atom, and the unit, Oxygen = 16 is now always employed. This value for the oxygen-atom was chosen so that the approximate atomic weights would in most cases remain unaltered by the change. Prout’s Hypothesis.