The development of the atomic theory.

  • Andrew Norman Meldrum
Date:
[1910-1911]
Catalogue details

Licence: In copyright

Credit: The development of the atomic theory. Source: Wellcome Collection.

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    general, though not the universal opinion was, that this vapour was present in a state of combination with the air. The evaporation of water was thought to be an act of chemical combination between air and water, whilst boiling was a physical action. For since the atmospheric pressure prevents water from boiling at ordinary tem- peratures, it was thought that boiling was something quite distinct from evaporation, which takes place at all temperatures and pressures of the air. This distinction had received the sanction even of Lavoisier.8 Dalton’s theory had a special bearing on this subject. For the theory meant that the pressure of a mixture of gases is the sum of the respective pressures of the gases in the mixture. Dalton saw that the water vapour in the atmosphere had to be considered in terms of the pressure of the vapour. Experimentally he showed that the evaporation of water is proportional to the pressure of the vapour which the water gives off. At any given temperature there is a maximum which this pressure can reach, and water, whether in contact with the air or not, can evaporate till the pressure of its vapour reaches this maximum and no further. On the other hand, air in which the water vapour is not at this maximum pressure •can be cooled till the maximum is reached, and then, on further cooling, the water is deposited as dew. This led to observations of the “ dew-point,” which Dalton was the first to institute. It was thus in the direction of Meteorology that Dalton’s theory first bore fruit. In this science, as Play- fair has pointed out, it is easier than in any other to “ accumulate observations, and more difficult to ascertain principles.” At the beginning of the nineteenth century, by pointing out the significance of the dew-point, Dalton 8 “Traite Elementaire de Chimie,” 3rd ed., pp. 7-11, 39,
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