Chemistry / [Raphael Meldola].

  • Meldola, Raphael, 1849-1915.
Date:
[1914?]
Catalogue details

Licence: In copyright

Credit: Chemistry / [Raphael Meldola]. Source: Wellcome Collection.

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    found. Methods of determining molecular weights had not been applied in Dalton’s time—the distinction between atoms and molecules had not been clearly appreciated. The Definiteness of Chemical Combination by Volume.—With the exception of the rela- tions between the weights and volumes of hydrogen and oxygen in water (p. 88), chemical combination by weight has alone been considered up to this stage. Now, the additional evidence required in order to decide definitely the relative weight of the atom of an element, when the relative weight of the mole- cule of a compound containing that element has been ascertained, is supplied, as hinted above, by the study of chemical combination by volume. This presupposes that we are dealing with elements or compounds which are or can be made gaseous—it is bulk as distinguished from mass that has now to be dealt with. But, before this part of the subject can be considered, the reader must be reminded of certain elementary physical principles. All gases expand when heated, or when the pressure upon them is reduced ; they contract when cooled, or when the pres- sure is increased. This is common knowledge ; but it can be stated in more precise terms. The actual change in volume which takes place on