Introduction to physical chemistry / by James Walker.

  • Walker, James, Sir, 1863-1935.
Date:
1919
Catalogue details

Licence: In copyright

Credit: Introduction to physical chemistry / by James Walker. Source: Wellcome Collection.

    34/462 (page 12)
    contain at least two atoms of nitrogen and two atoms of oxygen, the molecule of nitrogen must contain at least two atoms of nitrogen, and the molecule of oxygen two atoms of oxygen. The general adoption of this one principle has practically proved sufficient to fix the combining weights of the elements, and provide us with our present system of atomic weights. Although Avogadro formulated his hypothesis in 1811, the times were not ripe for it; and it was only after 1858, when Cannizzaro published a sketch of its con- sistent application to organic and inorganic chemistry, that its adoption became at all general. In the middle of the century the greatest confusion prevailed, different writers using different systems of com- bining weights, so that each compound had as many different formulae as there were competing tables of equivalents. So long as weights only were considered, one system was as good as another; but when volume relations, physical properties, and the nature of the substances produced in chemical actions are taken into account, the present system of atomic weights is the only one which gives a simple and consistent expression of the results. The hypothesis of Avogadro often goes under the name of Avogadro’s Law, but it must be borne in mind that it is a purely hypothetical statement, and not to be confounded with a generalised expression of fact such as the Law of Constant Proportions. This latter is independent of any theory, while the former is entirely theoretical—a fact which of course in no way impairs its usefulness. From another point of view Avogadro’s principle may be looked upon as giving a definition of the magnitude we know as the gram-molecular weight, or molar weight (p. 14). In the following pages a sketch is given of the mode employed in fixing our present system of atomic weights with the help of Avogadro’s principle. We are not for this purpose concerned with the absolute weight of atoms and molecules—all that we require is to express their weights relatively to each other. To begin with, a Standard must be chosen to which all chemical weights are to be referred. The standard is purely arbitrary, and in choosing it we only consult our own convenience. Two elements, hydrogen and oxygen, have been selected on practical grounds for this purpose. Dalton chose hydrogen because it had the smallest equivalent of the elements ; Berzelius chose oxygen because it was easy to compare the equivalents of the other elements with that of oxygen directly. Hydrogen does not form many well-defined and easily-analysed com- pounds with the other elements, so that comparison with the equiva- lent of hydrogen has usually to be indirect, frequently through the medium of oxygen. Oxygen has now been definitely chosen by inter- national agreement as the standard element, and its atomic weight has been fixed as 16 exactly. The actual number selected as standard magnitude is purely arbitrary ; Dalton adopted 1 as the atomic weight