Page:The Harveian oration 1903.djvu/47

 stem, spring nearly all the other accepted theories of chemistry." The conception of an atom as the smallest conceivable portion into which an element can be divided, or that can enter into combination, and attaching to the idea of the atom a definite relative weight constant for atoms of the same element but differing with different elements, gave a satisfactory explanation for the laws of definite proportions and of multiple proportions which previously had been but 1ncompletely recognised.

Since, with the exception of a few elementary gases, an atom is always combined with one or more atoms of the same or of other elements, some term is required to denote the smallest portion of the substance capable of a separate existence, and for this the word "molecule" is employed. Built up on these fundamental ideas there has developed among other great generalisations the chemistry of the carbon compounds, and the hypothetical recognition of the relative arrangements of the atoms within the molecule—in short, chemical constitution or chemical structure. Now "the greater the valency of an element the more complicated are its combining ratios and the greater the possibility of its atoms forming numerous compounds with similar and dissimilar atoms." The atoms of carbon, which is the chief element in so-called organic bodies,