CHAPTER XIV.
THE MODERN FORM OF THE ALCHEMICAL QUEST OF THE ONE THING.
The study of the properties of the elements shows that these substances fall into groups, the members of each of which are like one another, and form compounds which are similar.

The examination of the properties and compositions of compounds has shown that similarity of properties is always accompanied by similarity of composition.

Hence, the fact that certain elements are very closely allied in their properties suggests that these elements may also be allied in their composition.

Now, to speak of the composition of an element is to think of the element as formed by the union of at least two different substances; it implies the supposition that some elements at any rate are really compounds.
The fact that there is a very definite connexion between the values of the atomic weights, and the properties, of the elements, lends some support to the hypothesis that the substances we call, and are obliged at present to call, elements, may have been formed from one, or a few, distinct substances, by some process of progressive change.

If the elements are considered in the order of increasing atomic weights, from hydrogen, whose atomic weight is taken as unity because it is the lightest substance known, to uranium, an atom of which is 240 times heavier than an atom of hydrogen, it is found that the elements fall into periods, and the properties of those in one period vary from element to element, in a way which is, broadly and on the whole, like the variation of the properties of those in other periods.