Page:Outlines of Physical Chemistry - 1899.djvu/22

 2 OUTLINES OF PHYSICAL CHEMISTRY

2. The Law of Constant Proportions. — In order to form a substance, it is always necessary to have the same elements united in the same proportions.

8. The Law of Multiple Proportions. — When two elements can unite in several different proportions to give distinct compounds, there is always a simple numerical rela- tion between the different quantities of one of the elements which unite with the same quantity of the other element.

We may add :

4. The Law of Beciprocal Proportions. — The weights of different elements which unite with a constant weight of another element (taken as standard of comparison) express also, directly or after multiplication by a simple number, the proportional weights of these elements which combine amongst themselves.

These laws, which were known at the beginning of this century, still remain the exact expression of observed facts. They are best interpreted by the atomic hypothesis.

We assume that matter is made up of extremely small particles which we call atoms. The atoms constitute the limit of divisibility by chemical means, and exist in as many species as there are non-divisible substances (elements). Atoms may combine to form molecules. The molecule of a compound consists of atoms of different species. The mole- cule of an element may consist of a single atom, or it may be formed by the union of several atoms of the same species.

The Atoms have a Fixed and Invariable Weight. — With these ideas it is easy for us to comprehend the raison d'&tre of the above-mentioned laws.

In fact :

1. The weight of a molecule will be equal to the sum of the weights of the atoms which have united to form it. It is sufficient then to consider a conglomeration of n molecules, that is to say, a certain weight of a chemical compound, and we arrive at our first fundamental law.

2. The formation of any given molecule (the chemical individual of a substance) requires m atoms of one species,

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