Similarly, the element iron combines with oxygen to form two oxides, one of which is black and the other red. By analysis it has been shown that the former contains 1 part by weight of iron combined with 0.286 parts by weight of oxygen, while the latter contains 1 part by weight of iron combined with 0.429 parts by weight of oxygen. Here again we find that the amounts of oxygen which combine with the same fixed amount of iron to form the two compounds are in the ratio of small whole numbers, viz., 2:3. Many other examples of this simple relation might be given, since it has been found to hold true in all cases where more than one compound is, formed from the same elements. Dalton's law of multiple proportion states these facts as follows: _When any two elements,_ A _and_ B, _combine to form more than one compound, the amounts of_ B _which unite with any fixed amount of_ A _bear the ratio of small whole numbers to each other_. ~Hypothesis necessary to explain the laws of matter.~ These three generalizations are called _laws_, because they express in concise language truths which are found by careful experiment to hold good in all cases. They do not offer any explanation of the facts, but merely state them. The human mind, however, does not rest content with the mere bare facts, but seeks ever to learn the explanation of the facts. A suggestion which is offered to explain such a set of facts is called an _hypothesis_. The suggestion which Dalton offered to explain the three laws of matter, called the _atomic hypothesis_, was prompted by his view of the constitution of matter, and it involves three distinct assumptions in regard to the nature of matter and chemical action. Dalton could not prove these assumptions to be true, but he saw that if they were true the laws of matter become very easy to understand. ~Dalton's atomic hypothesis.~ The three assumptions which Dalton made in regard to the nature of matter, and which together constitute the atomic hypothesis, are these: 1. All elements are made up of minute, independent particles which Dalton designated as _atoms_. 2. All atoms of the same element have equal masses; those of different elements have different masses; in any change to which an atom is subjected its mass does not change. 3. When two or more elements unite to form a compound, the action consists in the union of a definite small number of atoms of each element to form a small particle of the compound. The smallest particles of a given compound are therefore exactly alike in the number and kinds of atoms which they contain, and larger masses of the substances are simply aggregations of these least particles.