In chemistry, the law of definite proportions, sometimes called Proust's Law, states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition. For example, oxygen makes up 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry. (wikipedia)

"The equivalent weight of an element is the weight which will combine with or displace one part by weight of hydrogen. Elements always combine with one another in definite proportions by weight, depending on their equivalent weights." (Dalton, 1806)

See Also

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3.13 - Reciprocals and Proportions of Motions and Substance
6.8 - Proportionate and Relative Geometries
9.12 - Velocity of Sound and its Propagation Rate are Proportional
12.00 - Reciprocating Proportionality
13.15 - Principle of Proportion
Figure 14.10 - Proportionate Tonal Relations dictate Contraction or Expansion
Figure 6.17 - Areas and Volumes - Relations and Proportions
Figure 6.19 - Sphere to Cube - Relations and Proportions
law of constant composition
law of multiple proportions
Laws
Proportion
proportional
Reciprocating Proportionality
Table 2 - Controlling Modes and Proportions