Qué concluyó Proust en relación a la ley de las proporciones definidas?

Proust concluded that the elements in a chemical compound always present in the same ratio by mass.

Explanation:

In English, this finding became known as the Law of Definite Proportions.

For example, a 100 g sample of copper(II) sulfide always contains 66.46 g of copper and 33.54 g of sulfur.  

The mass ratio in CuS is

Cu/S = 66.46/33.54 = 1.982/1

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.