The modern definition of for compounds was realized gradually over the course of the last three centuries. The term 'perfectly mixt' was meant to distinguish compounds from 'imperfect' mechanical mixtures.
Definite composition is an essential characteristic of compounds included in the modern definition.
Many quantitative studies of metallic compounds like silver chloride in the 1700's lead to the recognition that these compounds contained fixed, characteristic percentages of metal. French chemist Joseph Louis Proust painstakingly determined the elemental composition of a series of metal oxides, hydroxides, and sulfides. His results were remarkably consistent. A compound is a substance which Nature assigns fixed ratios. Proust's new law was immediately challenged by contemporary Claude Berthollet.