3.
In the natural sciences, and particularly in chemistry, generalities must come after the detailed knowledge of each fact and not before it.
Joseph Louis Gay-Lussac
5.
Compounds of gaseous substances with each other are always formed in very simple ratios, so that representing one of the terms by unity, the other is 1, 2, or at most 3 ... The apparent contraction of volume suffered by gas on combination is also very simply related to the volume of one of them.
Joseph Louis Gay-Lussac