The atomic model became undisputable after Jean Perrin's experiments in 1908 https://en.wikipedia.org/wiki/Jean_Baptiste_Perrin
And only after being prompted by the investigations of Einstein published in 1905 https://en.wikipedia.org/wiki/%C3%9Cber_die_von_der_molekula...
The size of this mystery gets forgotten in modern discussion - but getting to the bottom on how the world is built was among the biggest triumphs of modern world. And the people who lived before the 20th century should not be belittled for lacking this knowledge.
https://en.wikipedia.org/wiki/Atomic_theory#Avogadro
"Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various other elements, and made a clear distinction between molecules and atoms." (Since 1811).
Mach and similar simply denied what was already discovered, often not even wanting to consider that, sticking to their beliefs. Mach was "physicist and philosopher," not a chemist. Not having a definitive proof for physical existence doesn't mean that chemistry haven't already used the concepts.
Today too there are scientists who after they became prominent for some work start to spread the false beliefs, some realizing that at the moment they are too hard to disprove, even when most of the experts don't believe them. And some get more influence than it would be scientifically valid. The progress is not something automatically guaranteed, it's always the result of hard work and perseverance.
For the people reading this who know more about science than history of science (like me), this is referring to cathode rays and experimental verification of Einstein's "constant collisions from molecules" Brownian motion model.
>but getting to the bottom on how the world is built was among the biggest triumphs of modern world.
We are not at the bottom yet. :)
https://en.wikipedia.org/wiki/Ernst_Mach
Newton died in 1727. He followed however not only unscientific paths in alchemy, but also in his analysis of the ancient texts. Both reflect the time he lived and his own beliefs, which influenced these endeavors. With hindsight, we know that these investigations didn't advance humanity's knowledge, but are interesting for history. It can be that he even discovered something scientifically interesting while doing alchemy, but he had his reasons to try to hide that work and apparently some of these papers never survived to this day. And what survived wasn't public until 20th century, contrary to the principles of science.