Yesterday I was thinking about how they originally demonstrated that atoms exist. The traditional story is, of course, that Einstein showed in his 1905 paper that Brownian motion was explicable in terms of atoms, but it occurred to me that I remembered another experiment from when I was at school that also demonstrated the existence of atoms, and I wondered why noone used it as an argument before 1905.

The experiment in question was the one where you mix 50ml of one chemical 50ml of another (I can't remember which chemicals they were, and I imagine there are many that you could pick from and see the same effect) and find that the mixture only occupies 97ml. This can be explained by imagining that the molecules of one of the chemicals are smaller than those of the other, so could occupy some of the gaps between the bigger molecules. Just as if you mixed half a cup of coffee beans with half a cup of sugar, some of the sugar would fill in the gaps between the beans and so the mixture would not occupy the full cup.

Does anyone know if this effect had been noticed before the Einstein paper? Were there any explanations put forward that didn't require atoms?

Thanks for any answers.