An excerpt from this article:
The great puzzles of the mid nineteenth century drew Loschmidt in—questions like “How big is a molecule?” Ask yourself how you’d answer that one—obviously not with a ruler. Loschmidt realized he could deduce the size of molecules from two kinds of information. One was a relation connecting the size of gas molecules to the distance they travel between collisions. The other was the packed volume of molecules in a cold liquid. He correctly found that a small molecule is around one nanometer in size.
He deduced the size of molecules by doing what??
How was he able to detect the size of gas molecules, and how was he able to detect the distance they travel between collisions?
How was he able to deduce the size of molecule via a packed volume of molecules in cold liquid?