What were Einstein's reasons for the work on viscosity/Brownian motion? About 1905 Einstein published a work about diffusion of hard spheres and Brownian motion. One effluence of that is the so called "viscosity equation" which was/is very important for determining the molecular weight of macromolecules by the viscosity of solutions.

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*http://www.ias.ac.in/initiat/sci_ed/resources/chemistry/Viscosity.pdf (Link dead now, archived here)


*Wikipedia: Intrinsic viscosity
What I'd like to know: Was there any relation to the other topics Einstein worked on? Is something known on Einstein's reason to work on viscosity?
 A: From my studies of Einsteins work I am under the impression that the most important motive for his works stems from his interest in understanding physics from the point of view that matter exists of atoms and molecules that follow paths in a space-time continuum.  
In the sense that he was strongly influenced by the idea of a deterministic-materialistic universe - the Kant-Laplace universe. This point of view easily explains why he couldn't accept quantum physics where the concept of probablities - without underlying mechanism - supersedes determinism.
I believe the comment of user346 is important: "[...] the atomic hypothesis was not universally accepted before 1905 [...] a direct link between the atomic hypothesis and the observed macroscopic properties of matter".
In my view his work on the SRT has the same motive: understanding space-time relations from the point of view of light rays imagined as moving more or less like macroscopic objects.
A: Although the notion of viscosity in gases and liquids was known well before 1906, as well as its derivation from statistical mechanics, the atomistic structure of matter was still under experimentally discovering, so it was a hot topic at that time.
