On The Movement of Small Particles Suspended in Stationary Liquids Required By The Molecular-Kinetic Theory of Heat .
Einstein's paper on diffusion can be found here: 
In this paper, Einstein uses the kinetic-molecular theory of heat to describe how "bodies of microscopically visible size suspended in liquids" move randomly through the medium
By entertaining the possibility that the motion he described in his paper is the same as "Brownian molecular motion", Einstein makes a significant contribution to the debate between classical thermodynamics and the kinetic-molecular theory of heat (which later developed into the field of statistical mechanics).
"If it is really possible to observe the motion to be discussed here, along with the laws it is expected to obey, then classical thermodynamics can no longer be viewed as strictly valid even for microscopically distinguishable spaces, and an exact determination of the real size of atoms becomes possible. Conversely, if the prediction of this motion were to be provided wrong, this fact would provide a weighty argument against the molecular-kinetic conception of heat"Einstein, A.
Context for This Article
In 1905 (when this paper was written), there was a controversy between two schools of thought:
-a model that provided an exact description of the behavior of macroscopic materials by treating them as a continuum (as opposed to a set of discrete particles) -a deterministic model - could be used to determine how a material would behave with a change in pressure or temperature (for example)
Molecular Kinetic Theory of Heat:
- Boltzmann and Maxwell had developed a model that eventually developed into statistical mechanics -model that described matter as the movement and interrelations of a large number of discrete particles (on the order of Avogadro's number) -a statistical model - views everything (pressure, temperature, etc) as a statistical quantity Andelman, Diamant
Einstein begins his paper by pointing out the discrepancy between classical thermodynamics and the molecular-kinetic theory of heat in predicting the osmotic pressure produced by small suspended bodies
1. Einstein, A. "On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat", Annalen der Physik, 17, 1905, 549-560 2. Andelman, D., Diamant, H. "100 Years Since Einstein's Less Known Revolution: From the pollen dance to atoms and back"