Final answer:
The subject is Physics and the motion of visible particles due to unseen molecules is called Brownian motion.
Step-by-step explanation:
The subject you are referring to is Physics. In the study of physics, the motion of visible particles due to unseen molecules is known as Brownian motion. This phenomenon was first observed by Robert Brown in 1827 when he noticed pollen grains moving in water due to the random thermal motions of fluid molecules colliding with the particles. The motion is caused by the statistical fluctuations in the numbers of molecules striking different sides of the visible particle. Although the individual molecules cannot be directly observed, their effects on the particle can be seen.