  In what we today think of as chemistry and physics, Gassendi develops a great range of applications for his atomist hypothesis. In his optics, the atomist theory of light provides a counter to Descartes's view of light as pressure. For Gassendi, light is a property carried by particular atoms (atomi lucificae) that are identical with heat atoms. These tend to travel at greater than average velocity because they generally have fewer obstacles in their paths than most atoms (O I 422a-432b). Sound, too, is particulate, and travels as such particles travel. In contrast to a typical sound-wave view, the surrounding medium does not play an important role: Gassendi takes the velocity of sound particles, like that of light particles, to be invariant with respect to the air or wind in which they travel. In an experiment he models after a similar one of Mersenne's, he judges the velocity of sound to be 1,473 ft/sec [478 meters/sec] (his calculations are off by 435 ft/sec [146.7 meters/sec]) and invariant to the pitch of the sound (O I 418a-419a).  Even planetary motion has an ultimately atomist explanation, as the underlying forces driving the planets are magnetic forces borne by dedicated atoms (this is an atomist modification of a view developed, though later abandoned, by Kepler).