We describe an approach to parallel computation using particle propagation and collisions in a one-dimensional cellular automaton using a particle model -- a Particle Machine (PM). Such a machine has the parallelism, structural regularity, and local connectivity of systolic arrays, but is general and programmable. It contains no explicit multipliers, adders, or other fixed arithmetic operations; these are implemented using fine-grain interactions of logical particles which are injected into the medium of the cellular automaton, and which represent both data and processors. We give parallel, linear-time implementations of addition, subtraction, multiplication and division.