Boyle's law describes the inversely proportional relationship between the absolute pressure and volume of a gas, if the temperature is kept constant within a close system. Two of his friends discovered the law but Boyle confirmed it through experiments and published the results. His assistant, Robert Hooke, actually came up with the experiment apparatus. Boyle's law is based on experiments with air, which he considered to be a fluid of particles at rest in between small invisible springs. If an elastic fluid consisting of particles at rest, between which are repulsive forces inversely proportional to their distance, the density would be directly proportional to the pressure. This can relate to the real world. It is just like pushing in the plunger of a plugged-up syringe decreases the volume of air trapped under the plunger.