Boyle's Law is a statement of the relationship
between the pressure and volume of gasses. Specifically it states that
under isothermic conditions, i.e. when temperature remains constant, the product of the pressure and volume
remains constant, or
P1 x V1 = P2 x V2
where P1 is the pressure before some change, V1 is the volume before the change, P2 and V2 are the new values after the change.