A glass tube turned pressure into something measurable
The crucial trick was to trap a sample of air in one arm of a bent glass tube, then add mercury to the open arm. The mercury’s weight pushed on the trapped air. Because the tube was narrow and marked, Boyle could compare how much space the same sample occupied as the pressure changed.
The height of the mercury column served as a pressure gauge. A taller column meant a stronger push on the air; the difference in mercury levels also let the experimenter account for the pressure of the surrounding atmosphere. Mercury was useful because it is exceptionally dense: a manageable amount could create a substantial pressure in a glass apparatus.
Why the shrinking air mattered
The air did not vanish when compressed. Its molecules were crowded into a smaller volume, so they struck the container walls more often. Those collisions create gas pressure. Add pressure, and the gas settles into less space; reduce pressure, and it expands again.
The comparison only works cleanly when the temperature stays roughly constant. Heating a gas can make it expand, adding another effect that would blur the relationship Boyle was testing. Under steady-temperature conditions, pressure and volume move in opposite directions: double the pressure, and the volume is about half.
From a demonstration to a rule
The mercury experiment made an invisible property of air—the pressure it exerted—legible as a column’s height. Repeating the comparison at different pressures revealed a consistent pattern, not just a one-time squeeze. That pattern became Boyle’s law, a practical way to predict how a gas’s volume changes when pressure changes. It helps explain why a trapped air pocket compresses under a piston and expands when the load is lifted.
