20 Boyle’s Law
Two weeks later Robert Boyle stood before the society with a large glass tube that he
had shaped into a lopsided “U.” One side of the “U” rose over three feet high and was
skinny. The other side was short and fat. The short side was sealed at the top. The tall,
skinny side was open.
Boyle poured liquid mercury into his tube until it covered the bottom of the “U” and
rose just a little in both sides. A large pocket of air was trapped above this mercury in the
short fat side. A piston, Boyle explained, was any devise that compressed air. Since his used
mercury to compress air, there would be no friction to affect the results—as had been true in
the French experiment.
Boyle recorded the glass piston’s weight and etched a line in the glass where mercury
met the trapped air pocket. Boyle trickled liquid mercury down the long neck of the tall side
of his piston until he had filled the neck. Mercury now rose well over halfway up the short
side. The trapped air had been squeezed to less than half of its original volume by the weight
and force of mercury.
Boyle drew a second line on the short chamber to mark the new level of mer cury
inside—marking the compressed volume of trapped air.
He then drained mercury through a valve at the bottom of the “U” until the glass piston
and mercury weighed exactly the same as they had at the beginning. The mercury level returned to its exact starting line. The trapped air had sprung back exactly to where it started.
Air was perfectly springy. The French were wrong. Boyle was right.
Robert Boyle continued the experiments with his funny glass piston and noticed something quite remarkable. When he doubled the pressure (weight of mercury) on a trapped
body of air, he halved its volume. When he tripled the pressure, the air’s volume was reduced to one-third. The change in volume of air when compressed was always proportional
to the change in the pressure squeezing that air. He created a simple mathematical equation
to describe this proportionality. Today we call it “Boyle’s Law.” No other concept has been
more useful in understanding and using gasses to serve the needs of humankind.
Fun Facts: Oceanographer Sylvia Earle set the women’s depth record
for solo diving (1,000 meters or 3,281 feet). According to the concept
Boyle discovered, pressure at that depth is over 100 times what it is at the
surface!
More to Explore
Boyle, Robert. The Skeptical Chemist. New York: Dover, 2003.
Hall, Marie. Robert Boyle on Natural Philosophy. Bloomington: Indiana University
Press, 1995.
Hunter, Michael. Robert Boyle Reconsidered. New York: Cambridge University
Press, 2003.
Irwin, Keith. The Romance of Chemistry. New York: Viking Press, 1996.
Tiner, John. Robert Boyle: Trailblazer of Science. Fenton, MI: Mott Media, 1999.
Wojcik, Jan. Robert Boyle and the Limits of Reason. New York: Cambridge University Press, 2003.