Boyle's Law
Aiton's Encyclopedia (1910)
in physics, a principle relating to the elasticity of gases. Every boy who has a popgun knows that air can be compressed;--that the harder he pushes, the smaller the space into which the air is crowded. The smaller the space into which the air is compressed, the greater its expansive force, and the farther it will throw the pellet. About 1662 Robert Boyle, a member of the Royal Society of London for Improving Natural Knowledge, reduced these observations to a principle known as Boyle's Law. It may be stated in two ways. Both come to the same thing: The temperature remaining the same, the volume of a given body of gas varies inversely as the pressure upon it. Or, At constant temperature, the product of the pressure and the volume of a given mass of gas is a constant. Boyle means by this that if twice the pressure be exerted on a given mass of air, it will occupy half as much space, and that if four times the pressure be exerted, the gas will be crowded into onefourth the space. This statement seems exceedingly elementary at the present day, when every high school boy is expected to be familiar with it; but it should be remembered that in Boyle's day philosophers were not through yet with the search for some method of converting the baser metals into gold. The same principle was discovered about 1676 by a Frenchman named Mariotte; hence in French school books the principle is called Mariotte's Law. It should be stated that Boyle's and Mariotte's Law does not hold good for quite all cases; nor does it hold good at high pressures. Stated popularly, one hundred times the pressure does not reduce gas to one one-hundredth of its former volume. Boyle's Law holds good, however, for all ordinary purposes.
What Has Changed in the Last Century
Aiton's 1910 statement of the law is exactly what a physics class is taught today: at constant temperature, pressure times volume is a constant. Aiton's caveat that the law fails at high pressures had in fact already been accounted for by Johannes van der Waals, whose 1873 equation added correction terms for molecular volume and attraction and won him the Nobel Prize in 1910; Boyle's relation is now understood as the limiting behaviour of an ideal gas, and is taught alongside Charles's and Avogadro's laws as part of the combined ideal gas law. The main shift is in historical credit rather than physics: modern historians stress that the inverse relation was suggested to Boyle by Richard Towneley and Henry Power, and that Robert Hooke did much of the bench work, while French textbooks' preference for "Mariotte's Law" persists as Aiton noted.
Written for this edition, 2026.