R. Boyle, A Defence of the Doctrine Touching the Spring and Weight of the Air (London: Thomas Robinson, 1662). Available online at Spain's La Biblioteca Virtual de Patrimonio Bibliográfico. Boyle presents his law in "Chap. V. Two new experiments touching the measure of the force of the spring of air compress'd and dilated", pp. 57–68. On p. 59, Boyle concludes that "the same air being brought to a degree of density about twice as that it had before, obtains a spring twice as strong as formerly". That is, doubling the density of a quantity of air doubles its pressure. Since air's density is proportional to its pressure, then for a fixed quantity of air, the product of its pressure and its volume is constant. On page 60, he presents his data on the compression of air: "A Table of the Condensation of the Air." The legend (p. 60) accompanying the table states: "E. What the pressure should be according to the Hypothesis, that supposes the pressures and expansions to be in reciprocal relation." On p. 64, Boyle presents his data on the expansion of air: "A Table of the Rarefaction of the Air."
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In 1662, he published a second edition of the 1660 book New Experiments Physico-Mechanical, Touching the Spring of the Air, and its Effects with an addendum Whereunto is Added a Defence of the Authors Explication of the Experiments, Against the Obiections of Franciscus Linus and Thomas Hobbes; see J Appl Physiol 98: 31–39, 2005. (Jap.physiology.org Online.)
