Gay-Lussac's 1801 experiments establishing the law of volumes for gases are brilliantly simple, and he described them with a level of detail that was new to physics writing. But he did not present his actual measurements or tell us how he analyzed them to conclude that between 0 to 100 °C, a volume of any gas will expand by about 37.5%. We review his experiments and conclude that he measured initial and final volumes at slightly different pressures. By using the gas laws and his apparatus diagrams, we corrected his data so that they correspond to constant pressure. His corrected results give ∆V/V=36.6%, the currently accepted value for nearly ideal gases. Aside from their intrinsic interest, our analyses can provide students intriguing applications of the gas laws and Pascal's law and motivate them to consider Pascal's paradox. We also note the influence of ballooning and of the French Revolution on Gay-Lussac.