Charles law appears to imply that the volume of a gas will descend to zero at a certain temperature (−266.66 °C according to Gay-Lussacs figures) or −273.15 °C. Gay-Lussac was clear in his description that the law was not applicable at low temperatures:
At absolute zero temperature, the gas possesses zero energy and hence the molecules restrict motion.
Gay-Lussac had no experience of liquid air (first prepared in 1877), although he appears to have believed (as did Dalton) that the "permanent gases" such as air and hydrogen could be liquified. Gay-Lussac had also worked with the vapours of volatile liquids in demonstrating Charles' law, and was aware that the law does not apply just above the boiling point of the liquid:
The first mention of a temperature at which the volume of a gas might descend to zero was by William Thomson (later known as Lord Kelvin) in 1848:
However, the "absolute zero" on the Kelvin temperature scale was originally defined in terms of the second law of thermodynamics, which Thomson himself described in 1852. Thomson did not assume that this was equal to the "zero-volume point" of Charles law, merely said that Charles law provided the minimum temperature which could be attained. The two can be shown to be equivalent by Ludwig Boltzmann's statistical view of entropy (1870).
However, Charles also stated:
:The volume of a fixed mass of dry gas increases or decreases by  times the volume at 0 °C for every 1 °C rise or fall in temperature. Thus:
:where  is the volume of gas at temperature ,  is the volume at 0 °C.