Editing Charles's law (section)

==Relation to absolute zero==
{{Main|Kelvin}}
Charles's law appears to imply that the volume of a gas will descend to [[absolute zero|zero]] at a certain temperature (−266.66&nbsp;°C according to Gay-Lussac's figures) or −273.15&nbsp;°C. Gay-Lussac was clear in his description that the law was not applicable at low temperatures:
<blockquote>
but I may mention that this last conclusion cannot be true except so long as the compressed vapours remain entirely in the elastic state; and this requires that their temperature shall be sufficiently elevated to enable them to resist the pressure which tends to make them assume the liquid state.<ref name="GL02" />
</blockquote>
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's law, and was aware that the law does not apply just above the boiling point of the liquid:
<blockquote>
I may, however, remark that when the temperature of the ether is only a little above its boiling point, its condensation is a little more rapid than that of atmospheric air. This fact is related to a phenomenon which is exhibited by a great many bodies when passing from the liquid to the solid-state, but which is no longer sensible at temperatures a few degrees above that at which the transition occurs.<ref name="GL02" />
</blockquote>

The first mention of a temperature at which the volume of a gas might descend to zero was by [[William Thomson, 1st Baron Kelvin|William Thomson]] (later known as Lord Kelvin) in 1848:<ref name="Kelvin48">{{citation | author = Thomson, William | author-link = William Thomson, 1st Baron Kelvin | year = 1848 | title = On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations | url = http://zapatopi.net/kelvin/papers/on_an_absolute_thermometric_scale.html | journal = Philosophical Magazine | pages = 100–06}}.</ref>
<blockquote>
This is what we might anticipate when we reflect that infinite cold must correspond to a finite number of degrees of the air-thermometer below zero; since if we push the strict principle of graduation, stated above, sufficiently far, we should arrive at a point corresponding to the volume of air being reduced to nothing, which would be marked as −273° of the scale (−100/.366, if .366 be the coefficient of expansion); and therefore −273° of the air-thermometer is a point which cannot be reached at any finite temperature, however low.
</blockquote>
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.<ref>{{citation | author = Thomson, William | author-link = William Thomson, 1st Baron Kelvin | year = 1852 | title = On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule's equivalent of a Thermal Unit, and M. Regnault's Observations on Steam | journal = Philosophical Magazine | volume = 4}}. [http://web.lemoyne.edu/~giunta/KELVIN1.html Extract.]</ref> Thomson did not assume that this was equal to the "zero-volume point" of Charles's law, merely said that Charles's law provided the minimum temperature which could be attained. The two can be shown to be equivalent by [[Ludwig Boltzmann|Ludwig Boltzmann's]] [[Entropy (statistical thermodynamics)|statistical view of entropy]] (1870).

However, Charles also stated:

:The volume of a fixed mass of dry gas increases or decreases by {{frac|1|273}} times the volume at 0&nbsp;°C for every 1&nbsp;°C rise or fall in temperature. Thus:

::<math>V_T=V_0+(\tfrac{1}{273}\times V_0 )\times T</math>

::<math>V_T=V_0 (1+\tfrac{T}{273})</math>

:where {{mvar|V<sub>T</sub>}} is the volume of gas at temperature {{mvar|T}} (in degrees [[Celsius]]), {{mvar|V<sub>0</sub>}} is the volume at 0&nbsp;°C.<!-- and possibly also by Clausius, I haven't checked; you have to check it anyway -->