**P**. 211, **Paris**, 1869) **proposed** **an** **equation** **of** **the** **form** (**p**+**po**)(**v** - **b**) =**RO**, **in** **which** **the** **effect** **of** **the** **size** **of** **the** **molecules** **is** **represented** **by** **subtracting** **a** **quantity** **b**, **the** " **covolume**," **from** **the** **volume** **occupied** **by** **the** **gas**, **and** **the** **effect** **of** **the** **mutual** **attractions** **of** **the** **molecules** **is** **represented** **by** **adding** **a** **quantity** **po**, **the** **internal** **pressure**, **to** **the** **external** **pressure**, **p**. **This** **type** **of** **equation**, **was** **more** **fully** **worked** **out** **by** **van** **der** **Waals**, **who** **identified** **the** **internal** **pressure**, **po**, **with** **the** **capillary** **pressure** **of** **Laplace**, **and** **assumed** **that** **it** **varied** **directly** **as** **the** **square** **of** **the** **density**, **and** **could** **be** **written** **a**/**v** 2 .

**The** **introduction** **of** **the** **covolume**, **b**, **into** **the** **equation** **is** **required** **in** **order** **to** **enable** **it** **to** **represent** **the** **behaviour** **of** **hydrogen** **and** **other** **gases** **at** **high** **temperatures** **and** **pressures** **according** **to** **the** **experiments** **of** **Amagat**.

**B**, **Covolume** **of** **molecules** **of** **gas**.