text: The partial volume of a particular gas in a mixture is the volume of one component of the gas mixture. It is useful in gas mixtures, e.g. air, to focus on one particular gas component, e.g. oxygen.
  It can be approximated both from partial pressure and molar fraction:
  * V is the partial volume of an individual gas component X in the mixture
  * V is the total volume of the gas mixture
  * p is the partial pressure of gas X
  * p is the total pressure of the gas mixture
  * n is the amount of substance of gas X
  * n is the total amount of substance in the gas mixture