It is impossible to obtain in the market absolutely chemically pure
oxygen. All the oxygen that we have thus far been able to purchase
contains nitrogen and, in some instances, measurable amounts of
water-vapor and carbon dioxide. The better grade of oxygen, that
prepared from liquid air, is practically free from carbon dioxide and
water-vapor, but it still contains nitrogen, and hence with every liter
of oxygen admitted there is a slight amount of nitrogen added. This
amount can readily be found from the gasometric analysis of the oxygen
and from the well-known relation between the weight and the volume of
nitrogen the weight can be accurately found. This addition of nitrogen
played a very important rôle in the calculation of the oxygen
consumption as formerly employed. As is seen later, a much abbreviated
form of calculation is now in use in which the nitrogen admitted with
the oxygen does not influence the calculation of the residual oxygen.