By 1877 sprays of both liquid oxygen and liquid nitrogen had been produced, and a new field of research, low temperature physics, had been opened. The van der Waals equation played a part in all this especially with respect to the liquefaction of hydrogen and helium which was finally achieved in 1908. From measurements of p 1 , T 1 and p 2 , T 2 in two states with the same density, the van der Waals equation produces the values b = v − − R ( T 2 − − T 1 ) / ( p 2 − − p 1 ) ] and a = v 2 ( p 2 T 1 − − p 1 T 2 ) / ( T 2 − − T 1 ) . Thus from two such measurements of pressure and temperature one could determine a and b , and from these values calculate the expected critical pressure, temperature, and molar volume. Goodstein summarized this contribution of the van der Waals equation as follows: