William John Macquorn Rankine (1820-1872), a Scottish engineer and physicist who was a founding contributor to the science of thermodynamics. Rankine developed a complete theory of the steam engine and his manuals on the subject were used for many decades. In 1853, he stated, in the Law of Conservation of Energy, that "all different kinds of physical energy in the universe are mutually convertible." He also invented an absolute temperature based on the interval of one degree Fahrenheit termed the Rankine temperature scale. Absolute zero, or 0° R, is the temperature at which molecular energy is at a minimum, corresponding to a temperature of 459.67 °F. Another absolute temperature scale, the Kelvin temperature scale, is more commonly used for scientific measurements today.