"Carnot proved... that the ratio of the work done by a perfect (i.e., a reversible) engine to the heat taken from the source is a function of the temperatures of the source and condenser only. When these temperatures are nearly equal, this function is... called Carnot's Function. W. Thomson, so early as 1848, seized upon this remarkable proposition, and made it the basis of the earliest suggestion of an absolute Thermometric Scale; absolute in the sense of being based upon strict thermodynamic principles, and entirely independent of the properties of any particular substance."