Phlogiston and caloric

Before we look at Einstein's famous equation, we had better set the scene by describing how scientists had arrived at the concepts of energy and mass – the ‘E’ and ‘m’ in the equation. The gradual evolution of our present understanding of energy began with two ideas at least as curious as the infamous aether – ‘phlogiston’ and ‘caloric’. It provides us with some insight into the way that science progresses to look at why these two theories were invented and subsequently discarded.

Phlogiston was introduced towards the end of the seventeenth century by Georg Stahl, a German professor of medicine and chemistry, in an attempt to understand fire. Even in the latter part of the eighteenth century, many scientists still regarded fire as an element. Combustible materials were supposed to be made up of two parts -the calx, or ash, and the ‘phlogiston’. It was thought that, when a substance burned, the phlogiston was liberated, leaving the ash behind.