The logic of phlogiston

Science education should encourage students to make deductions and draw conclusions from experimental observations. However, at all levels, students often draw conclusions based on what they believe to be the expected ‘right answer’,¹ rather than by applying logic to their own observations. To try and teach students to look more critically at experimental results in the classroom, and beyond, the old theory of phlogiston can prove useful once again.

Good scientists apply logic to explain phenomena and develop theories, however, their inferences, arguments, and resulting conclusions, are not necessarily correct. The phlogiston theory, for example, was accepted for more than 100 years. The theory held that materials that burned contained a fire-like element that was released as the object burned. So well entrenched was the theory, that when Joseph Priestley discovered oxygen in 1774 he called it ‘dephlogisticated air’, believing that the mercuric oxide (or calx) that he heated with sunlight had adsorbed phlogiston, removing it from the surrounding air.

Throughout history, theories that fit the known evidence have been accepted, holding sway until further evidence either disproves them, or gives rise to an improved version. In such a fashion, science and knowledge progress, but to students learning about this progression, their own modern knowledge can colour how they view superseded theories, and even their own science. The theory of phlogiston may seem nonsensical nowadays, but suspending disbelief and examining the evidence can help students to appreciate why it was accepted. It also provides a useful exercise in applying logic. Banishing all thoughts of oxygen requires students to explain experimental observations without being influenced by the modern ‘right answer’.

Mike Tingle delves into the phlogiston theory and explains how it could be used as a teaching tool, suggesting a practical experiment which could aid teaching this topic.