Ever heard of Edward Craven Walker? Well, you probably don’t know him; but you would know of his invention, the lava lamp.
Invented in 1963, the ‘lamp’ vessel comprises of a wax mixture within a liquid, with the base of the vessel housing an incandescent light. When the light is switched on, the wax mixture heats up and becomes less dense than the liquid and hence rises to the top, where it cools and falls down again.
Fun fact – wax is naturally less dense than water and floats, so Craven came up with a special wax mixture that included the chemical tetrachloride, making the wax mixture heavier at room temperature. In the 1970s, the US banned the toxic substance, so Craven invented a new ‘compliant’ mix, which remains a trade secret of his company, Mathmos limited.