Lava lamps and their use

Liquid movement lamps, or popularly known as lava lamps, are a decorative element of great attraction. It was invented by the Englishman Edward Craven-Walker in 1963. Although it was very popular in the 1970s among young people, especially in the hippie subcultures. For the design of this lamp, Craven-Walker was inspired by the silhouette of an hourglass he saw in a tavern in Dorset (UK).
In the late 1960s, they began appearing in British television series, and in 1980 they arrived in Hollywood with the Superman II film. It was in the 1990s that the item became most successful and millions of lava lamps were manufactured in the UK.
This curious contraption works on the basis of fluid density. The secret lies in constantly changing the density of the lava. The heat given off by the hidden bulb is responsible for increasing the density of the lava. Therefore, once the lava is hot, it rises because it is less dense than the water it is in. Once it reaches the top, it loses its heat and descends, repeating this process continuously. When the lamp is turned off and cools down, the wax goes to the bottom of the vessel and can hardly be seen.
Every lava lamp consists of a metal base in the shape of a cone and inside this is the bulb and the electrical cable connection with its switch. The water and the wax are the two main fluids, contained in a glass container. At the top of this container there is a small amount of air and a metal lid, which are hidden by the small cone-shaped top cover.
The lamp should have a heating period of about 30 minutes until the wax starts to flow from top to bottom due to the density it acquires. It should be noted that this object is certainly dangerous, it should not be placed near cookers, fireplaces or any other heat source. The lid of the fluid container must not be opened under any circumstances. The mixture must not be shaken while the lamp is in operation, even if it is hot; it is advisable not to leave it on for more than 6 hours to prevent it from overheating. 
It must be said that the main function of this object is not to give light to a room, but to create a certain atmosphere of tranquillity and relaxation.