thumb|right|Floating and sinking demonstration (Cartesian diver). The tube is filled with water and air. When pressing the bottle, the additional water enters the test tube, thus increasing the average density of the system tube-water-air, resulting with negative buoyancy and the tube sinks.
A Cartesian diver or Cartesian devil is a classic science experiment which demonstrates the principle of buoyancy (Archimedes' principle) and the ideal gas law. The first written description of this device is provided by Raffaello Magiotti, in his book (Very firm resistance of water to compression) published in 1648. It is named after René Descartes as the toy is said to have been invented by him.
The principle is used to make small toys often called "water dancers" or "water devils". The principle is the same, but the eyedropper is instead replaced with a decorative object with the same properties which is a tube of near-neutral buoyancy, for example, a blown-glass bubble. If the tail of the glass bubble is given a twist, the flow of the water into and out of the glass bubble creates spin. This causes the toy to spin as it sinks and rises. An example of such a toy is the red "devil" shown here.
The device also has a practical use for measuring the pressure of a liquid.
Plastic divers were given away in American cereal boxes as a free toy in the 1950s, and "Diving Tony," a version of the toy modelled after Kellogg's Frosted Flakes mascot Tony the Tiger, was made available in the 1980s.
Experiment description
[[File:cartesian_diver.svg|thumb|left|upright|A Cartesian diver toy made from a drinking straw, paperclip and plastic bottle
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The experiment requires a large water-filled bottle, inside which is a "diver": a small, rigid tube, open at one end, very similar to an eyedropper with just enough air so that it is nearly neutrally buoyant, but still buoyant enough that it floats at the top while being almost completely submerged. Two alternative "divers" can be constructed. One sealed but a flexible bulb, and the other a sealed glass bulb (flashlight minus metal base) with wool threads trailing below. The flexible one will compress reducing volume, and the solid glass one will not change, but air bubbles will be trapped in the fibres, and be exposed to the pressure – thus will change volume.
The "diving" occurs when the flexible part of the larger container is pressed inwards, increasing the pressure inside the larger container, causing the "diver" to sink to the bottom until the pressure is released, when it rises back to the surface. If the container is rigid, as with a glass bottle, the cork sealing the bottle would be pressed inwards or drawn outwards.