Cartesian diver

The first written description of this device is provided by Raffaello Magiotti, in his book Renitenza certissima dell'acqua alla compressione (Very firm resistance of water to compression) published in 1648.

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.

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.

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.

When the pressure on the container is released, the air expands again, increasing the weight of water displaced and the diver again becomes positively buoyant and floats.

This positive reinforcement will amplify any departure from equilibrium, even that due to random thermal fluctuations in the system.

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 toy made from a drinking straw, paperclip and plastic bottle
  1. The trapped air in the straw makes the diver slightly buoyant, and it thus floats.
  2. Squeezing the bottle increases the water pressure; as the bubble shrinks, the diver's density increases and it sinks.