What is a Cartesian diver?
A Cartesian diver is an experiment used to demonstrate the relationship between density and buoyancy.
Density describes how much matter is in a certain volume. Imagine filling two measuring cups, one with vegetable oil and the other with water. Now imagine placing those cups on a kitchen scale. You would find that one cup of vegetable oil has a mass of 223 grams and one cup of water has a mass of 240 grams. Vegetable oil has less matter in one cup than water, so vegetable oil is less dense than water.
Buoyancy is the ability of an object to float in water. If you poured the vegetable oil and water into the same container, the vegetable oil would be buoyant and float on the water.
You can change the density and buoyancy of a Cartesian diver at will, making it float or sink (hence Cartesian “diver”).
Cartesian divers are thought to be named for René Descartes. You may know them by another name, such as Cartesian devils, water devils, water dancers, or bottle imps. A Cartesian diver is made up of several parts: a bottle or vessel filled with water, a lid or an air-tight membrane, and a “diver” (often a piece of a straw or a flameworked glass object).
How does it work?
On the table in front of you sits a Cartesian diver. The diver is floating because it is less dense than the water. If you apply pressure to the vessel, the gas within the diver is compressed, and the diver’s density increases to the point that is no longer able to float in the water. Thus, the diver lives up to its name and sinks to the bottom. However, if you release the pressure, the gas expands to its original volume and the Cartesian diver becomes a Cartesian floater!
How did itinerant glassworkers use Cartesian divers?
Today, you might make a Cartesian diver in science class, but for hundreds of years they were made and used by itinerant glassworkers. These artisans made all sorts of models and contraptions to entertain their audiences, including Cartesian divers.
One of the earliest known records of these glassworkers is an advertisement for a demonstration in Poland circa 1670. On the right-hand side of the handbill, a Dutch glassworker is using a Cartesian diver with three figures, or “divers,” inside. Based on the description below the image, the glassworker promoted the experiment as a magic trick rather than a science experiment. He claimed he could command each figure to move up or down in four different languages and the figure would obey.
Cartesian divers remained a popular part of itinerant glassworkers’ shows, whether billed as magic or science. During the 1800s, many middle-class Americans wanted to be educated while they were entertained, and went to scientific demonstrations, lectures, and museum exhibitions in droves. Glassworkers accordingly included a growing number of experiments and lectures in their shows, and Cartesian divers were often shown alongside pulse glass circulators, philosopher’s hammers, cryophorus deception glasses, and hydro-pneumatic fountains.
Itinerant glassworkers continued to perform into the 1900s, but the advent of mass entertainment drew audiences away from itinerant performers of all types. Glassworkers looked for alternative options, like starting their own stationary tourist attractions or joining a circus as a side show. Cartesian divers remained popular elements of their demonstrations.
How do you make a Cartesian diver?
You can make your own Cartesian diver with things you probably already have around the house and these step-by-step instructions.
This post was co-written by Kathryn Wieczorek; both Cartesian diver gifs are provided by her. A version of this post was originally published on the Corning Museum of Glass blog on April 4, 2017.