Charles Darwin’s expedition on The Beagle allowed the naturalist to gather evidence for his theory of evolution. The Beagle's commander, Admiral Robert Fitzroy, also had a theory. He used a special jar to help him predict weather.
A couple of scientists were on The Beagle when it underwent its voyages in the 1800s. One was a gentleman named Charles Darwin, whose later work was lost to obscurity. One, however, was the fortunate creator of Fitzroy’s Storm Glass. The glass was a sealed jar of a mix of materials. Under certain weather conditions, small crystals would appear in the jar, and under other conditions the crystals would melt away. Fitzroy used the glass, along with a lot of other meteorological instruments, to figure out what kind of weather was going to hit The Beagle next. A clear glass would mean a clear day. A jar of nothing but crystals would mean a cold, stormy day. Small specks of crystal floating around would mean cloudy and cool.
What Fitzroy really invented was a thermometer. Recent tests have shown that the Storm Glass is sensitive to temperature, but not to air pressure or humidity.
The glass itself was actually a sealed tube. In it was a mix of liquids and small solids; distilled water, ethanol, potassium nitrate, ammonium chloride, and camphor. The liquids were supersaturated – meaning the solids were dissolved, but were present to such a degree that the slightest change would let them precipitate out and form solids in the jar. When temperature changes, the solubility of liquid changes.
The hotter the liquid, the more solid can dissolve in it. Hot water increases entropy, and a solid that’s broken up, whizzing around a glass of water has more entropy than a solid that’s stuck together. As temperature increased, the water heated and broke up the little crystals, dissolving them. When the temperature dropped again, the water slowed down and crystals formed. The more the water cooled, the more solids got shaken out of it, and the more the crystals formed in the jar.
Fitzroy had hoped that things like humidity and barometric pressure might also be reflected in the crystals inside the glass. Sadly, he was mistaken, and so the voyage of The Beagle turned out to have little scientific merit.