This might be the time for a bit of myth-busting about old glass windows. It has sometimes been said that the reason that 300-year-old windows are thicker at the bottom than at the top is because the glass has flowed downward over time. This isn’t true; window glass isn’t a liquid and it isn’t flowing anywhere. It’s because these window panes were made using an incredibly ingenious method. A molten glass blob was stuck on an iron rod, and the rod was spun very quickly until the glass flowed outward into a flat disk.? This disk was cooled, and cut up to make window panes. The downside of this method is that the disk will always be thicker closer to its center. So the diamond-shaped window-pane pieces were cut with the thicker bit at one end, and when it was put into the window, the thicker end was often placed at the bottom to help rain run off. The glass didn’t move itself downward, it was put there.
Our glass blobs were not allowed to cool down straightaway. They were put in an oven overnight, one that would bring the temperature down slowly over the entire night until it matched room temperature in the morning. The reason for that is that even once the glass is solid, the atoms aren’t absolutely stuck in place. If you heat something up, the atomic arrangement changes slightly even if the temperature change isn’t enough to turn the solid into a liquid. The same thing happened as the glass blobs cooled down: The atoms shifted slightly. The reason for the oven is to allow this slight rearrangement to happen slowly and evenly over the whole structure. If it happened unevenly, unbalanced internal forces could shatter the glass. Once again, those extra internal stresses are the result of a very simple principle: The positions of the atoms may be fixed, but the distance between neighboring atoms isn’t. If you heat something up, it expands.
THE WORLD OF digital measuring devices has many advantages, but it has one definite downside: We’re disconnected from what the measurement really means. One of the saddest losses is the glass thermometer, an essential tool in science labs and homes for two and a half centuries. You can still buy them and I still use them in my lab, but in lots of places they’ve been superseded by digital alternatives. The shiny mercury thread that I remember from my childhood has been replaced by colored alcohol, but the modern version is essentially the same as the device that Fahrenheit invented in 1709. There’s a narrow glass rod with a skinny tube running all the way through the middle of it. At the lower end, the tube widens out into a bulb, a reservoir of liquid. Put this end of the thermometer in anything—bath water, your armpit, the sea—and what happens is both elegant and simple. The temperature of something is directly related to the amount of thermal energy that it has. In liquids and solids, thermal energy is expressed as the jiggling about of atoms and molecules. If you put your thermometer in the bath, you’re surrounding the cold glass with hot water. The molecules in the water are moving more quickly, so they’ll jostle the atoms in the glass, giving them the energy to jiggle faster as well. This is heat traveling by conduction. So when you put the thermometer in the bath water, thermal energy flows into the glass. The atoms in the glass don’t go anywhere—they just fidget on the spot, vibrating from side to side. The temperature of the glass is a measure of this fidgeting, and now the glass is hotter than it was. Then the atoms in the glass bump into the liquid alcohol until it starts jiggling faster too. That’s the first part—the bulb of the thermometer is heated up until it’s at the same temperature as its surroundings.
When atoms in a solid vibrate faster because of extra heat, they push the nearby atoms away, just a tiny bit. The glass takes up more space when it’s hot, just because its fidgeting atoms require it. This is why things expand when they get hot. But the alcohol molecules space out much more as they speed up; so alcohol expands about thirty times as much as glass for the same temperature change. Now the alcohol in the thermometer bulb is taking up more space than it was, but the only extra space is up the tube. So as the molecules in the alcohol vibrate and push each other apart, the liquid creeps up the tube. The distance it travels is directly related to the thermal energy of its molecules, and so the marks on the thermometer correspond to the amount of thermal energy in the liquid. It’s beautifully simple. When the liquid in the bulb cools, the alcohol takes up less space as its molecules slow down. When the liquid heats up, it takes up more space as its molecules vibrate more energetically. So a reading from a glass thermometer is a direct measurement of atoms jostling each other.
Different materials expand by different amounts when they’re heated. That’s why running hot water over a stuck jam jar lid can be helpful; both the glass and the metal lid will expand, but the metal expands far more than the glass. After it’s expanded, it’s easier to remove; even though the difference in its size is far too tiny for you to see, you can feel the result.
Generally, solids expand less than liquids as they’re heated up. The expansion is only a tiny fraction of the overall volume, but it’s enough to make a difference. Next time you cross a road bridge on foot, keep an eye out for a metal strip at either end of the bridge, running across the road. It’s likely to be made out of two interlocking comb-shaped plates. This is an expansion joint, and once you know what to look for, they’re pretty common. The idea is that as the temperature rises and falls, the combs allow the bridge materials to expand and contract without buckling or cracking. If the bridge sections expand, the fingers of the comb are pushed farther into each other, and if the bridge contracts, the fingers pull back but without generating a serious gap in the road.
Thermal expansion may be elegant and useful in a thermometer, but it can have serious consequences on larger scales. One of the problems caused by our emission of greenhouse gases is that sea level is rising steadily. The current global average sea-level rise is about 0.1 inch per year, and it’s rising more quickly as time goes on. As glaciers and ice sheets melt, water that was locked up on land is flowing back into the sea, so there’s more water in the global ocean. But that accounts for only approximately half of the current rise. The other half comes from thermal expansion. As the oceans warm, they take up more space. The current best estimate is that 90 percent of all the extra heat energy that the Earth has because of global warming has ended up in the oceans, and the extra sea-level rise is the consequence.