Science is the best tool humans have for  understanding the truth about the universe.

But sometimes, it takes a while for  science to catch on to the real truth. Every once in a while, scientists and the natural  philosophers who preceded them make mistakes.

Despite the best intentions,  and a lot of brainpower. From rays of coldness to a mass extinction-causing  star, here are five things we thought explained   some aspect of the world we live  in, but turned out to be bupkis. [♪ INTRO] In the late 18th century, natural philosophers  were just beginning to understand temperature. They knew that hot things cooled  down, and cold things warmed up,   and that heat could move through materials.

They also had systems like Fahrenheit to compare  temperatures and thermometers to measure it. But they didn’t have a great system for  understanding what was really going on. One of the leading theories was  something called caloric theory,   where heat is caused by a massless, invisible  fluid called caloric that permeates the universe.

Caloric was supposed to be attracted  to matter but would repel itself,   which would explain why heat  transfers and dissipates. It also explained why heating  something would make it expand:   the caloric was pulling the atoms apart! But in the 1790s, a Swiss man named  Marc-Auguste Pictet performed an   experiment that got people thinking  about coldness in a similar way.

He placed two concave mirrors  at opposite ends of a room,   then put a thermometer at one mirror’s focal  point, and a container of snow at the other’s. As a result, the thermometer  recorded a drop in temperature. Now, this was a pretty common  experimental setup to study something hot.

You could even make things burst into  flame at the other side of the room. But nobody had tried it for studying cold before. So when Pictet described this experiment,   it grabbed the attention of the enthusiastic  Benjamin Thompson, also known as Count Rumford.

Rumford performed a series of his own experiments,   and concluded there must be a cold version  of caloric, which he dubbed frigoric. His theory, which ran counter to the prevailing  caloric theory, was that heat was like sound. It had to do with how parts of an object  vibrate, and the frequency at which they do so.

Cold was something vibrating slowly,  and hot was it vibrating quickly. So coldness wasn’t just the absence  of heat, it was something different. He also claimed that frigorific  rays came from space,   which helped to explain why the tops of  mountains were colder than the foothills.

They were just closer to space. Of course, we now know that heat is caused by  the motion of atoms and molecules in a substance. The more motion, the hotter the substance.

So honestly? That’s not completely different from  the model Rumford had in his head. We also know that objects are always emitting  energy through electromagnetic radiation,   which is just a fancy word for light.

It was that insight… light is a major way heat  gets transferred… that Rumford was missing. Well, that and the concept  that not all light is visible. There is neither caloric nor frigoric,   and the kinetic theory of matter started taking  the place of these theories in the mid-1800s.

But if frigoric wasn’t the solution to  Pictet’s mirror experiment, what was? What you need to remember is  that everything is emitting   radiation all the time, including the thermometer. Without the mirrors, Pictet’s thermometer  would absorb radiation from all the   objects in the room pretty evenly and  display a typical room temperature.

But when you put a mirror right next to it,  the mirror effectively blocks a good chunk   of the radiation that would be coming from the  rest of the room…somewhere between a third and   a half…and replaces it with the radiation from  whatever is at the focus of the other mirror. If something hot is at the  focus of the other mirror,   then the thermometer receives a lot of  radiation and displays a rising temperature. But when there’s snow at the other mirror’s focus,   the thermometer receives much less radiation  than it would from the rest of the room.

It begins to radiate more heat than it  receives, which is why the temperature drops. Even though we have theories  that explain exactly what’s   going on, when you see these mirror  experiments, they can seem like magic. In fact, before we had an  atomic theory of chemistry,   the line between magic and  science was… a bit blurry.

The renaissance in Europe saw  many “alchemists” come and go,   searching for cures for ailments and mixtures  that could transform different materials. The Swiss alchemist Paracelsus, also known as…  Philippus Aureolus Theophrastus Bombastus von   Hohenheim…might be the guy who brought  the field closest to an actual science. He lived in the 1500s, and was intent on rewriting  the rules of medicine based on chemistry.

But like all good alchemists, he was  also very interested in astrology,   mysticism, and transmuting metals. He also made a less-well-known  alchemical discovery: the “Alkahest.” This was supposed to be a universal  solvent, which could dissolve anything. Or at least, that’s what the word  came to mean when it was taken up   by his followers in the decades after his death.

But if you go back to the one time  Paracelsus actually wrote down the word,   it was just a liver medicine made from mercury. Neither Paracelsus nor his students were ever able   to truly make a universal solvent,  probably because it’s not possible. Even the thing we nicknamed  “the universal solvent”,   water, is only good at dissolving  a lot of things, not all things.

Anyone who’s tried to wash  some butter off their hands   by just sticking them under a running  faucet knows how “universal” it is not. But over the years, scientists  have found solvents that can   dissolve some seemingly indestructible materials. Aqua regia is a great example.

It’s a mixture of nitric and hydrochloric  acids that can dissolve gold. On its own, nitric acid can entice a small number   of gold atoms away from the surface of  the gold and into solution in the acid. But the acid can’t hold the gold in solution  for very long before it returns back.

What you get is an equilibrium,   where an equal number of gold atoms  are going into solution and coming out. So if aqua regia was just nitric acid,   you wouldn’t even notice how it  was interacting with the gold. But it also has hydrochloric acid.

And after the nitric acid pops off those  surface gold atoms, groups of four chloride   ions from the hydrochloric acid will glom onto  one gold atom to form a new ion: tetrachloroaurate And tetrachloroaurate is soluble in water. The gold dissolves. This chemical one-two punch also works  on other fancy metals, like platinum.

But it doesn’t work on the glass  used to store your aqua regia. If you need to dissolve glass for some  reason, you can turn to hydrofluoric acid. Which itself can be stored in  containers made of polyethylene.

Or if you know an alchemist like Paracelsus, you  can ask him for some plain ol’ lead, instead. You’ll just have to convince him he  can’t use it to make gold, first. In the 1600s, many early modern physicists were   trying to come to grips with forces and  phenomena that defied easy explanation:   gravity, magnetism, electricity, light,  whatever it is that keeps atoms together.

And one explanation they  kept bringing up was tiny,   invisible particles, often  referred to as corpuscles. Robert Boyle used corpuscles to explain  how different kinds of matter…say,   a lump of gold versus a lump of  cheese…displayed different properties. Their corpuscles simply had different  shapes and were arranged differently.

Meanwhile, Isaac Newton thought  that light was made of corpuscles,   which is why rays of light  could bounce and be bent. And the different colors of light  were just different sized corpuscles. Eventually, after Newton got his  sticky physics hands all over gravity,   Georges-Louis Le Sage came along and built  a theory of gravity around corpuscles.

He proposed these particles  travelled quickly through   space but slowed down after hitting something. To Le Sage, this explained why large bodies  like the Earth and the Moon attract each other: Both the Earth and the Moon are constantly  getting hit by maximum-speed “Universe”   corpuscles from all directions, except  from the sides that face each other. Those sides receive slower “bounced” corpuscles.

This mismatch results in a net effect that  pushes the Earth and Moon towards each other. Le Sage did not have to explain why the  Moon was very slowly moving away from the   Earth over time, because we didn’t know that yet. We now know all of these  corpuscular theories were wrong.

But some weren’t too far off the mark. Boyle was right in that  matter is composed of atoms. But scientists eventually learned that  atoms are made of even smaller particles,   which can vary in properties  like mass, charge, and spin.

Newton was right that light is transmitted  by photons, which act as particles. But at the exact same time,  they also act as waves. On very small scales, matter  can also act like waves,   but we don’t have time to unpack  those quantum mechanics shenanigans.

As far as gravity, Einstein completely reinvented  our concept of gravity in the early 20th century. Instead of an attractive  force in the Newtonian sense,   it’s more of a manifestation of bodies  moving along curves in spacetime. But there are still some pretty  massive question marks about   gravity and its relationship to  the other forces in the universe,   and some scientists are still on the hunt  for hypothetical particles called gravitons.

While physicists try to explain the  fundamental workings of the reality   we live in, biologists have struggled  with the fundamentals of living itself. What differentiates the  living from the non-living? One quick answer is that living things  can move of their own accord, and seem   to be good at bringing order out of disorder.

They create complex structures like cells  and organs using raw materials like sunlight,   water, or the cells of other  living things that they consume. But what puzzled biologists was that this  ability to create order doesn’t make a   lot of sense when you think about  the second law of thermodynamics. Entropy, often described as a measurement  of disorder, should always be increasing.

So, many scientists from the 1600s all  the way through to the early 1900s came   up with theories about some kind of  “stuff” that gave living things the   energy to counteract thermodynamics,  at least until the thing was dead. The French philosopher Henri Bergson, in a 1907   book called “Creative Evolution,”  called his version Élan Vital. Other names include “anima sensitiva”,  and “Bildungstrieb”, and “entelechy”.

Or simply, “biotic energy”. But this force could never be  located or proved by experiment,   so despite its popularity, it never  really took off scientifically. There’s also the problem that it doesn’t really   answer the original question of  what makes living things alive.

The biologist Julian Huxley once pointed out that   it was equivalent to saying the “élan  locomotif” is what makes trains go. This concept falls firmly into the category  of a nice idea that makes humans feel special,   but ultimately isn’t backed up by evidence. For a long time, nobody knew what killed the  non-avian dinosaurs 66 million years ago.

But in 1980, a clever father-son duo named Luis   and Walter Alvarez published  a controversial new theory: A layer of iridium in the soil, found all  around the world and in just the right part   of the geological record, suggested it was  an asteroid impact that did the dinos in. But that’s not all! There was another layer of iridium,  dated to be 31 million years younger,   that corresponded to another mass  extinction in the fossil record.

These discoveries spurred another group of  scientists to look at the fossil record,   where they found that mass extinctions  tended to happen every 26-30 million years. To explain that regularity, they proposed that  there must be a small companion star to the Sun,   orbiting way outside the orbit of Pluto. Every 26 to 30 million years, its orbit would  take it through our solar system’s Oort cloud,   tossing a pile of comets  into the inner solar system.

They dubbed this star Nemesis, after  the Greek goddess of retribution. It was controversial at the time. Detractors argued that this whole   periodic-mass-extinction-thing  was just a statistical fluke.

A misinterpretation of the data. In the years that followed, several telescopes   and sky surveys would have turned up  evidence for Nemesis, if it existed. But they’ve constantly come up empty.

So these days, most astronomers  don’t believe Nemesis is real. Scientists are going to get things wrong as  they try to figure out how reality works,   but it’s the nature of science  to figure that out along the way. Concepts like “frigorific rays” or “gravity  corpuscles” might sound a bit funny now   that we have more evidence and know better,  but they were plausible in their own time.

So let’s meet up in the year, let’s say, 2265, to  see which of today’s theories have been upended. [♪ OUTRO]