At the time the United States was a toddler most chemists believed that when something was burned it would release a substance called terra pinguis or phlogiston. Phlogiston is lighter than air, odorless, colorless and was essential to the combustion process. Though this term is clearly no longer used it had a good life. When it was born science had only enough room for four elements: earth, fire, wind and water. During the time of it’s decline 150 years later, chemistry, as we know it, was coming into it’s own. One question that has been debated is how did chemists and other scientists go from using a term like phlogiston to not using that term? Did it go out of style? If so, surely it must have been replaced by another term. After all, phlogiston provided a good explanation for a handful of problems. It explained the oxidation of metals or rust, for instance. It also explained the loss of weight in substances that were burned. There were, however, a couple quirks that surrounded phlogiston. For one, magnesium weighs more after it is burned. So, phlogiston didn’t just go out of style. Something was wrong with it. History tells us that phlogiston was demoted after the discovery of oxygen, but because it goes from a weightless, colorless and odorless substance to a self-repellent fluid called caloric, I wouldn’t say it fared so badly. But the question remains, how do we go from phlogiston to caloric? Was it a preponderance of evidence against phlogiston and for caloric?
The fate of the caloric may provide a hint, but I should just say that a new theory emerged from the problems of an old one. That seems like a good thing when the new theory covers the old theory. But something lurks in going from one theory to another. It isn’t always the case that the new theory can explain everything the old theory could and more. A good example of this is the wave-particle duality that exists today. Originally, wave theory and particle theory were at odds with one another. Both are excellent theories with great explanatory power. But neither can cover enough of the explanatory power of the other in order to take it over (if you will). So after a little over 200 years of fierce competition, they were combined. So, if one theory can’t cover all of the explanations of another and more how can we say it is better? How do we know when a more specialized theory is more useful than one which covers more ground but is less specific or requires much more data, for instance?
We shouldn’t be so hard on ourselves to find a solution, after all, there seem to be several reasons why one theory concedes to another as you go through the history of science. One philosopher even suggested that there were no good reasons why one line of thought would prevail over another. That has always sounded like an interesting position; one that should keep everyone else honest, at least.