By Robert M. Hazen, Ph.D., George Mason University
The atomic theory is one of the transforming ideas in all of science. To get a historical sense of the inexorable buildup of evidence that led to our modern acceptance of the idea of the atom, let us look at how the assumption of the atomic theory led to important scientific advances by Daniel Bernoulli and John Dalton.
Daniel Bernoulli
The Swiss physicist Daniel Bernoulli lived in the mid-18th century, the time of incredible discoveries in electricity, magnetism, and so forth. Daniel Bernoulli’s most famous work had to do with the behavior of moving fluids, including the way the air moved over a bird’s wing. He found that air moved faster over the upper curved surface of the wing than the lower surface, and that’s what caused the lift. The upward pressure created a lift, and that explained how birds could fly.
In this context he was studying air pressure, and he realized that if atoms were real, they must have mass. They have to have velocity and, thus, they have kinetic energy.
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Volume, Temperature, and Pressure
Bernoulli successfully applied Newton’s second law of motion to this theoretical concept of atoms to explain the behavior of gases under pressure. For example, if you doubled the number of gas particles, or you have the volume of a container that contains a certain amount of gas, it doubles the number of collision and this doubles the pressure on the confining walls.
So, he measured pressure as a function of volume. For example, increasing temperature increases the average kinetic energy in a very predictable way, and once again, you can see how the change in temperature changes the pressure inside a vessel.
You could also just think about the behavior of a balloon when you blow it up. It’s easy to imagine individual particles flying around inside the balloon pushing outward on its surface, even as the elastic of the balloon pushes inward on the particles. This creates a balancing force. It’s easier to think about that when you have the concept of atoms than if you have just some abstract continuum. That’s the way Bernoulli thought about it.
A key point here is that many of these observations don’t prove that atoms exist. They just show that nature behaves as if atoms exist. And many of the early lines of evidence were like this.
Learn more about the nature of energy.
John Dalton
A second important line of evidence was advanced by the English meteorologist John Dalton, who lived from 1766 to 1844. It was Dalton who presented the first statement of the atomic theory from a chemist’s point of view, and this was in his three-volume treatise called A New System of Chemical Philosophy. Those volumes were published from 1808 to 1827.
Dalton was a poor country teacher. He gradually became known for his meteorological observations. He made meticulous studies of the weather. He was always curious, and he began to think about the nature of atoms through this study of the weather and how it varied from day to day, and week to week, year to year, and so forth. He said:
Having been long accustomed to make meteorological observations and to speculate upon the nature and constitution of the atmosphere, it often struck me with wonder how a compound atmosphere, or a mixture of two or more elastic fluids, should constitute apparently a homogeneous mass.
What Dalton was saying is how can you take nitrogen and oxygen and other compounds and mix them together if they were somehow indivisible fluids? How would they mix together? How would they combine with each other? He couldn’t really imagine that.
Dalton’s Determination
Dalton’s determined character, his drive to understand nature, can be sensed from the context of one of his more noted papers on colorblindness. This work stemmed from a very embarrassing incident in Dalton’s life.
It turns out he was invited to join the austere Manchester Literary & Philosophical Society. And at one of its meetings, he showed up wearing a brilliant scarlet coat. The other members of the society criticized him. “How could you come to this meeting wearing a brilliant scarlet coat like that?”
Only then did Dalton learn that he had red-green colorblindness. And, thus he began the first research that led to scientific publications on the phenomenon of this common variety of colorblindness, which is now called daltonism.
Dalton’s Atomic Theory
Dalton’s atomic theory states that matter is composed of atoms of perhaps several dozen different varieties. These are the different elements. These elements differ in their weights, and in their sizes, and in their properties.
He developed this theory at a time when the law of definite proportions in chemistry was first being developed. This empirical law notes that if you take any given set of elements, they often combine with other elements in very specific ratios. For example, water is always eight parts oxygen to one part hydrogen atoms, and you always have that ratio.
Learn more about the nature of materials.
The Law of Definite Proportions
And if you look at other chemical compounds you also see similar kinds of ratios developing over and over again, and you can see these patterns occurring. Furthermore, when two elements combine in more than one way, the ratio of weights for the two compounds are going to be a simple whole number: 12 pounds of carbon can thus combine with either 32 pounds of oxygen, or they can combine with 64 pounds of oxygen.
So, either 32 or 64, that’s a one-to- two type of ratio, simple ratios that were appearing over and over again. This suggested to Dalton the reality of atoms because you’re combining units of the substance in simple ratios.
However, many contemporaries still saw atoms as merely a convenient kind of fiction, just sort of a way of thinking about nature that made it easy to do calculations, but that atoms were not necessarily real.
Common Questions about Bernoulli and Dalton’s Application of Atomic Theory
Bernoulli successfully applied Newton’s second law of motion. He applied them to the theoretical concept of atoms to explain the behavior of gases under pressure.
Dalton presented the first statement of the atomic theory from a chemist’s point of view, in his three-volume treatise called A New System of Chemical Philosophy, published between 1808 and 1827.
Dalton’s empirical law notes that if you take any given set of elements, they often combine with other elements in very specific ratios.
