How Did Robert Boyle Discover a Law That Was Named After Him?

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Boyle’s Law is the first quantitative formula to describe the behavior of gasses. It is so basic to the understanding chemistry that it is taught to every student in beginning chemistry classes. Prof. Ashoka tells us how Robert Boyle discovered a law that is now known as Boyle’s Law, in the weekly column. A Different Truths exclusive. 

Year of Discovery: 1650

The concept Robert Boyle discovered (now called Boyle’s Law) laid the foundation for all quantitative study and chemical analysis of gasses. It was the first quantitative formula to describe the behavior of gasses. Boyle’s Law is so basic to the understanding chemistry that it is taught to every student in beginning chemistry classes.

A genius experimenter, Boyle also proved that gasses were made of atoms—just like solids. But in a gas, the atoms are spread far apart and disconnected so that they can be squeezed tighter. Through these experiments Boyle helped convince the scientific world that atoms existed—an issue still debated 2,000 years after their existence was first pro­posed by Democritus in 440 B.C. 

How Was It Discovered?

Robert Boyle was the son of an earl and a member of the British Scientific Society. During a 1662 society meeting, Robert Hooke read a paper describing a French experiment on the “springiness of air.” The characteristics of air were of great interest to scientists in the seventeenth century.

French scientists built a brass cylinder fitted tightly with a piston. Several men pushed down hard on the piston, compressing the air trapped below. Then they let go. The piston sprang back up, but not all the way back up. No matter how often the French tried this ex­periment, the piston never bounced all the way back up.

The French claimed this proved that air was not perfectly springy. Once compressed, it stayed slightly compressed.

Robert Boyle claimed that the French experiment proved nothing. Their piston, he said, was too tight to bounce all the way back up. Others argued that, if they made the piston looser, air would leak around the edges and ruin the experiment.

Boyle promised to create a perfect piston that was neither too tight nor too loose. He also claimed that his perfect piston would prove the French wrong.

Two weeks later Robert Boyle stood before the society with a large glass tube that he had shaped into a lopsided “U.” One side of the “U” rose over three feet high and was skinny. The other side was short and fat. The short side was sealed at the top. The tall, skinny side was open.

Boyle poured liquid mercury into his tube until it covered the bottom of the “U” and rose just a little in both sides. A large pocket of air was trapped above this mercury in the short fat side. A piston, Boyle explained, was any device that compressed air. Since his used mercury to compress air, there would be no friction to affect the results — as had been true in the French experiment.

Boyle recorded the glass piston’s weight and etched a line in the glass where mercury met the trapped air pocket. Boyle trickled liquid mercury down the long neck of the tall side of his piston until he had filled the neck. Mercury now rose well over halfway up the short side. The trapped air had been squeezed to less than half of its original volume by the weight and force of mercury.

Boyle drew a second line on the short chamber to mark the new level of mercury inside — marking the compressed volume of trapped air.

He then drained mercury through a valve at the bottom of the “U” until the glass piston and mercury weighed exactly the same as they had at the beginning. The mercury level re­turned to its exact starting line. The trapped air had sprung back exactly to where it started. Air was perfectly springy. The French were wrong. Boyle was right.

Robert Boyle continued the experiments with his funny glass piston and noticed some­thing quite remarkable. When he doubled the pressure (weight of mercury) on a trapped body of air, he halved its volume. When he tripled the pressure, the air’s volume was re­duced to one-third. The change in volume of air when compressed was always proportional to the change in the pressure squeezing that air. He created a simple mathematical equation to describe this proportionality. Today we call it “Boyle’s Law.” No other concept has been more useful in understanding and using gasses to serve the needs of humankind. 

Fun Facts: 

Oceanographer Sylvia Earle set the women’s depth record for solo diving (1,000 meters or 3,281 feet). According to the concept Boyle discovered, the pressure at that depth is over 100 times what it is at the surface!

Serialised from the book, Top 100 Scientific Discoveries of all Time (Chronological), by Prof. Ashoka Jahnavi Prasad

©Prof. Ashoka Jahnavi Prasad

Photos from the Internet

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Ashoka Jahnavi Prasad is a physician /psychiatrist holding doctorates in pharmacology, history and philosophy plus a higher doctorate. He is also a qualified barrister and geneticist. He is a regular columnist in several newspapers, has published over 100 books and has been described by the Cambridge News as the ‘most educationally qualified in the world’.