Once most scientists accept a scientific theory, it is difficult to do away with it. For about two thousand years, most philosophers and scientists thought there were four elementary substances from which everything else was made. These were fire, air, water, and earth. It was thought that they were in their simplest form and could not be broken down into anything else.
A version of this theory was accepted by most scientists/philosophers until sometime around the 1600s. During that time, a few European scientists, like Robert Boyle, began to doubt that fire, air, water, and earth were the only four basic elements. But the theory did not die easily or quickly.
Robert Boyle (1627-1691) was born into a wealthy family. The family wealth gave him the advantage of having time to study science. He was well-educated and even traveled to France with a tutor as a teenager where he studied with the elderly Galileo Galilei in 1641.
Boyle’s name is found in almost every chemistry book, because he was the first to mention that there is an opposite relationship between the volume of a gas and the pressure of the gas. This famous principle is known as Boyle’s Law and is usually expressed mathematically.
In 1661, Boyle published an influential book called The Sceptical Chymist. (This was the correct spelling in 1661.) In this book, he laid the foundations for the atomic theory of matter. He argued against limiting chemical elements only to fire, air, water, and earth. He also argued that scientists should back up their theories experimentally.
Although he loved studying chemistry, Boyle also loved to study the Bible. He believed that science and Scripture were harmonious. A Discourse of Things Above Reason was another book he wrote and published in 1681. He believed there were limits to reasoning about what God can or cannot do, that God’s attributes can be seen by studying nature, and that His wisdom can be seen in creation.
Eventually the four basic elements theory was updated to explain new research. Burning came to be explained in terms of a particular kind of “earth” that had a mysterious element, phlogiston, in it. Burning objects were said to give off phlogiston. The phlogiston theory explained many things about burning and was perfectly logical for many more years.
It is interesting to compare what three other famous scientists believed about elements. They all lived about 100 years after the time of Robert Boyle. They were Joseph Priestley (1733-1804), Henry Cavendish (1731-1810), and Antoine-Laurent Lavoisier (1743-1794). They all looked at the same evidence, and even conducted the same experiments, but they did not all reach the same conclusions.
Like many of the early scientists, Priestley was a Christian minister. He used a clever method for collecting gases that were given off during chemical reactions. The gases were collected in a glass container that was filled with water and inverted in a larger container of water. Tubing sent the gases into the inverted container and pushed the water out. This method allowed him to test and measure the gases.
In one of his experiments, he took some calyx of mercury (actually mercury oxide) and focused the sun’s rays on it with a magnifying glass. When it became very hot, it began to give off a gas. He collected the gas and did some tests on it. He found that the gas made a candle glow brightly, a mouse could live longer in this gas than in ordinary air, and he felt invigorated when he breathed it himself. His wrong conclusion was that he had isolated “dephlogesticated air”. We know today that he had isolated oxygen gas.
Cavendish also collected gases by the water displacement method. In his experiments, he added zinc to dilute sulfuric acid and then added it to hydrochloric acid (he didn’t call them acids). Both reactions resulted in the formation of bubbles of gas. He noted that the gas that had been collected burned with a pale blue flame. He repeated the acid tests using iron and then tin. Each reaction produced a gas that burned with a pale blue flame. According to the phlogiston theory, when metals are heated, they release phlogiston. Cavendish thought he might have produced phlogiston, but he wasn’t sure, so he just called it “inflammable gas”. We know today that he had isolated hydrogen gas.
Diagram of Cavendish’s apparatus for collecting gases LINK
Cavendish then repeated Priestley’s experiment with calyx of mercury to obtain “dephlogesticated air” (actually oxygen). He put this gas in a glass container with his “inflammable gas” (actually hydrogen). The two gases exploded when he added a spark of electricity, and water suddenly appeared in the container.