Until the English Chemist Steven Hales (1677-1761) collected gases over water by bubbling it into a jar, displacing the water within, little useful progress had been made in the area of gases and gas chemistry. This work was furthered by the Scottish chemist, Joseph Black (1728-99) when as part of his thesis in 1754, he strongly heated calcium carbonate to form calcium oxide and a gas (carbon dioxide). What he found interesting was that the gas could be encouraged to recombine with the calcium oxide, regenerating the calcium carbonate. He noted that the gas was identical to the "gas sylvestre" that was described by Van Helmont over 100 years earlier. Black called the gas "Fixed air" because of its ability to be fixed into a solid substance.
Blacks work had many consequences. He found that calcium oxide left to stand in the air returned to calcium carbonate. This was the first clear indication that air was not a simple pure substance and therefore was not an element by Boyle's definition since it must contain at least two distinct substances, ordinary air and carbon dioxide. He also noticed that the calcium carbonate was able to neutralize a given amount of acid, an idea that was to find its fullness with Lavoisier. He also noticed that a candle would not burn in the carbon dioxide. He passed this work to his student Daniel Rutherford (1749 - 1819, Son of Sir Walter Scott ) who kept a mouse in a confined volume of gas until it died. A candle and then phosphorus were burned to extinction in the remaining gas which was then passed through a solution that absorbed carbon dioxide. Despite their efforts gas still remained. They reported this experiment in 1772, identifying the remaining gas as "phlogisticated air". We call the gas "nitrogen", and give Rutherford credit for its discovery.
The discovery of hydrogen and oxygen is attributed to two other British chemists. The first, Henry Cavendish (1731 - 1810), was interested in the gas formed when acids reacted with certain metals. Although Boyle had actually collected the gas previously, Cavendish described its properties and was credited for the discovery of hydrogen. He was the fist to compare the weights of fixed volumes of gas and suggest this may be related to the density of the gas. The second, a more colorful character, was a Unitarian Minister. Joseph Priestly (1733 - 1806) practiced chemistry as a hobby and in the late 1760's took over a pastorate in Leeds, England. Beside the church was a brewery from which Priestly could obtain plentiful supplies of carbon dioxide for his experiments. In collecting the gas under water he noticed that some dissolved in the water giving a tart taste. Many believe that this can be recognized as the first ever soda pop and Priestly as the father of the modern soft drink industry. Priestly was more interested in the gases themselves and by collecting gases under other liquids such as mercury was able to isolate such gases as hydrochloric acid, nitrogen oxide, ammonia and sulfur dioxide. In 1774, he found that mercury when heated with air formed a red compound which he placed in a test tube and heated it in the path of a beam of focussed sunlight. The result was shinny globules of mercury and a colorless gas that was able to burn combustibles more brightly than air and was also able to relight a smoldering wooden splint. He applied the phlogiston theory and named the gas "dephlogisticated air". He found it had opposite effects to Rutherford's phlogisticated air in that mice became extra active and breathing the gas made him feel "light and easy". This discovery was later named oxygen.
Some other discoveries in this period were:
It was the work of French chemist Antoine Laurent Lavoisier (1743 - 94) who drew together the information into an overall theory. Lavoisier recognized the importance of accurate measurement and was far more systematic in his measurements than previous scientists. He was the first to identify diamond as a form of carbon and so relate it to coal. He also went on to produce many different forms of "calx" by heating metals in limited supplies of air. It was well known that calx weighed more than the metal itself, yet when he weighed the sealed vessel, it weighed precisely the same after heating as it did before heating. From these results it followed that if the metal gained weight in forming calx then something else in the vessel must have lost weight equivalent to that gained by the metal and that something else had to be air. If this was the case, Lavoisier predicted a vacuum would exist in the vessel. Sure enough, when he opened the vessel, air rushed in. From this he concluded that ores must be combinations of metals and gases and that when they were heated with charcoal, the charcoal took the gas from the metal forming carbon dioxide and leaving the metal behind.
It became clear to Lavoisier that in the course of his experiments if all substances taking part in a reaction and all products were taken into account then there would never be a change in weight. From this he maintained that mass was never created nor destroyed, but was merely shifted from one substance to another. This concept is known as "The Law of Conservation of mass." It is fundamental to all chemistry.
Lavoisier after meeting with Priestly in 1774 published a paper in 1775 where he claimed that air was a mixture of gases and that just one component which he called oxygen (priestly's "dephlogisticated air" ) was responsible for the formation of calx and was fundamental to life itself.
In 1783 Cavendish was working with his hydrogen gas and found that when he burned it and condensed the vapor it was water. This was very significant since it disproved the Greek theory of elements completely since water was not a simple substance but the product of two gases. It was Lavoisier who hearing of the experiment gave Cavendish's gas the name hydrogen ("water-producer") and added that hydrogen burned by combining with oxygen and that water was a hydrogen-oxygen combination.
In collaboration with other French chemists, Lavoisier set to work on producing a system of chemical names which was published in 1787, quickly bringing a unity of understanding in the chemical world that had been absent in the alchemical days. The naming was based on logical principles for example calcium oxide contained calcium and oxygen.
In 1789, Lavoisier produced a list of "Known Elements" of which only light and heat were completely wrong. Of the remaining 31, some were elements whilst 8 were later to be found to be capable of breaking down further into simpler substances. In the same year, the French revolution broke out and rapidly degenerated into mass murder. Lavoisier was connected to the Monarchies tax-collecting organization hated by the revolutionists and Lavoisier was unfortunately executed by guillotine in 1794. Lavoisier is remembered as "the father of modern chemistry".