Earth Air Water Fire
Greek philosohers believed in the four elements of earth, air, water, and fire.  Aristotle, the most influential of the Greek philosophers, proposed that elements also contained two of the following qualities: heat, cold, moisture, and dryness.  For example, fire was hot and dry; water was cold and moist; air was hot and moist; and earth was cold and dry.  Because qualities could vary, it was possible to change lead into gold.  Aristotle was the teacher of the alchemists and his ideas dominated chemistry for nearly two thousand years after his death.
 

Phlogiston 
Johann Becher (1625-1682) and Georg Stahl (1660-1734) had much in common: Both were German, physicians, university professors, and contributors to the phlogiston theory.  Phlogiston explained how air initially supports combustion and then does not.  It also addressed some of the shortcoming of Aristotle's theory, particularly its vague notions of chemical change.  Becher proposed that terra pinguis (fatty earth) was present in all flammable materials; this substance was given off during burning and the resulting ash was the true material.  Coining the term phlogiston from the Greek word phlogistos (burning), Stahl  believed living matter contained a soul that differed in composition from nonliving matter (vitalism theory).  Stahl outlined his medical theories in The True Theory of Medicine (1708) and the book had great influences throughout Europe.  The Becher/Stahl theory explained burning, oxidation, calcination (metal residue after combustion), and breathing in the following way:

• Flames extinguish because air becomes saturated with phlogiston.
• Charcoal leaves little residue upon burning because it is nearly pure phlogiston.
• Mice die in airtight space because air saturates with phlogiston.
• When heated, metals are restored because phlogiston transferred from charcoal  to calx.

First Chemists

 

Robert Boyle (1627-1691)
Boyle made precise measurements in studying the relationship between volume and pressure of gases (Boyle's Law).  In The Sceptical Chymist (1661), Boyle questions Aristotle's view of four elements and suggests matter consists of unique small particles.  Boyle regarded experimentation to be essential for scientific proof; this approach was to influence Isaac Newton and later scientists.
 
 
 
 
 
 
 
 
 
 
 
 
 

Joseph Black (1728-1799)
Black (physician & chemistry Professor) discovered carbon dioxide in 1750.  Black studied the reaction of MgCO3 with acids and was the first to perform careful gravimetric (weight) measurements.
MgCO_3(s) + H₂SO₄   MgSO₄ + CO_2(g) + H₂O
Black showed that the gas produced from the above reaction was the same gas produced in combustion, human breath, and fermentation. This suggested that combustion, fermentation and metabolism all have a common chemical basis.  Black quantitatively explained all the changes in weight for these processes without needing to invoke the mysterious phlogiston.  Black's work laid the foundation for modern chemistry and the arrival of Lavoisier.

Joseph Priestley (1733-1804)
Best known for discovering oxygen, Priestley (1774) subjected mercuric oxide to sunlight and obtained  air that allowed candles to burn brightly and a mouse to breathe longer.
2HgO  2Hg + O_2(g)
Priestly, a minister who dabbled in science, was a clever investigator but frequently did not see the importance of his discoveries.  In Priestley's own words, "I have discovered an air five or six times as good as common air."  This "good" air, which accounted for about 20% of atmospheric air, he named dephlogisticated air.  The failure to abandon phlogiston led to his downfall, but Priestley's discoveries and research methodology provided a foundation for chemists to follow.

Henry Cavendish (1731-1810)
Cavendish approached most of his investigations through quantitative measurements.  In order to establish that hydrogen gas was a substance entirely different from ordinary air, he calculated densities of several gases.  He found that common air is made up of nitrogen in a 4:1 ratio by volume.  In 1785 Cavendish made quantitative measurements on a closed container of air (fixed air) subjected to a burning candle followed by treatment with lime water.
C  +  O_2(g) CO_2(g)         Ca(OH)₂ + CO_2(g) CaCO_3(s) + H₂O

Antoine Lavoisier (1743-1794)
When informed by Priestley about dephlogisticated air, Lavosier repeated the experiments quantitatively.  But to make exact measurements Lavoisier developed a balance that could weigh to 0.0005g.   From his experiments Lavoisier proposed the Law of Conservation of Mass as well as the oxygen theory of combustion.   Phlogiston died on September 5 1775, the day Lavoisier presented a paper to the French Academy of Science.  Here are excerpts from the paper, Memoir on Combustion in General, published in 1777:

• I venture to propose to the Academy today a new theory of combustion. Materials may not burn except in a very few kinds of air, or rather, combustion may take place in only a single variety of air: that which Mr. Priestley has named dephlogisticated air and which I name here pure air. In all combustion, pure air in which the combustion takes place is destroyed or decomposed and the burning body increases in weight exactly in proportion to the quantity of air destroyed or decomposed.

• These different phenomena of the calcination of metals and of combustion are explained in a very nice manner by the hypothesis of Stahl, but it is necessary to suppose with Stahl that the material of fire, of phlogiston, is fixed in metals, in sulfur, and in all bodies which are regarded as combustible. Now if we demand of the partisans of the doctrine of Stahl that they prove the existence of the matter of fire in combustible bodies, they necessarily fall into a vicious circle and are obliged to reply that combustible bodies contain the matter of fire because they burn and that they burn because they contain the matter of fire. Now it is easy to see that in the last analysis this is explaining combustion by combustion.

• The existence of the matter of fire, of phlogiston in metals, sulfur, etc., is then actually nothing but a hypothesis, a supposition which, once admitted, explains, it is true, some of the phenomena of calcination and combustion; but if I am able to show that these phenomena may be explained in just as natural a manner by an opposing hypothesis, that is to say without supposing that the matter of fire or phlogiston exists in combustible materials, the system of Stahl will be found to be shaken to its foundations.

• Pure air, the dephlogisticated air of Mr. Priestley, is then, from this point of view, the true combustible body and perhaps the only one in nature, and we see that there is no longer need, in explaining the phenomena of combustion, of supposing that there exists an immense quantity of fixed fire in all bodies which we call combustible, that on the contrary it is very probable that little of this fire exists in metals, sulfur, and phosphorus and in the majority of very solid, heavy, and compact bodies; and perhaps even that only the matter of free fire exists in these substances by virtue of the property which this matter has of coming into equilibrium with neighboring bodies.

• Furthermore, I repeat, in attacking here Stahl's doctrine my object is not to substitute a rigorously demonstrated theory but solely a hypothesis which appears to me more probable, more conformable to the laws of nature, and which appears to me to contain fewer forced explanations and fewer contradictions.