Contribution to the History of Photosynthesis:
Joseph Priestley

Joseph Priestley (1733-1804). Priestley was an English chemist and a radical clergyman. Prior to his experiments and during most of the 18th century, the overwhelming theory in relation to combustion was the Phlogiston Theory. It was advanced by Johann Joachim Becher (1635-1682) and Georg Ernst Stahl (1660-1734). The theory was incorrect but Priestley believed in it and it became the basis of chemical investigations up to the 18th century. The experimental results ultimately lead to the theory’s demise which in turn paved the way to the present concepts of photosynthesis.

The Phlogiston Theory postulated that a burning body gives off a substance called phlogiston, leaving an ash or calx. Similarly, a metal consists of a calx and phlogiston. It has been pointed out that the heating of metal, as is done by metal workers, releases its phlogiston and a calx appeared, but heating of phlogiston-rich material (like charcoal or oil) with iron ore (without phlogiston) restores the metal (Cardwell 1995). It was also believed that the air absorbs a limited amount of phlogiston upon which combustion no longer occur (University of Illinois at Urbana-Champaign. n.d.).

Johann Becher was an English chemist who, in 1669, rejected Aristotle’s four-element theory (earth [soil], air, fire and water) and instead proposed the concept of five elements: water, air, and three earth-like elements in attempting to understand combustion and calcination, a phenomenon applied to the gain in weight of heated metals. Notable among these substances was Becher’s fatty earth or terra pinguis. In the later part of the 18th century Georg Stahl, a German chemist, gave the name phlogiston for Becher’s fatty earth and suggested that this material was lost during combustion (Windelspecht 2002).

Joseph Priestley started with Joseph Black’s “fixed air” (Egerton 2008). He lived within the neighborhood of a public brewery and became curious of the bubbling at the surface of fermenting vats. It induced him to conduct experiments on “fixed air” which readily formed on the surface of fermenting liquor. He noted that lighted candles or chips of wood are extinguished and sprigs of mint die when placed over the fermenting liquor (Priestley 1774). He was also puzzled why a burning candle in a closed container soon destoys the air inside, prevents further combustion, and unable to support life, but fires that have been burning since time immemorial seem to have no detrimental effect on the earth’s atmosphere (Cardwell 1995).

In and before 1772, Joseph Priestley conducted a series of experiments to determine the effects of gases on plants and animals. He observed that (1) if a candle was lighted in an airtight container, the flame would soon extinguish; (2) a mouse died soon when put in an airtight container in which a candle has burned out or the air inside has been spoiled by mice breathing and dying in it; and (3) the mouse lived if at the same time a live plant is placed inside the airtight container. He demonstrated that putting a sprig of mint in an airtight jar in which a candle had burned out restored the air. After only 10 days, another lighted candle burned again inside. Aside from mint, Joseph Priestley likewise found that the purifying effect of plants on “injured” air also applies to balm, groundsel which has an offensive smell, and spinach.

Moreover, he found that the burning of candles is not the only cause of “injury” to the air but also “the respiration of animals, the putrefaction of vegetables or animal substances, the effervescence of iron filings and brimstone, the calcination of metals, the fumes of charcoal, the effluvia of paint made of white-lead and oil, and a mixture of nitrous air." He concluded that “the diminution of the air was, in some way or other, the consequence of the air becoming overcharged with phlogiston, and that water, and growing vegetables, tend to restore this air to a state fit for respiration, by imbibing the superfluous phlogiston” (Priestley 1774).

Joseph Priestley published his results as Experiments and Observations on Different Kinds of Air in1774. Despite his adherence to the Phlogiston Theory, the paper demonstrated that green plants absorb “fixed air” (carbon dioxide) from the atmosphere, give off “gas” or “dephlogisticated air”, now known as oxygen, and that this gas is vital to animal life (Rook 1964).

(Ben G. Bareja. June 2012)

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