Lecture II - A Candle: Brightness of the Flame
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Outline of Faraday's Chemical History of the Candle, Lecture II
In this lecture, Faraday moves from the material in the candle to consider
the nature of the candle flame, and the possibility that the flame produces
something new from the candle material. Once he has show that candles combustion
does result in new chemical products, by changing the air and candle wax
into something else, he considers how other materials might burn to form
different products as well. Then he returns to the problem of candle combustion:
how do we identify the products, and what are their properties?
- Combustion is the process of changing materials from one type of matter
into another (chemical change)
- The parts of the flame
- Dark area: production of the vaporous form of wax (demonstration shows
this vapor burns when collected and ignited)
- Light area: vapor is consumed -- it becomes something else (so no vapor
is available for collection)
- Location of the hottest part of the flame is around the center, not
at the center
- Air is necessary
- Demonstration with candle and oil: air required for any flame
- Partial combustion (resulting in smoke) occurs if not enough air is
- Types of combustion
- Can occur with flame (gunpowder)
- Can occur without flame (glowing embers of iron filaments)
- Multiple flames from lycopodium
- Combustion products
- Soot (carbon) is produced in the "luminous part" of the flame
- Combustion of different materials produces different products
- Combustion of liquids and solids produces light (iron filings, platinum
- Combustion of gas does not produce much light--combustion of hydrogen
gas produces water
- Combustion of solid carbon produces gas (carbon dioxide)
- Conclusion: flame production involves the combining of oxygen with
some form of solid material
- Example: combustion of phosphorus produces gases and solids (as small
particles in smoke)
- Example: combustion of zinc (philosophic wool)
- Example: burning camphine gives off carbon particles which can then
be burned independently (producing flame)
- Example: adding air to the flame increases the heat so all the carbon
is consumed in the initial burning
- The products of candle combustion
- Large amount of some gas is produced (ascending balloon)
- Water is produced (vapor on side inside of glass tubing)
- vaporous fluid:
- Individual droplets suspended in air, as opposed to gas, which is individual
molecules or atoms moving freely
- fresh air:
- Air which has not supported either combustion (flame burning) or respiration
(lots of people breathing it); in either case, the oxygen is diminished.
- Argand lamp:
- See the diagram at the top of page 33. The main point is that the lamp
is fed fresh air from the bottom directly into the center of the flame,
which makes the flame much brighter than other lamps, and also provides
for complete combustion (no smoke).
- Powder made from the spore of certain club mosses and used for fireworks
and other explosives.
- Faraday's demonstration is based on the use of lime lanterns to produce
an intense white light which was used to light stages. The light was extremely
hot--and caused a lot of fires.
- The practice gave rise to the phrase "in the lime light"
-- meaning "at the center of attention."
- Philosophic wool:
- A form of zinc oxide as a white, fuzzy residue from burning zinc in
- What is combustion? What is happening to the wax in the dark and light
parts of the flame? How does chemical change differ from the physical change
(solid->liquid->gas) which Faraday demonstrated in the first lecture?
- Why is the hottest part of the flame not at its center?
- Faraday says that "all these things"--by which he means the
wicks and oil lamps that he is using for a demonstration--"are the
same as candles, after all." What does he mean? How are generalizations
like this important to the process of scientific thought?
- How does Faraday separate the products of combustion? What methods
does he use to identify the products?
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