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Chemical History of the Candle

Lecture V: The Gases in the Atmosphere

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Lecture VI: Respiration and Its Analogy to the Burning of a Candle

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In this lecture, Faraday takes the lessons learned from the candle and applies them to the world in general. Combustion of oxygen is a common chemical reaction, which not only occurs in flames, but also in the human body, although at a much slower rate.

Lecture outline

  1. Continuing from last lecture: the properties of carbonic acid
  2. The composition of carbonic acid
    1. Its source is in the smoke, which produces black particles (carbon)
    2. The carbon is consumed when exposed to pure oxygen
    3. The end product is carbonic acid; therefore carbonic acid is carbon and oxygen
  3. Demonstrations of oxygen and carbon interactions
    1. Burning flakes of heated carbon in oxygen
    2. Burning a chunk of charcoal (no flame)
    3. Collected gas reacts with limewater: it is carbonic acid
    4. Combination by mass: 6 parts carbon to 16 parts oxygen
    5. Combined with 28 parts lime forms calcium carbonate (carbonate of lime)
    6. Charcoal burns away completely
  4. Splitting carbonic acid into its constituent parts
    1. Combine phosphorus with carbonic acid gas: phosphorus can't burn
    2. Combine potassium with carbonic acid gas: heated potassium burns, producing potash and carbon flakes
    3. Generalization: carbonic acid is formed whenever carbon burns
  5. Producing carbon by burning various objects
    1. Partially burnt wood
    2. Coal gas (CO, or carbon monoxide)
  6. Properties of carbon
    1. Burns as a solid body, becoming a gas in combination with oxygen
    2. Comparison with lead pyrophorus, which forms a solid residue on burning
  7. Living combustion (respiration)
    1. Demonstrations that exhaled breath puts out candle (not by blowing it out, but because it lacks oxygen).
    2. Demonstration with air and limewater
      1. Limewater turns milky when exposed to air from lungs: such air contains carbonic acid (CO2)
      2. Normal air does not affect limewater
    3. Breathing is necessary: can't hold breath indefinitiely.
    4. Air drawn into body is combined with food eaten in the blood, forming carbonic acid: the oxygen in the air is consumed (i.e., food is effectively fuel that can be burned).
    5. Sugar composition (72 parts carbon to 11 parts hydrogen to 88 parts oxygen by mass): same elements, but different proportions, from candlewax.
    6. Reaction of sulfuric acid (oil of vitriol) with sugar: produces carbon
    7. Estimate of amount of carbon converted by humans due to respiration
    8. Earth could not support combustion of carbon by respiration if it produced solid
    9. Carbonic acid released into atmosphere by animal respiration is absorbed by plants, preserving balance
  8. Chemical affinity: attraction of one substance for another
    1. Lead reacts with oxygen upon exposure; carbon does not.
    2. Heat is required to set coal gas aflame
    3. Gunpowder and guncotton start burning at different temperatures
    4. Respiration in humans occurs at a little above room temperature

Reading notes

oil of vitriol:
Sulfuric acid
Ash residue produced by burning various substances

Discussion points