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