18. Plants, carbon that waits, and a candle's wish

The lesson, step by step

1. Hook question

   SayIf every person, animal and fire is pouring carbon dioxide into the air, why don't we run out of fresh air?

2. Read together

   Read pages 183 to 190, to the end of the six lectures.

   A man in twenty-four hours converts as much as seven ounces of carbon into carbonic acid; a milch cow will convert seventy ounces, and a horse seventy-nine ounces, solely by the act of respiration.

3. Plants take it back

   Faraday says the gas that is bad for us is food for plants: their leaves take carbon from the air and grow.

   what is disease to the one being health to the other.

   So are we made dependent, not merely upon our fellow-creatures, but upon our fellow-existers, all Nature being tied together by the laws that make one part conduce to the good of another.

   Go and look at a plant or tree. Say: Much of this leaf was built from carbon that came out of the air.

4. Carbon waits

   Faraday points out that carbon does not burst into flame the moment air touches it. It waits until it is heated. Ink made of carbon lasted about 1,800 years on the scrolls of Herculaneum, and a candle sits for years without changing.

   the matter which is appointed to serve the purpose of fuel waits in its action

   He touched gun-cotton and gunpowder with a hot wire to show they "wait" for different amounts of heat. Watch only: don't try this at home. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.

5. The whole story

   Ask your child to draw the chemical history of a candle as a big flow chart, from memory first: candle wax → melted in the cup → climbs the wick → turned to vapor → meets the air in a ring of flame (glowing carbon makes the light) → water + carbonic acid go up → plants take in the carbon. Then add a branch: food → breathing → water + carbonic acid.

   Now get out the list from Lesson 1. What did your child know then, and what do they know now?

6. Faraday's wish

   Read the last sentence aloud slowly:

   all I can say to you at the end of these lectures (for we must come to an end at one time or other) is to express a wish that you may, in your generation, be fit to compare to a candle; that you may, like it, shine as lights to those about you; that, in all your actions, you may justify the beauty of the taper by making your deeds honourable and effectual in the discharge of your duty to your fellow-men.

   Ask: What do you think he meant? What would it look like to be "a light to those about you"?

7. What we know now

   Plants use the energy of sunlight to make sugar from carbon dioxide and water, and they give off oxygen as they do it (photosynthesis). Today scientists also measure how burning coal, oil and gas adds extra carbon dioxide to the air, which is warming the Earth's climate. Faraday's big idea, that every part of nature is tied to every other, matters more than ever.