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A lighthouse on a rock in the open sea has to survive waves that destroyed earlier towers, and its mortar has to harden while wet. John Smeaton faced both problems when he rebuilt the Eddystone lighthouse in the 1750s, and his solution came partly from a simple acid test you could do in a school lab. This post covers the building, his lime experiments, the chemistry behind them with a worked calculation, and what to learn from his method.
Who was John Smeaton, and what was the Eddystone problem?
Smeaton was a Fellow of the Royal Society by 1759, when he read a long paper on the power of water and wind to turn mills in its Philosophical Transactions. That paper began with experiments "made on working models, which I look upon as the best means of obtaining the outlines in mechanical enquiries", but warned that "a model is more apt to lead us from the truth than towards it" unless you know how it differs from the full-size machine.
The lighthouse is described in his own book, A Narrative of the Building ... of the Edystone Lighthouse with Stone (1791). The previous lighthouse on the rock was destroyed by fire in December 1755. Smeaton's account records that the first stone of his tower was fixed on Sunday 12 June 1757, and that the new light was lit on 16 October 1759.
How did Smeaton find a lime that would set in water?
Lime mortar is made by heating limestone to make quicklime, then mixing it with water and sand. Ordinary lime hardens slowly in air. For a tower washed by the sea, Smeaton needed one that would set under water.
He noticed that the hardness of the stone did not predict the result. Chalk lime did about as well as lime from hard Plymouth marble, but lime from Aberthaw in Wales was "greatly superior" for buildings in water. To find out why, he analysed limestones with acid, a method his friend Mr Cookworthy taught him.
He took about "five pennyweights (or a guinea's weight)" of crushed limestone, poured on aqua fortis (nitric acid) a little at a time until it stopped fizzing, then washed and dried whatever did not dissolve. White chalk dissolved completely. Aberthaw limestone left a residue of fine, dark blue clay weighing "nearly one-eighth part of the original mass".
His conclusion: "the most pure limestone was not the best for making mortar, especially for building in water." The best water limes, he found, generally contained clay. A lime from Darking (Dorking) that London builders valued for water work was "a species of chalk, impregnated with clay" at about one seventeenth of its weight. For the tower, he found pozzolana, an earth found in great quantities near Mount Vesuvius, worked best with Aberthaw lime; his came from Civita Vecchia in Italy.
Worked example: the chemistry of Smeaton's test
The fizzing is carbon dioxide from calcium carbonate:
CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂
Clay does not react this way, so it stays behind. Here is a calculation with sample numbers, simplified by assuming the dissolved part is pure calcium carbonate.
A sample of 8.0 g leaves 1.0 g of clay, one-eighth, like Aberthaw. So 7.0 g of CaCO₃ dissolved.
Molar mass of CaCO₃ = 40 + 12 + 3 × 16 = 100 g/mol, so 7.0 g is 0.070 mol.
One mole of CaCO₃ gives one mole of CO₂ (44 g/mol). Mass of CO₂ = 0.070 × 44 = 3.08 g.
A Darking-type sample of 8.5 g with clay at one seventeenth would leave 8.5 ÷ 17 = 0.5 g of clay.
Our post on common mole concept errors covers the mole-ratio step in more detail.
What mistakes do students make here?
Using a 1 : 1 ratio for the acid. The equation needs 2 HNO₃ for each CaCO₃.
Assuming all mass loss is CO₂ in a real sample. Real limestones also contain sand and other minerals; Smeaton separated sand from clay by washing.
Confusing quicklime and limestone. CaO is made by heating CaCO₃; the acid test here is on the raw stone.
For the next step in this story, how cement became an industrial product, see our post on Joseph Aspdin and Portland cement. If balancing and mole ratios still trip you up, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What happened to the tower?
According to The Box, Plymouth's museum, which looks after Smeaton's Tower, the lighthouse cost £40,000. It was taken down in the early 1880s, when the sea was found to be eroding the rock beneath it, and about two thirds of it was rebuilt stone by stone on Plymouth Hoe.
Smeaton's book notes that the whole time spent working on the rock was 111 days and 10 hours, which "amounts scarcely to Sixteen Weeks". Check it: 111.4 ÷ 7 ≈ 15.9 weeks.
What can students learn from Smeaton's way of working?
Smeaton tested ideas on models, then on full-size work. When a property puzzled him, he found a measurement that could explain it, learned the chemistry he needed and kept careful numbers. He also reported a result that contradicted what he expected.
Pick one reaction from your syllabus this week, write its balanced equation, and calculate the mass of gas released from a 5 g sample.
