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Object lesson No 45

Cleanliness may be next to godliness, but when Jesus walked the earth you had to go a long way to find a bar of soap. Ancient Babylonians had discovered that boiling oils and alkalis produced a substance good for washing, and in Egypt they used a soda compound for cleaning; but in the years bc, body odour was not the social embarrassment it is today.

The Romans cleansed and conditioned their skin with olive oil and thought the rough red cakes of soap imported from Germany good only as hair dye.

But the famously fastidious Romans - who also collected stale urine to make an ammonia-based cleaner - were soon taking soap down to the baths.

With the fall of the Roman Empire, standards of sanitation declined and the Dark Ages were golden years for soap dodgers. Hygiene took a back seat to pestilence and plague and the only cleaning agent in town was soft soap, a pungent unguent made of wood ash and fish oil. Perfumed soap appeared in shops by the 18th century, but only the well-off could afford to wash their smells off.

Today, bars of soap still outsell recent inventions such as liquid soap and shower gel many times over. But the chemical giant nilever said last year it was washing its hands of world famous brand Pears, the distinctive smelling, amber, oval soap invented by hairdresser Andrew Pears in 1789.

But some spin-offs survive. Soap operas, named after the manufacturers who sponsored the first such serials, are still going strong, while politicians in search of the common touch still take to the poor man's podium of the soap box. Environmentalists get in a lather about the chemical contents of some modern soap powders which can harm marine life.

The defining characteristic of all soap is that it emulsifies grease and breaks the surface tension of water, enabling it to penetrate fabric or shift layers of dirt from the skin. Naturally occurring soap can be found in the fruit of the soapberry tree, whose latin name, saponaria, is where it gets its name.

But nature is not always so kind to your hands. During the second world war, soap shortages led to the German authorities advising people to wash their clothes in water which had been boiled with potato peelings or ivy leaves. But while this made the water feel soapy, it was no good at removing dirt. Suds law, obviously.

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