When it comes to the alcoholic strength of whisky, it would seem straightforward enough.Europeans have one way of measuring and the Americans are left seeing double.The European system of alcohol by volume results in a figure that is exactly half of the American proof system.Easy, isn’t it?Or is it? Why, then,does a bottle of Isle of Arran 100 Proof have a strength of 57% alcohol by volume? It would seem that there is more to this than meets the eye.Historically the word ‘proof’ comes from the tests that were applied to a spirit in Britain to determine how strong it was for tax purposes.Over the centuries various crude methods have been used, the most common being to mix the liquid with gunpowder and to try and light it.In theory the strength at which gunpowder could be ignited – when it was ‘proved’– would be 100 proof; anything above it was over proof and anything under was under proof.This was of course a load of old tosh – arbitrary at best and purely inaccurate at worst,because external temperature, atmospheric pressure conditions and the amount of spirit added all affect the outcome.Over time the science became more sophisticated, and improved chemistry tests played their part.Ethyl alcohol is lighter than water, and in 1818 a measure of pure alcohol was defined as weighing twelve thirteenths the weight of the same measure of pure distilled water when measured under the same pressure at a temperature of 51 fahrenheit – or a ratio of 0.9238 alcohol to 1.0 water.This is known as the specific gravity of proof and represents 100 proof – the theoretical strength when gunpowder would ignite – and the mix is shown to contain 57.1% alcohol by volume.This explains the Arran bottling.Using this method, 100 per cent pure alcohol has a proof of 175, and is sometimes referred to as 75 over-proof. Fifty per cent alcohol has a strength of exactly half that - 87.5 proof.The proof system in Britain was replaced by the alcohol by volume system in 1980 under a European directive.This simply calculates the percentage of the liquid that is alcohol, with pure alcohol being 100 per cent. Spirit that had been 70 or 75 proof in the bottle under the old British system was now 40% or 43% ABV.The Americans,never ones to make things complicated unless it involves sport,went for an altogether simpler system.They decided that a typical bottle of spirit would contain 50 per cent alcohol and 50 per cent water, so they ruled that a spirit that was 50%ABV was the equivalent to 100 proof.So a 40% ABV whisky has an old British proof of 70 and an American proof of 80.Nothing complicated about that, is there!GLOSSARY PEAT Naturally produced fuel created by the suppression of vegetation and burned to dry barley,imparting phenols that flavour some whiskies.