(1) If we talk about causes we may be tempted to list those we can do little or nothing about. For example, a source of ignition is often said to be the cause of a fire. But when flammable vapour and air are mixed in the flammable range, experience shows that a source of ignition is liable to turn up, even though we have done everything possible to remove known sources of ignition (see Chapter 4). The only really effective way of preventing an ignition is to prevent leaks of flammable vapour. Instead of asking, ‘What is the cause of this