We often hear of fires apparently "starting themselves." Such cases are due to accumulation of heat produced by slow oxidation. If a pile of oily rags, cotton waste, etc.,be allowed to stand for a time, the oily matter will begin to combine slowly with oxygen. This may occur in the inner part of the heap, and the outer layers retain the heat until, perhaps, the kindling point of some of the inflammable oils is reached, when the whole mass will burst into flame. This is much more likely to happen with linseed oil and certain other vegetable "drying oils," as they unite readily with oxygen, and so become hard and varnishlike. The mineral oils (paraffine oil) do not combine with oxygen at ordinary temperatures, and probably will not cause spontaneous combustion. Still, all oily cloths should be burned or disposed of in some safe fashion.