Firefighters warned on Saturday that a tank of toxic chemicals in California is heating up, adding to fears of a catastrophic explosion that has already forced tens of thousands of Californians to evacuate.About 40,000 residents were ordered to leave their homes in the Garden Grove area of Orange county, southeast of Los Angeles, on Friday after the tank began to leak, sending fumes over a heavily populated area.The tank contains 26,000 litres (7,000 gallons) of methyl methacrylate, a volatile...