Complex sentence: If the thermometer contains nitrogen, the gas may flow down into the column when the mercury solidifies and be trapped there when the temperature rises, making the thermometer unusable until returned to the factory for reconditioning.
Split into simpler sentences: If the thermometer contains nitrogen, the gas may flow down into the column and be trapped there when the temperature rises. If this happens, the thermometer will be unusable until returned to the factory for reconditioning.