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The flat stone was a decoy, concealing the most advanced timing system in this world, along with several chemical substances including white phosphorus.
The timing system was driven by a spring and constituted by multiple sets of gears, which would trigger a small explosion by fusing potassium chlorate and phosphorus when a set time was reached.
The explosive power of potassium chlorate and phosphorus was not substantial, but it was enough to ignite a large amount of white phosphorus mixture combined with multiple chemical substances.
The white phosphorus mixture inside the flat stone was made according to the proportions used in military white phosphorus grenades, and if Li Shiming hadn't been worried about the power being too great, he would have prepared to use the white phosphorus grenades he made directly.