This demonstration is a direct descendent of Otto von Guericke's observation in the sixteen fifties that a pressure difference of one atmosphere would drive water through the walls of a wooden barrel. Precautions have to be taken to keep mercury out of the pump mechanism, which is made of brass and therefore amalgamates easily. We are very unlikely to do this demonstration today because of the well-documented toxicity of mercury and mercury vapor. Scientists of the nineteenth century (and earlier) handled mercury in this and other experiments with no apparent realization of its dangers. Electrical apparatus routinely used exposed mercury contacts to secure freedom of motion, along with low contact resistance. Mercury poisoning was regarded as an acceptable, if unfortunate, industrial hazard. Remember that the Hatter in "Alice in Wonderland" was not mad; he suffered from "hatter's shakes", a tremor caused by the mercury compounds which he used in his trade.
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