This classic chemistry demo involves the use of toxic metallic mercury, so it’s one of those that is best to just watch on YouTube instead of trying yourself. The pulsing action is caused by surface tension effects–metallic mercury is oxidized at the surface of the drop to form a film of mercury (I) sulfate, which lowers the drop’s surface tension and causes it to flatten under its own weight. The flattening brings the drop into contact with the tip of a carefully-positioned iron nail, which reduces the mercury (I) sulfate back to metallic mercury, which in turn increases the drop’s surface tension and causes it to contract away from the nail. The solution contains an electrolyte and an oxidizing agent, in this case weak sulfuric acid and potassium dichromate, respectively. Thanks to YouTuber sciencevidds for sharing it with us. [via Boing and then some more Boing]