The next division of the work relates to potassium. Davy had hitherto produced that metal only in minute quantities by the action of the galvanic battery upon potash. But Gay-Lussac and Thenard contrived a process by which it can be prepared on a large scale by chemical decomposition. Their method was, to put into a bent gun-barrel, well coated externally with clay, and passed through a furnace, a quantity of clean iron-filings. To one extremity of this barrel was fitted a tube containing a quantity of caustic potash. This tube was either shut at one end by a stopper, or by a glass tube luted to it, and plunged under the surface of mercury. To the other extremity of the gun-barrel was also luted a tube, which plunged into a vessel containing mercury. Heat was applied to the gun-barrel till it was heated to whiteness; then, by means of a choffer, the caustic potash was melted and made to trickle slowly into the white-hot iron-filings. At this temperature the potash undergoes decomposition, the iron uniting with its oxygen. The potassium is disengaged, and being volatile is deposited at a distance from the hot part of the tube, where it is collected after the process is finished. Being thus in possession, both of potassium and sodium in considerable quantities, they were enabled to examine its properties more in detail than Davy had done: but such was the care and industry with which Davy's experiments had been made that very little remained to be done. The specific gravity of the two metals was determined with more precision than it was possible for Davy to do. They determined the action of these metals on water, and measured the quantity of hydrogen gas given out with more precision than Davy could. They discovered also, by heating these metals in oxygen gas, that they were capable of uniting with an additional dose of oxygen, and of forming peroxides of potassium and sodium. These oxides have a yellow colour, and give out the surplus oxygen, and are brought back to the state of potash and soda when they are plunged into water. They exposed a great variety of substances to the action of potassium, and brought to light a vast number of curious and important facts, tending to throw new light on the properties and characters of that curious metallic substance. By heating together anhydrous boracic acid and potassium in a copper tube, they succeeded in decomposing the acid, and in showing it to be a compound of oxygen, and a black matter like charcoal, to which the name of _boron_ has been given. They examined the properties of boron in detail, but did not succeed in determining with exactness the proportions of the constituents of boracic acid. The subsequent experiments of Davy, though not exact, come a good deal nearer the truth.