me be your bottle-washer”—that the good-hearted Cornishman took the poor blacksmith’s son, then twenty-one years old, after eight years of book-stitching, and made him his assistant, “keeping him in his place,” nevertheless, which, for an assistant in those days, meant feeding with the servants, except by special invitation. This was in 1823, and next year Faraday had liquefied chlorine, and soon did the same for a dozen more gases, among them protoxide of nitrogen, to liquefy which, at a temperature of fifty degrees Fahrenheit, was needed a pressure of sixty atmospheres—sixty times the pressure of the air—i.e., nine hundred pounds on every square inch. Why, the strongest boilers, with all their thickness of iron, their rivets, their careful hammering of every plate to guard against weak places, are only calculated to stand about ten atmospheres; no wonder then that Faraday, with nothing but thick glass tubes, had thirteen explosions, and that a fellow-experimenter was killed while repeating one of his experiments. However, he gave out his “Law,” that any gas may be liquefied if you put pressure enough on it. That “if” would have left matters much where they were had not Bussy, in 1824, argued: “Liquid is the middle state between gaseous and solid. Cold turns liquids into solids; therefore, probably cold will turn gases into liquids.” He proved this for sulphurous acid, by simply plunging a bottle of it in salt and ice; and it is by combining the two, cold and pressure, that all subsequent results have been attained. How to produce cold, then, became the problem; and one way is by making steam. You cannot get steam without borrowing heat from something. Water boils at two hundred and twelve degrees Fahrenheit, and then you may go on heating and heating till one thousand degrees more heat have been absorbed before steam is formed. The thermometer, meanwhile, never rises above two hundred and twelve degrees, all this extra heat becoming what is called latent, and is probably employed in keeping asunder the particles which when closer together form water. The greater the expansive force, the more heat becomes latent or used up in this way. This explains the paradox that, while the steam from a kettle-spout scalds you, you may put your hand with impunity into the jet discharged from a high-pressure engine. The high-pressure steam, expanding rapidly when it gets out of confinement, uses up all its heat (makes it all “latent”) in keeping its particles distinct. It is the same with all other vapors: in expanding they absorb heat, and, therefore, produce cold; and, therefore, as many substances turn into steam at far lower temperatures than water does, this principle of “latent heat,” invented by Black, and, after long rejection, accepted by chemists, has been very helpful in the liquefying of gases by producing cold.