Iron, although one of the imperfect metals, is susceptible of a very high polish, and more capable than any other metal of having its hardness increased or diminished by certain chemical processes. It is often manufactured in such a way as to be _one hundred and fifty times_, and, as will now be seen, in some cases, to be even above _six hundred and thirty times_ more valuable than gold. On weighing several common watch-pendulum springs, such as are sold for ordinary work by the London artists, at half a crown, ten of them were found to weigh but one single grain. Hence one pound avoirdupois, equal to seven thousand grains, contains ten times that number of these springs, which amount, at half a crown each, to eight thousand, seven hundred and fifty pounds sterling. Reckoning the troy ounce of gold at four pounds sterling, and the pound equal to fifty-seven hundred and sixty grains, at forty-eight pounds sterling, the value of an avoirdupois pound of gold is over fifty-eight pounds sterling. The above amount of the value of the watch-springs weighing an avoirdupois pound, being divided by that sum, will give a ratio of somewhat more than one hundred and fifty to one. But the pendulum-springs of the best kind of watches sell at half a guinea each; and at this price the above-mentioned value is increased in the ratio of four and one-fifth to one; which gives an amount of thirty-six thousand, seven hundred and fifty pounds sterling. This sum being divided by the value of the avoirdupois pound of gold, gives a quotient of more than six hundred and thirty to one. It is one of the valuable properties of iron, after it is reduced into the state of steel, that, although it is sufficiently soft when hot, or when gradually cooled, to be formed without difficulty into various tools and utensils, still it may be afterward rendered more or less hard, even to an extreme degree, by simply plunging it, when red-hot, into cold water. This is called tempering, the hardness produced being greater in proportion as the steel is hotter and the water cooler. Hence arises the superiority of this metal for making mechanics’ instruments or tools, by which all other metals, and even itself, are filed, drilled and cut. The various degrees of hardness given to iron, depend on the quantity of ignition it possesses at the moment of being tempered, which is manifested by the succession of color exhibited on the surface of the metal, in the progress of its receiving the increasing heat. These are, the yellowish white, yellow, gold-color, purple, violet, and deep blue; after the exhibition of which the complete ignition takes place. These colors proceed from a kind of scorification on the surface of the heated metal.