The Story of Alchemy and the Beginnings of Chemistry

We are accustomed to-day to use the words fire and water with different meanings, according to the ideas we wish to express. When we say "do not touch the fire," or "put your hand into the water," we are regarding fire and water as material things; when we say "the house is on fire," or speak of "a diamond of the first water," we are thinking of the condition or state of a burning body, or of a substance as transparent as water. When we say "put out the fire," or "his heart became as water," we are referring to the act of burning, or are using an image which likens the thing spoken of to a substance in the act of liquefying. As we do to-day, so the alchemists did before us; they used the words fire and water to express different ideas. Such terms as hardness, softness, coldness, toughness, and the like, are employed for the purpose of bringing together into one point of view different things which are alike in, at least, one respect. Hard things may differ in size, weight, shape, colour, texture, &c. A soft thing may weigh the same as a hard thing; both may have the same colour or the same size, or be at the same temperature, and so on. By classing together various things as hard or soft, or smooth or rough, we eliminate (for the time) all the properties wherein the things differ, and regard them only as having one property in common. The words hardness, softness, &c., are useful class-marks.