The great philosophers of ancient Greece believed that all the elements are modifications of one common substance, called the primary matter, which they demonstrated to be devoid of all quality and form, but susceptible of all qualities and forms. It is everything in capacity, but nothing in actuality. Matter is eternal; the elements are the first matter arranged into certain distinguishing forms. Some of the early philosophers held that all the materials which compose the universe existed in a fluid form; they understood by fire, matter in a highly refined state, and that it is the element most intimately connected with life, some even considering it the very essence of the soul. “Our souls are fire,” says Phornutus. “What we call heat is immortal,” says one of the Hippocratic writers, “and understands, sees, and hears all things that are or will be.”[369] Bacon explains the ancient fable of Proteus as signifying matter, a something which, being below all forms and supporting them, is yet different from them all. Sir Isaac Newton is not widely different from Strabo when he says that all bodies may be convertible into one another. Commenting upon these opinions of the Greek philosophers, Dr. Adams says, in his introduction to the works of Hippocrates:[370] “If every step which we advance in the knowledge of the intimate structure of things leads us to contract the number of substances formerly held to be simple, I would not wonder if it should yet turn out that oxygen, carbon, hydrogen, and nitrogen are—like what the ancients held the elements to be—all nothing else but different modifications of one ever-changing matter.” The theories of the Greek philosophers on the elements are poetically summed up in Ovid’s _Metamorphoses_:— “Nor those which elements we call abide, Nor to this figure nor to that are ty’d: For this eternal world is said of old But four prolific principles to hold, Four different bodies; two to heaven ascend, And other two down to the centre tend. Fire first, with wings expanded, mounts on high, Pure, void of weight, and dwells in upper sky; Then air, because unclogged, in empty space Flies after fire, and claims the second place; But weighty water, as her nature guides, Lies on the lap of earth; and mother Earth subsides. All things are mixed of these, which all contain, And into these are all resolved again; Earth rarifies to dew; expanding more, The subtle dew in air begins to soar; Spreads as she flies, and, weary of the name, Extenuates still, and changes into flame. Thus having by degrees perfection won, Restless, they soon untwist the web they spun, And fire begins to lose her radiant hue, Mixed with gross air, and air descends in dew! And dew condensing, does her form forego, And sinks, a heavy lump of earth, below. Thus are their figures never at a stand, But changed by Nature’s innovating hand.”[371]