What was the problem that the mother was working over even while she sewed for her little girl, or rocked her to sleep to the gentle crooning of an old Polish folk-song whose melody Chopin has wrought into one of his tenderest nocturnes? [Illustration: Marie Sklodowska Curie] The child who used to delight in experiments with light-waves in her father's laboratory, was interested in the strange glow which Prof. Becquerel had found that the substance known as uranium gave off spontaneously. Like the X-rays, this light passes through wood and other bodies opaque to sunlight. Madame Curie became deeply interested in the problem of the nature of the Becquerel rays and their wonderful properties, such as that of making the air a conductor for electricity. One day she discovered that pitchblende, the black mineral from which uranium is extracted, was more _radioactive_ (that is, it gave off more powerful rays) than the isolated substance itself, and she came to the conclusion that there was some other element in the ore which, could it be extracted, would prove more valuable than uranium. With infinite patience and the skill of highly trained specialists in both physics and chemistry, Madame Curie and her husband worked to obtain this unknown substance. At times Pierre Curie all but lost heart at the seemingly insurmountable obstacles in the way. "It cannot be done!" he exclaimed one day, with a groan. "Truly, 'Nature has buried Truth deep in the bottom of the sea.'" "But man can dive, _cher ami_," said his wife, with a heartening smile. "Think of the joy when one comes up at last with the pearl--the pearl of truth!" At last their toil was rewarded, and _two_ new elements were separated from pitchblende--polonium, so named by Madame Curie in honor of her native Poland, and radium, the most marvelous of all radioactive substances. A tiny pinch of radium, which is a grayish white powder not unlike coarse salt in appearance, gives out a strange glow something like that of fireflies, but bright enough to read by. Moreover, light and heat are radiated by this magic element with no apparent waste of its own amount or energy. Radium can also make some other substances, diamonds for instance, shine with a light like its own, and it makes the air a conductor of electricity. Its weird glow passes through bone almost as readily as through tissue-paper or through flesh, and it even penetrates an inch-thick iron plate. The Curies now woke to find not only Paris but the world ringing with the fame of their discovery. The modest workers wanted nothing, however, but the chance to go on with their research. You know how Tennyson makes the aged Ulysses look forward even at the end of his life to one more last voyage. The type of the unconquerable human soul that ever presses on to fresh achievement, he says: All experience is an arch where-thro' Gleams that untravel'd world, whose margin fades Forever and forever when I move.