Even greater surprises were yet in store, for this power of uranium to give out a new kind of rays which impressed a photographic plate was found to be quite indestructible. No matter if the uranium was melted up in a crucible, pounded to a fine powder, mixed with other substances, melted or boiled, or treated with all kinds of acids, the effect was always the same so long as the quantity of uranium remained the same. The new and strange power seemed to lie in the very atoms of the uranium themselves. This power was quite new, and therefore had to be called by a new name. The wife of another French professor of the name of Curie, who greatly distinguished herself in studying this new power, called it “radio-activity,” and this name is now used generally. Professor Curie and his wife went to a great deal of trouble to concentrate the uranium and make it as highly radio-active as they could. They found that some preparations of uranium were more active than others, and, therefore, they found that a certain sediment of the uranium ore was quite extraordinarily radio-active. They extracted this sediment from several tons of the ore. It was an extremely expensive operation, but it was richly rewarded by another marvellous discovery, for at the end of a great deal of work they found a new substance which was a million times more radio-active than uranium. This substance they called “radium.” So intensely active was this new substance, that if shown in the dark, a small piece of it made a phosphorescent screen brightly luminous. But the radium proved as dangerous as the X-rays, for Madame Curie, who wore a small quantity of it in her sleeve, found that it gave her a sore which took several months to heal. After the tragic death of her husband in a motor accident Madame Curie was appointed Professor of Chemistry in the Paris University, being the first lady ever appointed to such a post in that ancient city of learning. CHAPTER XVII AIR-SHIPS AND FLYING MACHINES WHEN we read of the great inventions of the past, we are apt to think that they would in any case have been invented some time or other, even though the great inventors themselves had never lived. But to tell the truth, each inventor had before him a great number of difficulties which he could only overcome by patient and persistent effort. It often happened that some attempt to solve the problem had been made before him, and had failed. Such a failure would lead men to believe that success was impossible. Somebody would try to prove that the problem could not possibly be solved, and that one might as well give it up as hopeless. It is just this kind of attitude which is most disturbing to inventors and discoverers. They are surrounded by friends who never cease to tell them that they are simply wasting their time in trying to solve a problem which cannot be solved.