It was discovered in 1896 by Henri Becquerel that uranium and its salts, when placed on a photographic plate, wrapped in black paper to exclude light, were capable of impressing the paper so as to give an image on development. This power is not exhausted by repetition, but resides permanently in the uranium. Some years later, Madame Curie, on examining certain minerals containing uranium, found that their action on a photographic plate, and also their capacity for discharging an electroscope, was much greater than the amount of uranium they contained could account for; for Becquerel had found the action of the uranium salts to be proportional to the amount of uranium they contain. The greatest activity was found by Madame Curie to reside in the residues from the uranium works at Joachimsthal in Bohemia,-residues from which the uranium had in great part been removed.

After a long and arduous research, aided by her husband, she separated from these residues a considerable quantity of barium in the form of sulphate, of many hundred times the activity of the original residues. The sulphate was converted into chloride, and the chloride was fractionally crystallised many times, the less soluble portions being set on one side. After some years' work she succeeded in separating a hitherto undiscovered element, to which she gave the name of radium.

This discovery was followed by that of actinium by Debierne; Giesel also separated a similar substance under the name emanium, which has proved to be identical with actinium. It is allied in its reactions to the elements of the yttrium group.

About the same time, Schmidt observed that thorium salts also possess the power of discharging an electroscope when they are placed in the vessel containing the gold leaves.

It was soon discovered that the power of discharging an electroscope was due to the emission of three kinds of " rays." The first of these, " a-rays," were recognised by Rutherford and Soddy to be due to the presence of a gas, condensable at a low temperature. After they had determined this fact for thorium and radium, Debierne and Giesel found that the a-rays from actinium are due to the same cause. The second kind of rays, "/3-rays," have a much greater penetrating power, for they pass through glass and thin sheets of metal. They have been recognised as free electrons, travelling with a velocity approaching that of light. Their mass is about yinnr^¹ °^ ^t^^iat °^ hydrogen, atom for atom. The third variety of rays are almost certainly perturbations of the ether, somewhat like those which cause light and heat.

The most completely investigated of these curious elements is radium; the amount obtainable from the Joachimsthal residues, however, is exceedingly minute; from 1 ton, i.e. 1000 kilograms, it is possible to extract about 1 gram of radium in the form of bromide, or 1 part per million. Had it not been that the test for the presence of radium, viz., its power of discharging an electroscope, greatly surpasses in delicacy all other tests, even the most sensitive spectroscopic tests, it would never have been discovered.