The metal radium has not yet been prepared ; its salts resemble those of barium, the sulphate being even less soluble than barium sulphate ; its carbonate forms a white crystalline precipitate ; its halides are soluble, and when freshly prepared, white and crystalline; its nitrate, like that of barium, is somewhat sparingly soluble, and even less in presence of excess of nitric acid. It is best separated from barium by recrystallisation of the bromide; the crystals which separate first from the mixed solution are much richer in radium than the mother-liquor. After twenty or thirty recrystallisations, a practically pure product is obtainable.
Its atomic weight was determined by Madame Curie, and found to be 225 ; but a revision of this constant has recently been carried out by Dr. Brill, who finds that on recrystallisation the atomic weight of the less soluble portion rises to 24O, and may possibly be higher; spectroscopic evidence points to an even higher number, about 256. Final evidence on this point will soon be obtained.
The name "emanation " was given by Rutherford to the gas which radium salts continuously evolve. It is best collected by enclosing a solution of a salt of radium in a small glass or silica bulb, in communication with a Toplers pump, from which every four days a mixture of hydrogen and oxygen may be extracted, carrying with it the emanation. The actual volume of the emanation is very small-0.2 cubic millimetre for 0.1 gram of radium bromide. That the emanation is a gas is shown by its obeying Boyle's law; it possesses a spectrum similar in character to the gases of the argon group, chiefly consisting of green lines. It is unattacked by any chemical reagent; like argon, it withstands the action of a red-hot mixture of magnesium and lime, and also sparking with oxygen over caustic potash. It condenses at about -1500, probably to a solid. Immediately after it evaporates it begins to contract to half-volume; and on heating to 8o° it again acquires its original volume; in this it resembles NO0 which polymerises to N2O4; but the process of polymerisation, in the case of the emanation, lasts a considerable time. Its atomic weight is unknown, but it appears not unlikely to be about 215, in which case it would be the second element after xenon.
It possesses, like radium, the extraordinary property of changing into other elements. While the time necessary for a given weight of radium to be reduced to half by conversion into its emanation is about 250 years, the emanation undergoes half-change in 3.8 days. This change is accompanied by an enormous evolution of heat; indeed, for a cubic centimetre of emanation, the amount of heat is 3,500,000 times that evolved during the explosion of an equal volume of a mixture of two parts of hydrogen with one of oxygen. It has also the power of decomposing water into its constituent gases, and it appears to be fairly soluble in water.
If its spontaneous change occurs in the gaseous state, it disappears, and leaves in its place about three times its volume of helium; if its solution in water is allowed to stand, the product is neon; and if there is present in its solution sulphate or nitrate of copper, the product appears to be argon. In fact it is not improbable that the energy evolved during the change undergone by the major part of the emanation is the cause of the degradation of a smaller part into lower members of the same group. At the same time, the copper appears to suffer a similar change ; for the spectrum of lithium has been observed in the alkaline residue left after removal of the copper; and copper and lithium belong to the same group of elements.
During the spontaneous change of the emanation, a-rays alone are emitted. By electrical measurements of the periods of decay of the various products, their existence has been inferred; thus radium A emits only a-rays, and its period of half-change is 3 minutes; it changes into radium B in 26 minutes, evolving only f3- and y-rays; the next change is into C, half of B being converted in 19 minutes, with evolution of a-, /?-, and y-rays. Next comes radium D, which is half-changed into radium E in 40 years, without the emission of any recognisable rays; E has a period of half-change of 6 days, and F of 143 days. F is supposed to be identical with polonium, a metallic substance with much analogy to bismuth, the first radioactive body separated by Madame Curie from pitchblende residues. At least the times of half-decay of radium F and polonium are nearly the same. These substances, although present in quite invisible amount, yet show chemical and physical differences; some are soluble in weak acids, some in strong ; some are precipitable by nickel, others even by bismuth; some are volatile at a red heat, others not. They deposit on the walls of the vessel in which the emanation is contained.
The /2-rays, or electrons from radium C, cause soda-glass to turn purple, potash-glass brown, gold-glass crimson-pink, etc. This is probably to be ascribed to the fact that the sodium, for example, in soda-glass is in the form of ions, and colourless, as it is in salt. The electrons de-ionise the sodium atoms, or, in other words, combine with them, and the product is metallic sodium in a state of atomic division; and just as metallic gold colours glass crimson, so metallic sodium colours it purple; and so with the rest.