7 o [SCIENCE PROGRESS 



the crystallised bromide, assumed to be RaBr 2 , 2H2O, is some- 

 what less soluble than BaBr 2 , 2H 2 0. The salts colour a flame 

 crimson, as do strontium salts. The spectrum was first examined 

 by Demarcay, and has more recently been studied by a number 

 of other physicists. In its general aspect it resembles the 

 spectra of the other alkaline earth metals. 



The salts are quite white when freshly prepared ; they 

 closely resemble barium and calcium salts. On long standing 

 they turn first yellow, and finally brown. It is found that 

 the colour is much deeper with impure preparations ; pure 

 salts are scarcely coloured after a considerable period. This 

 can be used as a test of the degree of purity. 



All the radium salts are radioactive ; it is practically certain 

 that the metal would exhibit the same radioactive properties 

 and disintegrate in the same way. There are several grounds 

 for this statement. Uranium has the same radioactive properties, 

 whether free in the state of metal or combined in its salts. 

 When a solution of radium bromide is electrolysed, the mercury 

 becomes permanently active, showing the formation of radium 

 amalgam. This shows the same radioactive properties of its 

 salts. Several radioactive elements have been deposited on 

 metals, that is, they have been deposited in metallic form 

 themselves. One example is polonium or radium F, separated 

 by placing a rod of bismuth in a solution of its salt; it 

 possesses the same activity as metal as it does in solution. 



Direct determinations of the atomic weight of radium have 

 been made by Madame Curie. In her initial experiments, by 

 successive crystallisations of barium chloride she obtained 

 fractions increasing in atomic weight from 137*5 (barium) to 

 225. The final fraction weighed only 0'i gram. Much later, 

 in 1907, she worked up 0*4 gram of chloride of such purity that 

 its spark-spectrum showed that only minute traces of barium 

 were present. By precipitating the chloride as silver chloride 

 with warm silver nitrate solution the ratio Ra : Ag was obtained. 

 Three successive determinations of the atomic weight with the 

 same fraction gave as mean value 226*18. Madame Curie con- 

 cluded that the atomic weight is 226*2 ± 0*5. 



Quite recently Thorpe has published the results of his 

 determinations, using Madame Curie's methods. The spectrum 

 of his final preparation of radium chloride scarcely showed 

 the presence of the strongest barium lines. Three successive 



