THE TRANSFORMATIONS OF ELEMENTS 543 



A short account of the individual elements will now be 

 given, in the order shown opposite. 



Uranium was discovered by Klaproth in the mineral pitch- 

 blende in 1789. It is of comparatively rare occurrence, and 

 widely scattered. The composition of uranium minerals varies 

 largely ; many are complex silicates, phosphates, or arsenates. 

 Pitchblende and uraninite consist largely of uranium oxide, and 

 contain from 50-80 per cent, uranium. The metal was isolated 

 by Peligot in 1842 by the action of potassium on the fused oxide. 

 Thus formed it is a black powder. In the ordinary metallic 

 state it closely resembles iron and nickel. Heated to redness 

 it burns brilliantly. It dissolves in dilute acids, with evolution 

 of hydrogen ; combines directly with chlorine, and, on heating, 

 with sulphur. Its atomic weight is 238-5 ; it is thus the heaviest 

 metal yet isolated. A great number of its compounds have been 

 prepared. The discovery of radioactivity by Becquerel, in 1896, 

 was made with these compounds; it has been shown that the 

 metal is also radioactive. The life of uranium is extremely 

 great. Its half period is many millions of years. 



In 1900 Sir William Crookes showed that when excess of 

 ammonium carbonate is added to a uranium solution, until the 

 uranium precipitate has all redissolved, a light cloudy pre- 

 cipitate remains: this contains all the photographic activity; 

 the separated uranium is without effect on a photographic 

 plate (a rays do not affect a photographic plate ; uranium gives 

 off a rays only) ; Crookes called the new product uranium X. 

 The precipitate consists largely of impurities from the uranium 

 salt, and insufficient uranium X has been obtained to give a 

 distinctive spectrum. A mixture of ether and water effects 

 a partial separation. The ether layer contains most of the 

 photographic or /3-ray activity. Becquerel effected a separa- 

 tion by adding barium chloride and precipitating the barium as 

 sulphate ; uranium X is carried down along with it. Recently 

 (in 1906) Moore and Schlundt have described further methods of 

 separation. When crystallised uranium nitrate is dissolved in 

 acetone, or several similar organic solvents, the slight precipi- 

 tate which remains contains most of the uranium X. Complete 

 separation is effected by adding moist ferric hydroxide to such 

 a solution ; the hydroxide carries down all the active matter. 

 Moore and Schlundt state that uranium X gives both a and 

 fi rays ; 34 per cent, of the activity is due to the a-rays. Levin 



