50 A. IV. Stewart on 



broken fragment of an incandescent mantle, containing oxide of 

 thorium, upon a i»hotograi)liic plate and leaving it in the dark for 

 a week. On development the plate will show the reticulations 

 of the mantle. Soon the new element actinium was discovered, 

 and Mme. and M. Curie isolated the salts of radium from pitch- 

 blende. 



After months of laborious work it was shown that radium 

 was chemically closely related to barium, since it was extremely 

 difficult to separate these t\ro elements, though it was com- 

 paratively easy to isolate the radium-barium group from all the 

 other elements in pitch-blende. 8ome idea of the labour involved 

 may be got from the fact that fiom one ton of the material less 

 than the thirtieth of an ounce of very impure radium salt was 

 obtained. 



The chemical resemblance between radium and barium 

 shows they belong to the same group in the Periodic System, and 

 it is seen that radium lies just below barium in Group II of this 

 classification. The other radio-active elements have positions in 

 the table between lead and uranium, so that their atomic 

 weights are all high. Potassium and rubidium form exceptions, 

 and investigation in their case is still incomplete. 



It is found that radium compounds have properties different 

 from those of other elements. They break up water just as 

 electricity does ; their temperature is always slightly higher than 

 that of their surroundings ; they colour glass vessels in which 

 they are kept, and they give off the " rays " which are called 

 Becquerel rays. Finally, radium salts emit a radio-active gas, 

 niton, which will be dealt with in the next lecture. 



A simple experiment shows that no less that three types of 

 " ray " are emitted by radio-active materials. A sample of a 

 radio-active salt is allowed to radiate upon au electroscope, and 

 the rapidity of discharge of the electroscope is observed. If, 

 now, a sheet of cigarette paper is wrapped about the salt, and 

 the observation repeated, it is found that there is a marked 

 diminution in the rate of discharge. Thus a fraction of the 



