484 POPULAR SCIENCE MONTHLY. 



ducing precisely the same effects whicli Becquerel had discovered with 

 uranium. After this discovery the rays from all this class of sub- 

 stances began to be called Becquerel rays, in honor of Becquerel, and 

 all substances which emitted such rays were called radio-active sub- 

 stances. 



But in connection with this investigation, Madame Curie noticed 

 something which appeared to her very noteworthy. It was that pitch- 

 blend, which is the crude ore from which uranium is extracted and 

 which consists chiefly of uranium oxide, would produce an effect upon a 

 photographic plate, or would discharge an electrified body, in about 

 one fourth the time in which the same weight of a pure uranium salt 

 would produce the same effect. She inferred, therefore, that the 

 activity of pitchblend in emitting rays could not be due solely to the 

 uranium contained in it : that, on the contrary, pitchblend must con- 

 tain some hitherto unknown element which had the property of 

 emitting Becquerel rays more powerfully than uranium itself. She 

 therefore immediately set about the task of separating as carefully as 

 possible the dozen or so of substances which are contained in pitch- 

 blend, such for example, as uranium, barium, lead, copper, arsenic, 

 antimony, and so on, and after each separation, testing the two por- 

 tions separated to find which part carried with it the activity, that is, 

 the ability to affect a photographic plate or to discharge an electrically 

 charged body. The methods employed were the ordinary ones used in 

 qualitative chemical analysis. The search was a long and difficult one, 

 but ended triumphantly in the separation from several tons of pitch- 

 blend of two or three grains of the new element which has now become 

 one of the wonders of the world. 



The successive steps in this discovery were as follows: Madame 

 Curie found, first, that in this process of separation of the constituents 

 of pitchblend, the reagent which separated the barium out of the 

 solution also brought down in the barium precipitate a large part of 

 the activity. The barium chloride precipitate obtained in this way 

 had about sixty times the activity of pure uranium chloride. She next 

 found that when alcohol was added to a solution of this barium 

 chloride, the first precipitate which was thus formed was more active 

 than that which came down later. By retaining only this first pre- 

 cipitate and discarding the rest, and again redissolving and repeating 

 the process over and over again (this process is called fractional 

 precipitation) she succeeded in obtaining a sample of barium chloride 

 which was 4,000 times as active as uranium chloride. Further, since 

 the weight of the barium chloride for a given weight of contained 

 chlorine was greater in the ratio 140 to 137 than the weight of ordinary 

 inactive barium chloride for the same weight of contained chlorine, she 

 concluded that the apparent activity of the barium chloride could not 

 be due to barium at all, but must be due to this unknown element which 



