Page:Popular Science Monthly Volume 64.djvu/488

484 precisely the same effects which Becquerel had discovered with uranium. After this discovery the rays from all this class of substances began to be called Becquerel rays, in honor of Becquerel, and all substances which emitted such rays were called radio-active substances.

But in connection with this investigation, Madame Curie noticed something which appeared to her very noteworthy. It was that pitchblend, 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 contain 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 pitchblend, such for example, as uranium, barium, lead, copper, arsenic, antimony, and so on, and after each separation, testing the two portions 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 pitchblend 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 precipitate 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