CH. VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 179 
all the activity when examined by the photographic method, while 
the other fraction was almost inactive. These results, taken by 
themselves, pointed very strongly to the conclusion that the 
activity of uranium was not due to the element itself, but to 
some other substance, associated with it, which had distinct 
chemical properties. 
Results of a similar character were observed by Becquerel’. 
It was found that barium could be made photographically very 
active by adding barium chloride to the uranium solution and 
precipitating the barium as sulphate. By a succession of precipi- 
tations the uranium was rendered photographically almost active, 
while the barium was strongly active. 
The inactive uranium and the active barium were laid aside; 
but, on examining them a year later, it was found that the wraniwm 
had completely regained its activity, while that of the bariwm had 
completely disappeared. 'The loss of activity of uranium was thus 
only temporary in character. 
In the above experiments, the activity of uranium was examined 
by the photographic method. The photographic action produced 
by uranium is due almost entirely to the @ rays. The a@ rays, in 
comparison, have little if any effect. Now the radiation from Ur X 
consists entirely of @ rays, and is consequently photographically 
very active. If the activity of uranium had been measured 
electrically without any screen over it, the current observed would 
have been due very largely to the « rays, and little change would 
have been observed after the removal of Ur X, since only the con- 
stituent responsible for the 8 rays was removed. This important 
point is discussed in more detail in section 189. 
119. Thorium X. Rutherford and Soddy?, working with 
thorium compounds, found that an intensely active constituent 
could be separated from thorium by a single chemical operation. 
If ammonia is added to a thorium solution, the thorium is precipi- 
tated, but a large amount of the activity is left behind in the 
filtrate, which is chemically free from thorium. This filtrate was 
evaporated to dryness, and the ammonium salts driven off by 
IC. h. 130) p. 137, 1900); 133, p: 977, 1902: 
2 Phil. Mag. Sep. and Nov. 1902. Trans. Chem. Soc. 81, pp. 321 and 837, 1902. 
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