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{X.^yR, or about 4.1 times R. And if (for instance) air at normal tem- 

 perature and pressure is bombarded by the alpha-particles of radium 

 C which in this gas have a range of seven centimeters, and scintillations 

 are observed on a fluorescent screen beyond, the observer must reckon 

 with the chance that they may be due to the nuclei of hydrogen mole- 

 cules mixed with the air, so long as the distance to the screen is less than 

 4.1 times seven, or say thirty, centimeters. 



On the principle that the best way to deal with a possible source of 

 trouble is to examine it minutely, Rutherford prepared for his attempt 

 at transmutation by a study of the nuclei which are struck and which 



* Fig. 1 — -Rutherford's apparatus for detecting transmutation of gases by the 

 scintillation method. Source of alpha-particles at D; gas in the tube; fluorescent 

 screen (transparent) at 5; microscope at M. 



* From Sir Ernest Rutherford, James Chadwick and C. D. Ellis, " Radiations from 

 Radioactive Substances," 1930. By permission of The Macmillan Company, 

 publishers. 



"recoil," as we say, when alpha-particles are fired into hydrogen. His 

 pupil Marsden had begun on such a study in 1914, and had observed 

 that scintillations appeared on a screen set up far beyond the ultimate 

 reach of the alpha-rays — more than a hundred centimeters, inasmuch as 

 in hydrogen the range of either kind of charged corpuscle is about four 

 times as great as in air. Resuming the research in 1919 (one never 

 needs to ask why things begun in 1914 should have lain so long un- 

 continued) Rutherford counted the scintillations and plotted their 

 number for what, in effect, were various distances of the screen from 

 the source. I must pause to say that in practice one does not draw the 

 screen back so as to interpose thicker and thicker layers of gas between 



