CONTEMPORARY ADVANCES IN PHYSICS 



635 



and the point of entry of the alpha-particles; instead one leaves it 

 fixed and varies the pressure of the gas, or else interposes a series of 

 thin foils of aluminium or some other metal or of mica, each of which 

 slows down the particles to the same extent (in the technical language, 

 has the same "stopping-power") as a known thickness of air. (For 

 instance, a thickness of mica of weight 1.43 mg. per square cm. is 

 equivalent in stopping-power to 1 cm. of air at 15° C. and 760 mm. Hg.) 

 In curves of the sort in which we shall be interested, number-of-scintil- 

 lations is usually plotted along the vertical axis, number-of-centi- 

 meters-of-air along the horizontal ; but in general some other substance 

 did duty for the air, and its thicknesses were translated into equivalent 

 thicknesses of this standard gas (at normal temperature and pressure) 

 before the curve was drawn. 



Curves of this sort appear in Fig. 2. All of them were obtained with 

 gases bombarded by alpha-particles of seven-centimeter range. No- 



'lO 18 26 34 42 50 58 66 74 82 



RANGE IN EQUIVALENT CENTIMETERS OF AIR 



* Fig. 2 — Number of protons falling on fluorescent screen, plotted as function of 

 thickness of air which they have traversed since leaving the disrupted atoms. 

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

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

 publishers. 



tice first the curve marked B\ it corresponds to hydrogen, mixed with 

 carbon dioxide; and it testifies that the scintillations did not cease until 

 the screen was shielded by the equivalent (in mica) of thirty centi- 

 meters of air, the amount computed for the range of hydrogen nuclei 



