REVIEW OF EXPERIMENTAL INFORMATION 141 



whole being immersed in Avater, and the range of the Po alpha particles 

 was determined from the contour of blackening of the emulsion. A 

 careful study under different experimental circumstances led to a value 

 for the range of Po alpha particles (5.298 Mev) of 32.0 ± 0.5 microns in 

 liquid water. Michl dealt as best he could with a number of possible 

 complications— among them swelling of the gelatin and depression of the 

 gelatin by the wire, as well as solubility of the Po in water— but it is 

 virtually impossible today to assess the accuracy of his final result. 



Michl's value for the range in liquid water is 20 per cent smaller than 

 that calculated for water vapor at (hypothetically) equal density using 

 modern data (cf. Table 1, p. 143). 



The ranges of Po alpha particles in alcohol and in six other organic 

 liquids were also determined. All ranges were smaller (by 10-20 per 

 cent) than values predicted for the corresponding vapor, reduced to the 

 same density. 



B. PHiLipp (1923) (2, 3) 



By means of visual scintillation observation, with the radioactive 

 source mounted below a Avater surface and the fluorescent screen just 

 above it, the range of RaC alpha particles (7.680 Mev) in liquid water 

 was found to be 59.5 ± 0.8 microns, 16 per cent smaller than that cal- 

 culated for the vapor (Table 1). Ranges in alcohol and in two other 

 organic liquids were also determined. Philipp measured the ranges in 

 the corresponding vapors as well. For water vapor of density 0.532 mg 

 per cm^ he found an extrapolated range of 13.0 cm, which corresponds 

 to a mean range of approximately 12.7 cm; the above value for liquid 

 water, when reduced to the same density, is 12 per cent smaller than this. 

 In contrast to the abnormally small range in the liquid, which also was 

 found to occur for alcohol (for which the difference was 11 per cent), 

 measurements on aniline and pyridine gave more closely equivalent 

 ranges for the liquid and vapor. Philipp attributed this difference to 

 the circumstance that, whereas the latter liquids are ''normal," both 

 water and alcohol are "associated." Even if his results are correct, this 

 is a correlation but certainly not an explanation. 



C. APPLEYARD (1949) (4, 5) 



In this ingenious experiment a very dilute solution of polonium in 

 0.5-1.5 N HCl acted as a thick source, the alpha particles being counted 

 by a thin-window Geiger counter mounted at varying distances in air 

 above the liquid. From the variation of the number of alpha-particle 

 counts with distance, the stopping power corresponding to an energy 

 close to the initial energy of the alpha particles could be determined. 



