354 Prof. McCoy and Dr. Viol : Chemical Properties and 



but 65 alpha particles of range S'G cm. and 35 of range 

 4' 8 cm.*. The detailed discussion of this matter is given in 

 Section VI. 



Hahn t also determined the ranges of ThX and Em, 

 finding 5*7 and 5*5 cm., and arbitrarily ascribed the 5*7 

 value to ThX. We now know that ThX must contain the 

 very short-lived A as well as the emanation. Marsden and 

 Barratt have found the ranges of Em and A to be 5*0 and 

 5*4 cm. respectively. Furthermore, if we calculate the 

 range of ThX by the law relating ranges and periods, first 

 suggested by Rutherford, and later tested by Geiger and 

 Xuttall for the radium and actinium series, we find the value 

 4*1 cm, We have made range determinations of ThX which 

 show very clearly the presence ol an alpha-ray product of 

 much shorter range than that of the emanation, The longer 

 ranges correspond with those given by Marsden and Barratt 

 for Em and A, namely 5'0 and 5*4 cm. (at 0° and 760 mm. 

 pressure), while our lowest value is 4'1 cm. (at 0° and 760 

 mm.). The ThX was prepared as was described in Section II., 

 special care being taken to have a very active preparation, 

 obtained by the use of only one milligram of barium chloride, 

 to precipitate the ThX. A part of the active barium sulphate 

 was made into a very thin film 4 mm. in diameter on the 

 centre of the brass disk of the silvered ionization flask, of 

 internal radius 9"075 cm. Measurements of the activity were 

 then made at pressures between 75 and 23 cm. of mercury. 

 Table IV. gives the observed results, at a room temperature 

 of 24°'3. 



Table IV. 



Pressure 74-93 6603 57*11 52-15 4957 4843 47'6S 46*56 



Activity 1-000 1003 1022 1-034 1-010 1-048 1-054 1-041 



Pressure 45-40 44'41 43*12 42-38 41*40 40-13 39-06 38-26 



Activity 1-028 1-023 1*014 1-006 '988 -969 *948 *930 



Pressure 3726 3620 35-21 3414 3301 31*67 2782 23*77 



Activity -914 *S88 *864 -841 -804 *760 -647 *526 



The graph obtained from these results (fig. 1) changes 

 direction at points corresponding to pressures of 36*0, 42*7, 

 and 47*6 cm. These points indicate ranges of 31)4, 4 - 68, and 

 5*22 cm. respectively at 0° and 76 cm. pressure. But ihese 

 figures must be increased somewhat on account of a slight 



* See also, Barratt, Lc Radium, ix. p. 81 (1912). 

 t Phil. Mag. [6] xi. p. 792 ; xii. p. 82 (1906). 



