336 



Prof. Bragg and Mr 



Kleeman 



Ethyl chloride was tried before we used the method of 

 weighing a definite quantity of the gas. But the vessel was 

 emptied and filled three times, and there must have been very 

 little air in it. It cannot give a decisive answer as to the 

 validity of the square-root law, because, as it happens, the 

 square-root law and the simple density law give nearly the 

 same value. On that account we have not yet repeated 

 the experiment. 



Hydrogen presented some special difficulties. The range 

 (on the square-root law) of the a particle of Ea C must be 

 about 30 cm., and our apparatus only measures up to 8 cm. 

 We therefore placed a piece of silver-foil over the radium, 

 which was equivalent to 3'3 cm. of air. This cut down the 

 range of the Ra C particles to 3*63, and the range of the 

 swiftest of the others to 1'43. The former when multiplied 

 by four would still be out of range, but the latter would be 

 within it. 



Fig. 2 (p. 331) shows that this was the case. With a mixture 

 of hydrogen and air in the proportion (by pressure) of 9 to 1 

 it was at 4" 6. With mixtures containing a greater proportion 

 of hydrogen it rose somewhat, but a small proportion of air 

 made little relative difference. The curve is of course very 

 much drawn out vertically, and drawn in horizontally. 



The results obtained for all the above substances are collected 

 in the following table : — 



Table, showing " stopping power " of various atoms and 

 molecules as compared with that of air. The atomic weight 

 of the supposititious atom of air is taken as 14*4, and the 

 " atomic square root " as 3" 79. 





Stopping power 

 of atom or 

 molecule. 



Ratio of 

 square 



roots. 



Ratio of atomic 



or molecular 



weights. 



Hvdroffen 



•246 

 1 

 1-53 



2-42 

 311 

 3 42 

 4-12 

 4-45 

 209 

 230 

 2-49 

 3-30 

 38 



•265 

 1 



1-3S 

 2-1 

 2-75 

 2'88 

 3-7 

 3-7 

 2-09 

 2-36 

 2-35 

 368 

 3-61 



•069 

 1 



1-88 

 453 

 7-5 

 8-2 



13-5 



13-7 

 3-28 

 2 23 

 4-9 

 2-56 

 5-41 



Air 



Aluminium 



Copper 



Silver 



Tin 





Gold 



Methyl bromide 



Ethvl chloride 



Methyl iodide 



Ether 



Carbon tetrachloride ... 



