413-416] 



Numerical Values 



341 



For instance, Dewar* has found the density of solid hydrogen at 131 

 absolute to be '0763. The mass of a cubic centimetre of solid hydrogen 

 is accordingly '0763 grammes, while the mass of each molecule (assuming 

 4 x 10 19 for Avogadro's number) is known to be 2*25 x 10~ 24 grammes. The 

 number of molecules in a cubic centimetre is accordingly 3'39 x 10 22 . If the 

 molecules are spheres of diameter cr, these molecules, if packed as closely as 

 possible, would occupy a volume 



3-39 x 10 22 x 



V2' 



This volume, then, is certainly less than a cubic centimetre. Or, what 

 comes to the same thing, the value of the molecular radius of hydrogen is 

 certainly less than the value of ^<r which makes the above expression equal 

 to a cubic centimetre. This value is 



<r = 1-734 xlO- 8 . 



In this way we obtain a superior limit to the value of \a for hydrogen. 

 The similar limits for other gases can be obtained in the same way, and we 

 arrive at the following table : 



416. Considering the general crudeness of the supposition upon which 

 we have been working, the comparative agreement of these two sets of 

 figures cannot be regarded as otherwise than satisfactory. In regarding all 



* Dewar, Royal Soc. Proc., LXXIII. p. 251. 



t The density is taken at the point of maximum density in the liquid state, this being greater 

 than the density in the solid state. 



J Cailletet and Mathias, Jour, de Phys. 1886, v. p. 555. 



Ramsay, see Lord Kelvin in Phil. Mag. iv. p. 197. 



|| Meyer, Kinetic Theory, p. 319. U Knietsch, Ann. Chem. 1890, CCLIX. p. 100. 



** Mendeleef, Chemistry, p. 47 (English Trans.). tt At 203 C. 



