28 



MAXWELL'S LAW 



57 



shown by the following numbers. Of 1,000 molecules of 

 oxygen at 



13 to 14 molecules have a speed below 100 m. per sec. 



81 



166 



214 



202 



151 



91 



76 



82 



167 



215 



203 



152 



92 



77 



from 100 to 200 

 200 300 

 300 400 

 400 500 

 500 600 

 600 700 

 above 700 



Since this one example will suffice to give a clear repre- 

 sentation of the nature of the law, I shall limit myself to 

 giving only the mean values of the molecular speeds for 

 other gases and vapours, and these I shall tabulate together 

 with the values of the specific gravity which have been used 

 in their calculation. I have in this case not referred the 

 density p of a gas to that of water as unity, but instead of 

 this I have introduced, as in 13, the specific gravity s 

 referred to atmospheric air. This is a procedure which 

 would not be admissible for exact scientific calculations, 

 since atmospheric air is, as Jolly 1 has shown, by no means 

 always of the same composition. Still, for the purpose in 

 hand, this inexact procedure is justified by there being a 

 still greater uncertainty in the values by reason of the 

 deviations of the gases from Boyle's law. It is on this 

 account that most observers have referred their numerical 

 values to air ; it would have served no purpose to reckon them 

 with respect to water. The references appended to the 

 table relate to the determinations of the specific gravities. 



Values at C. 



1. Calculated from Lord Eayleigh's observations, ' Proc. Eoy. Soc.' xxiii. 

 1888, p. 356. 



2. Th. Thomson, 'Ann. of Phil.' i. 1813, p. 178. 



3. Biot and Arago, ' Mem. de 1'Inst.' vii. 1806, p. 320. 



4. Gay-Lussac, ' Ann. Chim. Phys.' [2] i. 1816, p. 218 ; ii. p. 135. 



1 Abh. d. Akad. zu Miinchen, xiii. ; Wied. Ann. vi. 1879. 



