ll-l Transactions of the Royal Microscopical Society. 



conclusions we can from the various properties of gases. This 

 problem has been attacked by Stoney,* Thomson,! and Clerk- 

 Maxweiy who, from various data, and by various methods of 

 reasoninfy, have endeavoured to determine the number of ultimate 

 atoms in a given volume of any permanent and perfect gas. In 

 order to avoid inconveniently long rows of figures, I have reduced 

 all their results to the number of ultimate atoms contained in a 

 space of Tiroo of an inch cube, that is to say, in xoo-o oWoo-s- of 

 a cubic inch, at 0'' C. and a pressure of one atmosphere. These 

 numbers are as follows : 



Stoney 1,901,000,000,000 



Tliomson 98,320,000,000,000 



Clerk-Maxwell 311,000,000,000 



Mean 50,260,000,000,000 



As will be seen, there is a very great discrepancy between the 

 numbers given by Thomson and Clerk-Maxwell. This is in part 

 due to the fact that Thomson gives the greatest probable number, 

 whilst Clerk- Maxwell has endeavoured to express the true number 

 indicated by the phenomena of inter-diffusion of gases. The deter- 

 minations do to a great extent depend on the measurements of 

 length, and any differences are of course greatly increased when 

 the number of atoms in a given volume is calculated, since that 

 varies as the cube of the linear dimensions. Extracting the cube 

 root of each of the above numbers, we obtain the number of atoms 

 that would lie end to end in the space of joVo of an inch in length. 

 These are as follows : 



Stonev 12,390 



Tliomson 46,100 



Clerk-Maxwell 6,770 



Mean 21,770 



The cube of this mean is about 10,317,000,000,000, and, taking 

 into consideration the various circumstances named above, it appears 

 to me a far more probable approximation to the truth than the 

 mean of the numbers in a cubic yoV o of an inch as given by the 

 authors. As will be apparent from the wide differences, even this 

 mean result can be looked upon in no other light than a very 

 rough approximation ; but still, when we bear in mind that 

 Thomson's result is given as a limit, it must be admitted that 

 the numbers belong sufficiently to one general order of magnitude 

 to justify our looking upon the mean as a tolerably satisfactory 

 ground on which to form some provisional conclusions. 



* ' Philosophical Magazine,' 1868, vol. xxxvi., p. 132. 

 t 'Nature,' March 31, 1870, vol. i., p. 551. 

 X Ibid., August 11, 1873, vol. viii., p. 298. 



