228 NOTES AND MEMORANOA. 



of a cubic inch, at 0° 0. and a pressure of one atmosphere 

 These numbers are as follows : 



Stouey .... 1,901,000,000,000 



Thomson .... 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 determinations 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 ^-qV^ of an inch in length. It 

 also appears desirable to give double weight to the determi- 

 nation by Clerk-Maxwell, since it is founded on the best data. 

 We thus obtain as follows : 



Stoney .... 12,390-) oq „„. 



Thomson .... 46,160) ^^''^'^ 



Clerk-Maxwell .... 6,770 



Mean 18,022 



Calculating from this mean, we may perhaps conclude that 

 the number of atoms in a cubic -p'o^th of an inch is about 

 6,000,000,000,000. As will be apparent from the wide 

 difference in the determinations, this 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. 



Now, if the gas containing the above-named number of 

 atoms consisted of two volumes of hydrogen to one volume of 

 oxygen, when combined to form vapour of water there would 

 be a condensation of volume from three to two, and on con- 

 densing into a liquid a further contraction to -paVT of the 

 bulk of the vapour. Each molecule of water would however 

 consist of three atoms of gas, and hence in order to determine 

 the number of molecules of liquid water in -roVo of an inch 

 cube, it is necessary to multiply the number in a gas by 

 -1 X 1234 x4 = 617. This gives for the number of molecules 



