418 Dr. G. J. Stoney on the 



last measure by taking into account the degree in which the 

 gas falls short of being " perfect," i. e. of accurately fulfilling 

 the law 



pv _ T 



P 'v f " r p 



where jo is pressure, v volume, and T absolute temperature. 

 Judged in this and other ways, it appears that the average 

 interval between the molecules of any of the more perfect 

 gases, when at atmospheric pressures and temperatures, is 

 something like the ordinate of our gauge at the distance of 

 one centimetre from its apex*. If the vacuum in a Sprengel- 

 pump be carried so far as to reduce the pressure to one 

 millionth of an atmosphere (which is not very far from the 

 greatest exhaustion that can be attained), the average spacing 



(which may "be called the average spacing of the molecules) is nearly the 

 same in all nearly " perfect" gases when compared at the same pressure 

 and temperature. This is, in fact, the truth that underlies and gives its 

 value to Avogadro's erroneous hypothesis that at the same temperature 

 and pressure the size of the gaseous molecules of all substances is the 

 same. In the present state of science it is desirable that every practicable 

 effort should be ma de to determine with more exactness the value of this 

 important physical quantity. 



* Phil. Mag. for August 1868, p. 140. If we assume, in conformity 

 with the estimate in the text, that the molecules of a gas at, say, 21° C. 

 and 760 millim. pressure, are as numerous within a given space as would 

 be a number of points cubically disposed at intervals of a ninethet-nietre 

 asunder (this being the ordinate of our gauge at the distance of one centi- 

 metre from its apex) ; then the number of molecules of the gas in every 

 cubic millimetre of its volume is a uno-eighteen — the number repre- 

 sented by 1 with eighteen 0's after it. Hence, in a litre of the gas there 

 will be a million times more, i. e. a uno-twentyfour of molecules. Now 

 at the above-mentioned temperature and pressure a litre of hydrogen 

 weighs just one twelfth of a gramme (•083'). Hence the mass of each 

 molecule is the twenty fourth et of this (i. e. the fraction represented by 1 

 in the numerator, and 1 followed by twenty-four 0's in the denominator), 

 i. e. it is =8' # 3 xxvi ets of a gramme; and according to this computation 

 the chemical atom of hydrogen, being the semi-molecule, has as its mass 

 4'*16 xxvi ets of a gramme. This is probably somewhere in the neigh- 

 bourhood of the true value ; so that we may regard the mass of a 

 chemical atom of hydrogen as a mass probably not more than a few times 

 more or a few times less than the twenty-fifthet or twenty-sixth et of a 

 gramme. This seems the best approach that can at present be made to 

 estimating the mass of a chemical atom. 



The determination depends upon the average spacing of the centres of 

 the molecules of a gas at standard temperature and pressure (see last 

 footnote); and if this very important physical magnitude, which is 

 common to all perfect gases, can be ascertained with more accuracy, we 

 shall get a proportionally better estimate of the mass of a chemical atom. 

 Of course if the mass of the atom of any one element, e. g. hydrogen, be 

 determined, the masses of all the others become known by the chemical 

 tables of atomic weig-hts. 



