121 DIRECT PEOPERTIES OF MOLECULES 335 



121. Absolute and Specific Weight of Molecules 



Remembering this possibility, it will not appear an un- 

 allowable boldness to attempt also, as vander Waals and 

 Maxwell have done, to calculate the mass of a molecule in 

 absolute measure, or at least to determine a limiting value 

 of it. 



From the number of molecules in 1 cubic centimetre 

 of a gas which we now know, we obtain very simply the 

 mass of a single molecule by dividing by this number the 

 mass of the gas contained in 1 cubic centimetre, which is 

 the same number as that representing the density of the gas 

 relatively to that of water. Since air under atmospheric 

 pressure is 770 times lighter than water, it follows that 

 there will be about 770 x 60 or 46,000 trillion molecules 

 of air in 1 gram, or 46 trillions in 1 milligram ; or the mass 

 of a molecule of air is about 2 x 10 ~ 23 gram. 



From this we can also calculate the density of a mole- 

 cule of air, since we know the size of the molecules ; for we 

 have the mass of a 46-trillionth of a milligram contained in 

 a volume of 0*004 trillionth of a cubic millimetre, so that 

 the specific gravity of the actual substance of an air mole- 

 cule referred to water is 5, which is considerably greater 

 than the specific gravity of air in the liquid state. 



Hydrogen is about fourteen times lighter than air, and 

 there are therefore about 640 trillions of hydrogen molecules 

 in 1 milligram ; the unit of the usual atomic weights is thus 

 equal to about a 1,300-trillionth of a milligram, or, as we 

 may more shortly express it, a quadrillion of hydrogen atoms 

 weigh about f gram. 



122. Comparison with other Limiting Values 



Having arrived at the end of these investigations, we will 

 not omit to compare the last remarkable conclusions that 

 have been deduced from the kinetic theory of gases with 

 the results of other methods by which attempts have been 

 made to compass the weighing and measuring of molecules 

 and atoms. 



