MATTER, SPACE, AND TIME 209 



This, as we have indicated, is a maximum 

 estimate ; it is possible than the number is 

 less. 



As already suggested, the interdiffusion of 

 gases also leads to a molecular conception of their 

 structure, and from the observed values of the 

 coefficients of diffusion, and of the allied property 

 viscosity, it is possible, from the principles of 

 the kinetic theory, to calculate more exactly the 

 number of molecules in a cubic centimetre of a 

 gas. The results of the investigation indicate 

 about 2.5 X 10^^ molecules per cubic centimetre. 

 Since water, the liquid, is about 1200 times 

 denser than its vapour, it follows that a cubic 

 centimetre of water contains about 3 x 10^^ 

 molecules, a number which may profitably be 

 compared with the maximum estimate given 

 above. Such figures do indeed convey little to 

 the mind ; but it may be useful to remember that 

 the thinnest line clearly visible in a good micro- 

 scope — a line with a thickness approaching the 

 hundred-thousandth of a centimetre — would need 

 about three hundred molecules to stretch across it 

 from side to side. Thus the molecular structure of 

 matter is not immeasurably finer than magnitudes 

 which, with the aid of modern instruments, our 

 senses are enabled to apprehend. 



Our mental picture of matter, then, is that 

 of a discontinuous substance; we can, moreover, 

 form some notion of the number of grains in a 

 given volume, and we know some of the chemical 

 properties of the individual grains. But what is 

 the nature of these particles ? Are they similar 

 in kind to the matter-in-bulk they compose, or 



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