igo8.] THE PHYSICS OF THE EARTH. 175 



maintained at the same temperature ; for as it expands it cools, and 

 the heat that would have to be supplied to it would be more than 

 sufficient to vaporize it. Now it is found by observation that the 

 intensity of the surface tension of the film of water falls off before 

 the thickness is reduced to 1/200,000,000 cm., and hence there prob- 

 ably are but few molecules in that thickness. 



3. By the phenomenon of dispersion in the wave theory of 

 light. Cauchy showed that dispersion of colors implied a granular 

 structure in refracting media, and that the grains could not be 

 indefinitely small, but must exceed 1/10,000 of the shortest wave 

 length ; and to produce the observed effect Lord Kelvin concluded 

 that the number of molecules in a wave length would have to be 

 from 200 to 600. Nobert ruled lines on glass at the rate of 40,000 

 to the centimeter,^ or about two to the wave length of blue light 



(about 4/100,000 centimeter) ; and as this left the ruled surface 

 capable of reflection, the number of molecules in the ridges between 

 the grooves must have been sufficient to give solid body to the 

 sculptured mass, and thus not less than several hundred to the wave 

 length. If the mean free path in a solid like glass be 25 times the 

 diameter of the atom itself, this will make the diameter of the 

 atoms of the order of 1/400,000,000 of a centimeter. 



4. By calculating the length of the average free path of a mole- 

 cule in a gas, according to the kinetic theory. Loschmidt in 1865, 

 Stoney in 1866, and Lord Kelvin in 1870, independently reached 

 similar results, namely, for the average free path about 1/100,000 

 of a centimeter, and for the diameter of the gaseous molecule about 

 1/500.000,000 of a centimeter. 



These four methods of estimating the diameter of atoms thus 

 agree very closely among themselves ; and moreover a similar result 

 on the average distance of molecules deduced by entirely different 



' Referring to Xobert's lines ]\laxwell says : " A cube, whose side is the 

 400th of a millimetre, may be taken as the minimum visible for (microscopic) 

 observers of the present day. Such a cube would contain from 60 to 100 

 milhon molecules of oxygen or nitrogen" (cf. The article "Atom," Ency- 

 clopedia Britannica, ninth edition, p. 42). If there be 400 molecules in a 

 line the length of the edge of the cube just considered, the cube would con- 

 tain 64,000,000, which agrees with ^laxwell's estimate. A line equal to the 

 wave length of blue light would thus contain 250 molecules. 



