174 SEE-FURTHER RESEARCHES ON [April 24, 



with irresistible power. Geikie remarks that the cavities in quartz 

 have all sizes from the coarse pores visible to the naked eye to 

 minute spaces less than 1/10,000 of an inch in diameter, which can be 

 seen only under high magnifying power. 



Now it is worth while to remember that small as are the least 

 cavities and fissures which we can see with the microscope, they are 

 very large and coarse compared to the molecular structure of a fluid 

 such as water or of a solid like glass. It is useful to remember that 

 the limit of naked eye vision is about 1/250 of an inch, and of the 

 most powerful microscope about 1/100,000 of an inch. The micro- 

 scope therefore increases our power of vision about 400 times. 

 (Cf. Prof. A. A. Michelson's " Light Waves and their Uses," p. 30.) 

 § 10. On Lord Kelvin's Determination of the Size of Atoms. — 

 In order to form a clear conception of the physical constitution of 

 the matter composing the crust of the globe, we must recall the lines 

 of research by which Lord Kelvin has determined the size of atoms. 



1. By determining the work done or heat produced in bringing 

 thin plates of zinc and copper together. The observed amount of 

 heat evolved when the plates are made of given thickness and after- 

 wards imagined to be thinner and thinner, limited only by the con- 

 dition that the mass shall not be melted, under the heat of combina- 

 tion, which is not indefinitely great even when brass is produced by 

 fusing zinc and copper, but corresponds to the mutual attraction of 

 a number of plates not more numerous than 100,000,000 to the milli- 

 meter; hence it follows that the molecules are at least 1/1,000,000,000 

 cm. and probably more than 1/400,000,000 cm. in diameter. Lord 

 Kelvin concluded that " Plates of zinc and copper 1/300,000,000 of 

 a centimeter thick, placed close together alternately, form a near 

 approximation to a chemical combination if indeed such thin plates 

 could be made without splitting atoms." He fixed 1/1,000,000,000 

 of a centimeter as the minimum diameter of the atoms found in this 

 way. It is to be remembered here that 2.54 centimeters = i inch. 



2. By the study of Newton's rings on soap bubbles as they 

 become thinner and thinner, the thickness of the film being reckoned 

 from the known wave-length of the reflected light. Unless the 

 film materially weakened when a certain limit is attained, it could 

 not be stretched beyond a certain thickness without volatilizing, if 



