654 THE POPULAR SCIENCE MONTHLY. 



ruling plates covering 224,000 lines per inch, such as would aggregate 

 * in superficial areas to over 50,000,000,000 to the square inch ! Such 

 minute divisions are wholly beyond the resolving power of the most 

 elaborate of modern microscopic appliances ; for it has been shown by 

 Sorby 'that the ultimate power of the microscope for distinct defini- 

 tion is limited to the examination of magnitudes not less than one-half 

 of the average wave-length of the luminous spectrum ; and it is shown, 

 upon the authority of Helmholtz, that when the amplitude of the ob- 

 ject is less than this half wave-length or somewhat in excess of 80,- 

 000 to the inch the dark interference-fringes impair the definition of 

 the instrument, except in the case of striated markings, which may be 

 clearly defined, or resolved, by so arranging the illumination as to 

 mask the fringes, and bring out a good definition even in excess of 

 100,000 to the inch. Hence, the main difficulty attending the possible 

 amplification of objects less than about the yo'sto^ ^^ ^" mch in diam- 

 eter is a purely physical one, and depends upon the constitution of 

 light itself. 



The various phenomena of chemical physics teach us that matter 

 is not homogeneous, but is made np of infinitesimal particles or atoms, 

 the term atotn meaning indivisible 2^0,^11016 ; and that the term mole- 

 cule meaning literally a little mass refers to an aggregation of two 

 or more atoms. Thus, a crystal of common salt may be pulverized until 

 one of its fragments is barely discernible to the highest range of micro- 

 scopic power, and still this fragment will retain all the characteristics 

 of salt. This same microscopic portion is susceptible of a further 

 subdivision by solution in water, when the spectroscope will detect 

 its presence in the still minuter quantity of the one-hundred-millionth 

 part of a grain. Here, in the case of salt, physical analysis ends, and, 

 aside from chemical analysis, any further subdivision must be by the 

 process of abstraction, until by its means we arrive at the mental con- 

 ception of a portion so minute as to consist of an atom of sodium 

 united by the bonds of chemical affinity to an atom of chlorine. This 

 is now a molecule of common salt. Any further division destroys the 

 entity of the compound, and results in the decomposition of the salt 

 into the atoms of its elements. Hence a simple molecule is the small- 

 est portion of any chemical compound that is not susceptible of subdi- 

 vision without destroying its entity, or, in other words, the smallest 

 number of atoms that can cohere to form a compound constitute the 

 molecule of that compound. An atom is designated as the ultimate 

 particle of any elementary body, and is not susceptible of any further 

 division within the range of human analysis. 



Were it possible to magnify the atoms of matter to a diameter 

 available for distinct vision, we should be met at the outset by a dif- 

 ficulty too astounding for realization. It is a matter of easy proof 



' H. C. Sorby, F. R. S., in his anniversary address to the Royal Microscopical Society, 

 in the Quarterly Journal of Microscopical Science for April, 1876. 



