

NATUKAL PHILOSOPHY. 



SIZE OF ATOMS. 



SIR WILLIAM THOMPSON, who has been investigating regarding 

 the probable size of molecules, has derived an important argu- 

 ment from electrical phenomena. He had previously proved that 

 " a plate of zinc and a plate of copper, kept in metallic connec- 

 tion with one another, act electrically upon electrified bodies in 

 their neighborhood, and upon one another, as they would if they 

 were of the same metal and kept at a difference of potentials equal 

 to about three quarters of that produced by a single cell of 

 Daniell's. Hence, in connection with certain measurements on 

 the electrostatic effects of that battery, it is found that plates of 

 zinc and copper held parallel to one another at any distance, D, 

 apart, which is a small fraction of the linear dimensions of their 

 opposed surfaces, and kept in metallic communication with one 

 another, exercise a certain force of attraction which can be calcu- 

 lated. From this we can calculate the work done if the plates 

 approach each other from a distance, D', till they arc at this dis- 

 tance, D. Suppose, now, a pile composed of a great number of 

 plates of zinc and copper kept in metallic connection while they 

 are made to approach each other, their positions in the pile being 

 parallel, and suppose for simplicity that their thickness is also D. 

 We calculate the work done, estimating it by the height through 

 which this energy would raise the whole pile, and giving different 

 values to D we get varying values of this height according as the 

 energy calculated by the formula varies. Down to values of D 

 equal to one two hundred-thousandth of a centimetre the values of 

 the energy computed are such that we may suppose that the laws 

 of electrical attraction and induction are essentially the same as 

 have been found for plates of measurable thickness at measurable 

 distances from each other. But if D equals a very small amount, 

 as one four-hundred-millionth of a centimetre, the computed 

 energy is greater than that which converted into heat would melt 

 the zinc and copper. So we must admit that as zinc and copper 

 filings have no tendency to inflame on approaching one another, 

 the electric attraction between plates one four-hundred-millionth 

 of a centimetre thick, at a distance of one four-hundred-millionth 

 of a centimetre from each other, must be much less than the 

 amount calculated, supposing the ordinary laws still to be fol- 

 lowed ; " or, in other words, plates of zinc and copper, so thin as a 

 four-hundred-millionth of a centimetre from each other, form a 



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