256 Mr. William Sutherland on the 



and so the mass of an atom of H is 



3-5/74 x 10 19 =5/10 21 grm. nearly. 



It would lead us too far from our present purpose to discuss 

 other estimates of molecular distance, especially as Reinold 

 and Riicker's measurement of the black film is the most defi- 

 nite and striking yet made of these minute distances ; hut the 

 question of the range of molecular force is of special import- 

 ance to us. 



Quincke (Pogg. Ann. cxxxvii.) determined what thickness 

 of silver it is necessary to deposit on glass so that the capil- 

 lary effect on water may be the same as that of solid silver; 

 that is, at what distance the difference between the molecular 

 attractions of glass and silver for water becomes too small to 

 be measured. He found the thickness to be about 50 micromms. 



Now, according to the law of the inverse fourth power, the 

 attraction of a cylinder of radius c, length h, and density p on 

 a particle of mass m on the axis at a distance z from the 

 nearest end is easily calculated as 



OA /i i 1 1 \ 



r \z z + h ^c 2 + z 2 */c s + (* + A)V' 



If the cylinder consists of a length 7^ of silver with a length 

 h 2 of glass, the silver being near the particle, then, the suffixes 

 1 and 2 applying to silver and glass, the attraction of the 

 composite cylinder is 



Making the circumstances correspond to Quincke's experi- 

 ment, we have z nearly equal to the mean molecular distance 

 in water, about 10 micromms. ; h x is small compared to li 2 and c, 

 and, according to Quincke, is 50 micromms. when the composite 

 cylinder exerts the same force on m as if it were all silver ; 

 accordingly the last expression reduces to the two terms 



2A 1 mp l ir/z-2ni7r(A lPi - A 2 p 2 )/(z + \), 



which permit us to compare the molecular force range h Y with 

 the molecular distance z. That the second of these terms 

 should become negligible when h ± is 50 micromms. is a result 

 quite in accordance with the value 10 micromms. for z. 



Let us briefly compare the magnitudes of molecular and 

 gravitational force. The most convenient plan will be to 

 compare the two forces in the case of two single ethyl-oxide 



