190 Sir William Thomson [Feb. 2, 



Now I am just going to enter a very little into detail regarding 

 the reasons that those four lines of argument give us for assigning a 

 limit to the smallness of the molecules of matter. I shall take 

 contact electricity first, and very briefly. If I take these two pieces 

 of zinc and copper and touch them together at the two corners, they 

 become electrified, and attract one another with a perfectly definite 

 force, of which the magnitude is ascertained from absolute measure- 

 ments in connection with the well-established doctrine of contact 

 electricity. I do not feel it, because the force is very small. You 

 may do the thing in a measured way ; you may place a little metallic 

 knob or projection on one of them of 1-1 00,000th of a centimeter, 

 and lean the other against it. Let there be three such little metal feet 

 put on the copper ; let me touch the zinc plate with one of them, and 

 turn it gradually down till it comes to touch the other two. In this 

 position, with an air-space of 1-100, 000th of a centimetre between 

 them, there will be positive and negative electricity on the zinc and 

 copper surfaces respectively, of such quantities as to cause a mutual 

 attraction amounting to 2 grammes weight per square centimetre. 

 The amount of work done by the electric attraction upon the plates 

 while they are being allowed to approach one another with metallic 

 connection between them at the corner first touched, till they come to 

 the distance of 1,100,000th of a centimetre, is 2-100,000ths of a centi- 

 metre-gramme, supposing the area of each plate to be one square 

 centimetre. 



I will now read you a statement from an article which was pub- 

 lished thirteen years ago in ' Nature.' * 



" Now let a second plate of zinc be brought by a similar process 

 to the other side of the plate of copper ; a second plate of copper to 

 the remote side of this second plate of zinc, and so on till a pile is 

 formed consisting of 50,001 plates of zinc and 50,000 plates of copper, 

 separated by 100,000 spaces, each plate and each space 1-100, 000th 

 of a centimetre thick. The whole work done by electric attraction in 

 the formation of this pile is two centimetre-grammes. 



" The whole mass of metal is eight grammes. Hence the amount 

 of work is a quarter of a centimetre-gramme per gramme of metal. 

 Now 4030 centimetre-grammes of work, according to Joule's dynamical 

 equivalent of heat, is the amount required to warm a gramme of zinc 

 or copper by one degree Centigrade. Hence the work done by the 

 electric attraction could warm the substance by only 1-16, 120th of 

 a degree. But now let the thickness of each piece of metal and 

 of each intervening space be l-100,000,000th of a centimetre, 

 instead of l-100,0i 0th. The work would be increased a million- 

 fold unless 1-100,000, 000th of a centimetre approaches the smallness 

 of a molecule. The heat equivalent would therefore be enough to 



* See article "On the Size of Atoms," published in 'Nature,' vol. i, p. 551 ; 

 printed in Thomson and Tait's * Natural Philosophy,' second edition, 1883, vol. i. 

 part 2, Appendix F. 



