H POPULAR LECTURES AND ADDRESSES. 



gravity. You see we are not troubled by any 

 trickling down of these liquid films but I must 

 not say down, we have no up and down here they 

 do not trickle along these surfaces towards the 

 table, at least you must imagine that they do not 

 do so. I now turn one or both of these pieces 

 of matter till they are so nearly in contact all 

 over the surfaces A and B, that the whole inter- 

 stice becomes filled with water. My metrical 

 sense of touch tells me that exactly four and a 

 half centimetre-grammes of work has again been 

 done ; this time, however, not by a very great 

 force through a space of less than fifty micro- 

 millimetres, but by a very gentle force acting 

 throughout the large space of the turning or fold- 

 ing-together motion which you have seen, and 

 now see again. We know, in fact, by the ele- 

 mentary principle of work done in a conservative 

 system, that the work done in the first case of 

 letting the two bodies come together directly, 

 and. .in the second case of letting them come 

 together by first bringing two points into contact 

 and then folding them together, must be the 



