16 POPULAR LECTURES AND ADDRESSES. 



whole surface had been coated over with an in- 

 finitely thin contractile film possessing a uniform 

 contractile force of 3/40 of a gramme weight, or 

 seventy-five milligrammes, per lineal centimetre. 



It is now convenient to keep to our ideal film, 

 and give up thinking of what, according to our 

 present capacity for imagining molecular action, 

 is the more real thing namely, the mutual attrac- 

 tion between the different portions of the liquid. 

 But do not, I entreat you, fall into the paradoxical 

 habit of thinking of the surface film as other than 

 an ideal way of stating the resultant effect of 

 mutual attraction between the different portions 

 of the fluid. Look, now, at one of the pieces of 

 water ideally rigidified, or if you please, at the 

 two pieces put together to make one. Remember 

 we are at the centre of the earth. What will 

 take place if this piece of matter resting in the 

 air before you suddenly ceases to be rigid ? Im- 

 agine it, as I have said, to be enclosed in a film 

 everywhere tending to contract with a force equal 

 to 3/40 of a gramme or seventy-five milligrammes 

 weight per lineal centimetre. This contractile film 



