174 POPULAR LECTURES AND ADDRESSES. 



tractile force before it is reduced to a thickness of 

 i/io,ooo,oooth of a millimetre. It is scarcely 

 possible, upon any conceivable molecular theory, 

 that there can be any considerable falling off in 

 the contractile force as long as there are several 

 molecules in the thickness. It is therefore probable 

 that there are not several molecules in a thickness 

 of i/io,ooo,oooth of a millimetre of water. 



Now when we' are considering the subdivision of 

 matter, look at those beautiful colours which you 

 see in this little casket, left, I believe, by Professor 

 Brande to the Royal Institution. It contains 

 polished steel bars, coloured by having been 

 raised to different degrees of heat, as in the 

 process of annealing hard-tempered steel. These 

 colours, produced by heat on other polished metals 

 besides steel, are due to thin films of transparent 

 oxide, and their tints, as those of the soap-bubble 

 and of the thin space of air in " Newton's rings," 

 depend on the thickness of the film, which, in 

 the case of oxidisable metals, forms, by combina- 

 tion with the oxygen of the air under the influ- 

 ence of heat, a true surface-burning. 



