194 Sir William Thomson [Feb. 2, 



Centigrade (0*57°) of elevation of temperature of the substance. The 

 thickness to which the film is reduced on this supposition is very 

 approximately l-10,000th of a millimetre. The commonest observa- 

 tion on the soap-bubble shows that there is no sensible diminution 

 of contractile force by reduction of the thickness to l-10,000th of a 

 millimetre ; inasmuch as the thickness which gives tlie first maximum 

 brightness, round the black spot seen where the bubble is thinnest, is 

 only about l-8000th of a millimetre. 



The very moderate amoimt of work shown in the preceding esti- 

 mates is quite consistent with this deduction. But suppose now 

 the film to be farther stretched until its thickness is reduced to 

 l-10,000,000th of a millimetre (1.100,000,000th of a centimetre). 

 The work spent in doing this is two thousand times more than that 

 which we have just calculated. The heat equivalent is 280 times the 

 quantity required to raise the temperature of the liquid by 1° Centi- 

 grade. This is far more than we can admit as a possible amount of 

 work done in the extension of a liquid film. It is more than half the 

 amount of work which, if spent on the liquid, would convert it into 

 vapour at ordinary atmospheric pressure. The conclusion is un- 

 avoidable, that a water-film falls otf greatly in its contractile force 

 before it is reduced to a thickness of 1-10,000,00 0th of a millimetre. 

 It is scarcely possible, upon any conceivable molecular theory, that 

 there can be any considerable falling ofi" 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 

 l-10.000,000th 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 difierent defrrees 

 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 combination with tlie oxygen of the air under the 

 influence of heat, a true surface-burning. 



You are all familiar with the brilliant and beautifully distributed 

 fringes of heat-colours on polished steel grates and fire-irons escaping 

 that unhappy rule of domestic aesthetics which too often kee2)s those 

 articles glittering and cold and useless, instead of letting them show 

 the exquisite play of warm colouring naturally and inevitably brought 

 out when they are used in the work which is their reason for exist- 

 ence. The thickness of the film of oxide which gives the first 

 perceptible colour, a very pale orange or buff tint, due to the en- 

 feeblement or extinction of violet light and enfeeblement of blue, and 

 less enfeeblement of the other colours in order, by interference of the 

 reflections from the two surfaces of the film, is about l-100,000th of 



