July 12, 1883] 



NA TURE 



251 



" one-half or three-quarters of an inch in breadth before the 

 " bubble broke. At first I thought there had been no light 

 " reflected from the water in that place, but observing it 

 " more curiously I saw within it several smaller round 

 " spots, which appeared much blacker and darker than the 

 " rest, whereby I knew that there was some reflection at the 

 " other places which were not so dirk as those spots. And 

 " by farther trial I found that I could see the images of 

 " some things (as of a candle or the sun) very faintly re- 

 jected, not only from the great black spot, but also from 

 " the little darker spots which were within it. 



"Obs. 18. — If the water was not very tenacious, the 

 " black spots would break forth in the white without any 

 " sensible intervention of the blue. And sometimes they 

 " would break forth within the precedent yellow, or red, 

 " or perhaps within the blue of the second order, before 

 " the intermediate colours had time to display themselves." 



Now I have a reason, an irrefragable reason, for saying 

 that the film cannot keep up its tensile strength to 

 1/100,000,000 of a centimetre, and that is, that the work 

 which would be required to stretch the film a little more 

 than that, would be enough to drive it into vapour. 



The theory of capillary attraction shows, that when a 

 bubble — a soap-bubble for instance — is blown larger and 

 larger, work is done by the stretching of a film which 

 resists extension, as if it were an elastic membrane with a 

 constant contractile force. This contractile force is to be 

 reckoned as a certain number of units of force per unit 

 of breadth. Observation of the ascent of water in capil- 

 lary tubes, shows that the contractile force of a thin film 

 of water, is about sixteen milligrammes weight per milli- 

 metre of breadth. Hence the work done in stretching a 

 water film to any degree of thinness, reckoned in milli- 

 metre-milligrammes, is equal to sixteen times the number 

 of square millimetres by which the area is augmented, 

 provided the film is not made so thin that there is any 

 sensible diminution of its contractile force. In an article 

 "(in the Thermal Effect of Drawing out a Film of 

 Liquid," published in the Proceedings of the Royal 

 Society for April, 1858, I have proved from the second 

 law of thermodynamics that about half as much more 

 energy, in the shape of heat, must be given to the film, to 

 prevent it from sinking in temperature while it is being 

 drawn out. Hence the intrinsic energy of a mass of 

 water in the shape of a film kept at constant temperature, 

 increases by twenty-four milligramme-millimetres for 

 every square millimetre added to its area. 



Suppose then a film to be given with a thickness of a 

 millimetre, and suppose its area to be augmented ten 

 thousand and one fold : the work done per square milli- 

 metre of the original film, that is to say, per milligramme 

 of the mass, would be 240,000 millimetre-milligrammes. 

 The heat equivalent of this is more than half a degree 

 Centigrade (o'57°) of elevation of temperature of the sub- 

 stance. The thickness to which the film is reduced on 

 this supposition, is very approximately 1/10,000 of a milli- 

 metre The commonest observation on the soap-bubble, 

 shows that there is no sensible diminution of contractile 

 force, by reduction of the thickness to 1/10,000 of a milli- 

 metre; inasmuch as the thickness which gives the first 

 maximum brightness, round the black spot seen where 

 the bubble is thinnest, is only about 1/8,000 of a millimetre. 



The very moderate amount of work shown in the pre- 

 ceding estimates, is quite consistent with this deduction. 

 But suppose now the film to be farther stretched, until its 

 thickness is reduced to 1/10,000,000 of a millimetre 

 (1/100,000,000 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 2S0 times the 

 quantity required to raise the temperature of the liquid 

 by one degree Centigrade. 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 

 unavoidable, that a water-film falls off greatly in its con- 

 tractile force, before it is reduced to a thickness of 

 1/10,000,000 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 1/10,000,000 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 Prof. Brand to the 

 Royal Institution. It contains polished steel bars, 

 coloured by having been raised to different degrees of 

 heat, as in the process of annexing 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 spice of air in " Newton's rings," depend on the 

 thickness of the film, which, in the case of oxidisable 

 metals, forms by combination with the 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 keeps those articles glittering 

 and cold and useless, instead of letting them show the ex- 

 quisite play of warm colouring, naturally and inevitably 

 brought out, when they are used in the work which is 

 their reason for existence. The thickness of the film of 

 oxide which gives the first perceptible colour, a very pale 

 orange or buff tint, due to the enfeeblement or extinction 

 of violet light and enfeeblement of blue, and less en- 

 feeblement of the other colours in order, by interference of 

 the reflections from the two surfaces of the film, is about 

 1 100,000 of a centimetre, being something less than a 

 quarter wave-length of violet light in the oxide. 



The exceedingly searching and detective efficacy of 

 electricity comes to our aid here, and by the force as it 

 were spread through such a film, proves to us the exist- 

 ence of the film when it is considerably thinner than that 

 1/100,000 of a centimetre, when in fact it is so very thin 

 as to produce absolutely no perceptible effect on the re- 

 flected light, that is to say, so thin as to be absolutely 

 invisible. If in the apparatus for measuring contact 

 electricity, of which the drawing is before you (NATURE, 

 vol. xxiii. p. 567), two plates of freshly polished copper be 

 placed in the Volta condenser, a very perfect zero of 

 effect is obtained. If, then, one of the plates be taken 

 out, heated slightly by laying it on a pie e of hot iron, 

 ani then allowed to cool again and replaced in the Volta 

 condenser, it is found that negative electricity becomes 

 condensed on the surface thus treated, and positive 

 electricity on the bright copper surface facing it, when 

 the two are in metallic connection. If the same process 

 be repeated with somewhat higher temperatures, or some- 

 what longer times of exposure to it, the electrical difference 

 is augmented. These effects are very sensible before 

 any perceptible tint appears on the copper surface as 

 modified by heat. The effect goes on increasing with 

 higher and higher temperatures of the heating influence, 

 until oxide-tints begin to appear, commencing with buff, 

 and going on through a ruddier colour to a dark-blue 

 slate colour, when no farther heating seems to augment 

 the effect. The greatest contact-electricity effect which I 

 thus obtained between a bright freshly polished copper 

 surface and an opposing face of copper, rendered almost 

 black by oxidation, was such as to require for the neutra- 

 lising potential in my mode of experimenting 1 about one- 

 half of the potential of a Daniell's cell. 



■ First described in a letter to Joule, published in the Proceedings of the 

 Literary and Philosophical Society of Manchester of Jan. 21, 1662, where 

 also I first pointed out the demonstration of a limit to the size of molecules 

 from measurements of contact electricity. The mode of measurement is .core 

 fully described in the article of Natike (vol. xxiii. p. 567). referred to above. 



