August 1 6, 1877J 



NATURE 



331 



ON BLACK SOAP FILMS 



TT is a matter of general knowledge that a very thin 

 ■* plate of a transparent substance exhibits colours, the 

 tints of which depend upon the angle of incidence of the 

 light by which it is viewed, and also upon its thickness 

 and refractive index, and that these " colours of thin 

 plates " were classified by Newton and divided by him 

 into orders. If, however, such a plate is of less than a 

 certain thickness, it appears black whatever the angle of 

 incidence of the light may be, and its colour, therefore, 

 gives us no indication of its thickness, except that it must 

 be less than a particular value. 



With a well-made soap-solution it is by no means 

 difficult to obtain a film thin enough to appear black, and 

 special interest attaches to observations made on it when 

 in this state of extreme tenuity, as the thinner it 

 becomes the more nearly must its thickness be compar- 

 able with the distance at which the forces exerted upon 

 one another by the molecules of which it is composed 

 cease to be sensible Now under whatever conditions a 

 film which is part black may have been formed, and 

 whatever its shape may be, a very rapid and remarkable 

 change of thickness invariably (as far as the writer's 

 experience goes) occurs at the boundary which divides 

 th° black from the coloured portion of the film. 



It is indeed true that contiguous portions of films are 

 often of different thicknesses. Thus, when a bubble is 

 first blown, streams of various colours circulate rapidly 

 over all its surface, but as a rule, these miniature convul- 

 sions cease in a short time, and the colours appear in 

 regular order from top to bottom, each tint shading off 

 gradually into those which precede and follow it. Some- 

 times, too, and especially when a film becomes very thin, 

 specks of a shghtly different colour from the rest are 

 obserTcd moving upon its surface in paths which are 

 often very regular. Thus in a cylindrical film which had 

 been formed several hours and the top of which had be- 

 come black, while the rest exhibited the blue of the second 

 order, fakes of a deeper blue than the main body of the 

 film were seen to move rapidly for many minutes in such 

 a way as to indicate that a regular circulation was kept up 

 in the film. Two horizontal streams moved in opposite 

 directions round the base of the cylinder ; at their 

 meeting-point they united, and the direction of the 

 current became vertical, but on approaching the boundary 

 of the black, the stream again bifurcated, and separate 

 currents flowed in opposite directions round the upper 

 edge of the blue, which again uniting on the other side of 

 the cylinder, flowed down to the base and resumed their 

 original directions. Slight differences in the temperature 

 of different parts of the film would sufficiently explain 

 these currents, which are only mentioned here! because 

 the coloured flakes by which they were rendered conspi- 

 cuous proved the existence of inequalities in the thickness 

 of the film. These and the other variations in thickness 

 referred to above, are, however, slight compared with 

 those which are often observed at the edge of the black, 

 as to the naked eye whole orders of colours (which can, 

 however, in general be seen by means of a microscope) 

 may appear to be missing between the black and the 

 portitn of the film which is in contact with it. Thus, in 

 one case which has been noted, the latter must have been 

 fourteen times, and very probably was more than forty 

 times, thicker than the black itself. 



But e\en when the colour next the black is the white 

 of the first order, we do not observe, as we should if the 

 change ir thickness were very gradual, an undefined 

 boundary aetween the two, shifting with every variation of 

 the angle of incidence of the light by which it is viewed, 

 but rather a definite line of demarcation ruled, as it were, 

 across the fJm, the position of which is independent of 

 the direction of the incident light, and at which the 

 change of thickness, if much less than that observed 



in the extreme case above cited, is nevertheless very 

 considerable. 



IJotb, then, by its comparative magnitude and by the 

 constancy with which it recurs, this phenomenon seems 

 to suggest some special connection between the form- 

 ation of the black portion of the film and the molecular 

 forces which are at play in the liquid of which it is 

 composed. 



At the edge of the black itself, however, optical 

 methods fail, as above explained, to give us any further 

 help in investigating the form of the film, but some 

 information has been obtained on this point in the course 

 of a series of experiments on the electrical resistance of 

 soap films, upon which Prof. Reinold and the author of 

 this article are engaged, which they have recently made 

 the subject of a communication to the Royal Society. To 

 determine the thickness of a uniform soap film by measur- 

 ing its electrical resistance and (having previously deter- 

 mined the specific resistance of the hquid of which it is 

 composed) applying Ohm's law that the resistance of a 

 homogeneous conductor varies inversely as its section may 

 at first appear a simple problem ; but at the outset the diffi- 

 culty arises that the molecules of a liquid which lie very 

 near the surface are necessarily in a different state from 

 those in the interior. The latter are subjected to the action 

 of other molecules which are on the average distributed 

 symmetrically around them, while the greater number of 

 molecules which can exert any influence on the former 

 are situated on one side only. Hence the surface of a 

 hquid differs in its physical properties from the interior, 

 and there is no guarantee that the electrical resistance of 

 the liquid in mass is the same as that of a very thin film 

 skimmed off its surface. But although Ohm's law can- 

 not be applied to the case of a very thin film with any 

 certainty that the numbers obtained by its aid will be 

 correct, and the electrical method fails to give thoroughly 

 reliable information as to the thickness of a black tilm, 

 yet it is possible by experimenting on a film as it gra- 

 dually thins and the black portion increases in size, to 

 learn from its electrical resistance whether the thickness 

 of the black portion is constant, or whether, as is ordi- 

 narily the case with the rest of the films, it is thinner 

 above than below, and by repeating the investigation with 

 different films to determine whether the thickness of a 

 black film depends upon the varying circumstances of its 

 formation. To investigate this question fully will require 

 more time than the authors of the paper above referred 

 to have, as yet, been able to devote to it, but they have 

 obtained very consistent and definite results in the expe- 

 riments they have already made. The liquid used was 

 M. Plateau's " liquide glyceriquc" with three parts of 

 potassium nitrate added to every 100 parts of water to 

 improve the conductivity. 



The apparatus employed consisted essentially of a glass 

 cylinder about 16 cm. high fitted with an ebonite cover, 

 through the centre of which passed a brass tube which 

 could be raised or depressed at pleasure, and was retained 

 in its position by friction. 



To the lower end of the tube was attached a circular 

 brass plate, to which was soldered a ring of stout 

 platinum foil 33'5 1 mm. in diameter. 



A platinum crucible, the mouth of which was of the 

 same diameter as the ring was placed at the bottom 

 of the glass vessel in a little dish containing mercury. 

 When the apparatus was used, a little of the liquid was 

 poured into the glass vessel and into the platinum crucible, 

 in order to prevent the film thinning by evaporation ; a 

 piece of india-rubber tubing provided with a pinch-cock 

 was attached to the upper end of the brass tube, and a 

 plane film having been formed on the platinum ring was 

 blown out, through the tube, into a bubble which, when 

 large enough, adhered to the rim of the crucible. The 

 quantity of air inside the bubble and the position of the 

 various parts of the apparatus were then adjusted so as 



