392 



NATURE 



[August 23, 1883 



case of the cylindrical films are closely similar. Specks 

 of black also readily form in the neighbourhood of the 

 solid in contact with the film. They, too, rise through 

 the surrounding liquid, and the growth of the black ring 

 at the top of the film is sometimes caused as much by 

 additions of black spots from below as by a downward 

 motion of the lower edge. These phenomena are only 

 observed on a large scale shortly before the rupture of the 

 film. 



The bla:k appears at times in other ways. Sometimes 

 when the while of the first order was in contact with and 

 below the black, a small portion of it would rapidly 

 disintegrate. It would become streaked with black lines, 

 the white portions would fall doun through the rifts thus 

 formed, and a sudden extension of the black would thus 

 take place. In films containing small quantities of 

 glycerine this phenomenon is sometimes observed on a 

 very large scale. 



There is also a third way in which the black appears, 

 namely, in cases where there is no discontinuity between 

 the white and black. Here the thinning takes place in 

 the normal way, but, as in Newton's observations, specks 

 of a deeper black frequently appear. This phenomenon 

 may easily be shown as a lecture experiment. If a few 

 drops of water be placed on the surface of a piece of 

 yellow soap, and the end of a glass tube ground plane be 

 dipped into them, a film can be removed. On throwing a 

 magnified image of this on the wall, it is observed to thin 

 rapidly. The white often passes through gray into black, 

 and then the deeper black spots appear and rise to the 

 top of the film. Within our experience, however, this 

 phenomenon occurs only in the case of transient films 

 formed of a liquid which does not allow any high degree 

 of persistence. It is for this reason that in the summary 

 of results with which we conclude our paper, and which 

 is given in the abstract (see Nature, vol. xxviii. p. 142), 

 we limit our statements to "persistent soap films." It is 

 on these only that we have been able to make measure- 

 ments, and of these only that we have any certain know- 

 ledge. 



While, therefore, in answer to Sir Wm. Thomson, we 

 are able to say that we have often observed the same phe- 

 nomenon as Newton, viz. that of a deeper black separated 

 by a line of apparent discontinuity from the less intense 

 black which surrounds it, this observation has only been 

 made in the case of liquids like that used by Newton, 

 which he describes as " water made tenacious by dis- 

 solving a little soap in it." 



We have made use of two liquids in the experiments 

 on which our published results are based. In the case of 

 the "liquide glycerique" the black was under continual 

 inspection, the colours of the remainder of the film 

 being frequently noted during the experiments, and when 

 the film became very thin and uniform in colour, the 

 observer had plenty of time to study its appearance. We 

 have no recollection of ever having observed any black 

 specks deeper than that of the main mass of black, either 

 stationary, or moving about in it. Had they been 

 forme 1 in large quantity, our electrical measurements 

 must have detected them. They would have risen through 

 the thicker black as the white or black specks do through 

 the coloured parts of the films, and would have congre- 

 gated in the upper part and formed a ring of greater 

 tenuity at the top. If, as analogy would lead us to sup- 

 pose likely, they had appeared in greatest quantity to- 

 wards the end of the film's existence, the resistance of the 

 black area would have increased more rapidly than its 

 length. We tested this by grouping our experiments 

 according to the length of the black area (Proc. Roy. Soc, 

 June 21, 1877, p. 344), and found that the resistance per 

 millimetre was, to within the limits of the errors of experi- 

 ment, constant, whether the black was less than two or 

 more than ten millimetres in length. 



The second liquid, which was formed only of oleate of 



soda and water, was more similar to Newton's and more 

 likely to give similar results. With this we could obtain 

 such large areas of black that the electrometer method 

 enabled us to measure the resistance of a portion of the 

 black alone, without regard to that of the coloured por- 

 tions of the films. These films were therefore observed 

 much less closely than those formed of "liquide gly- 

 ceVique," but no eye observation or electrical measure- 

 ment ever gave any indication of more than a single 

 thickness of the black for each particular film. 



Coming now to the optical observations, we have indeed 

 noticed in the earlier stages of the history of the black 

 films a bending of the interference fringes in the lower 

 parts of the black region, which might indicate that near 

 the coloured part of the film it was somewhat thicker 

 than at some distance from it. It is, however, very 

 doubtful whether in this part of the field the light was 

 passing through black films only. The area of the black 

 was not exactly the same for all the fifty or sixty films 

 inclosed in the tube, and thus near the boundary of the 

 black the light might pass through a few white films, 

 which would account for the apparent thickening. We 

 were unable to satisfy ourselves as to which of these ex- 

 planations is the true one, though the latter is the more 

 probable. The question is fully discussed in our paper, 

 in which we show that if the apparent thickening were 

 really in the black, that colour must begin to show itself 

 at a far greater thickness than that ordinarily assigned to 

 the "beginning of the black," which is unlikely, though 

 not, in view of the great uncertainty which attaches to 

 this part of Newton's scale, impossible. 



On the whole, then, we incline to the opinion that the 

 number given by our experiments is the least thickness of 

 the black in the liquids we observed. We also think 

 that the tint our persistent films displayed is decidedly 

 deeper than that of the less intense black shown by com- 

 paratively non-persistent films, though to make certain of 

 this would require careful comparative observations. It 

 is possible that the spots of deeper black in non-persistent 

 films may be thinner than that we have measured, and 

 the very fragility of the films in which they appear gives 

 some colour to the supposition that it is so. It is, how- 

 ever, significant that, in two liquids differing so much in 

 composition as those we employed, the one containing 

 two parts of glycerine out of five, and the other no 

 glycerine at all, the means of the optical and electrical 

 measurements give results differing so little as iri3X io -6 

 and 1 19X io" 6 mm. It would be very interesting to settle 

 the question by direct experiment, but the nature of the 

 films which show the two kinds of black would make it 

 no less difficult. We are, however, at present studying 

 the composition of what we may perhaps call black- 

 forming liquids in the hope of extending our investiga- 

 tions further, and if we can obtain one suitable for the 

 purpose we will certainly attempt the measurement sug- 

 gested by Sir William Thomson. 



In conclusion we may point out two deductions from 

 our measurements. The first refers to their connection 

 with the subject of Sir William Thomson's lecture. If 

 the size of the molecules of which the liquid is composed 

 is between 2XlO" 6 and I X 10 s mm. (the limits given by 

 him), it follows that the thinnest film measured by us, 

 which was 72 X io- 6 mm., must contain not less than three 

 and not more than 720 molecules in its thickness. The 

 smallness of the smaller of these numbers tends to show 

 that the real size of the molecules is considerably below 

 Sir W. Thomson's superior limit. 



The second deduction is a good illustration of the mag- 

 nitude of the stress in a liquid surface. The surface 

 tension of Plateau's "liquide glycerique" is about fifty- 

 seven dynes per linear centimetre (cf. " Statique des 

 Liquides," t. i. p. 200). This force must not be considered 

 as acting on a mathematical line, but as the resultant of 

 forces which are in play in the thin la) er of liquid which 



