286 



NATURE 



[July 17, 1890 



tilled water could not be reduced below lo, and the behaviour 

 ■of motes before the advancing needle was quite different from that 

 observed upon a clean surface. A nearly saturated solution of 

 chloride of sodium could not be freed from superficial viscosity ; 

 while, on the other hand, an addition of \ per cent, of alcohol 

 did not modify the behaviour of distilled water. 



The films of grease that may be made evident in Tlateau's 

 apparatus are attenuated in the highest degree. In a recent 

 paper [supra, p. 364) I have estimated the thickness of films of 

 olive oil competent to check the movements of camphor frag- 

 ments as from one to two micro-millimetres ; but these films are 

 comparatively coarse. For example, there was never any diffi- 

 culty in obtaining from tap-water surfaces upon which camphor 

 was fully active without the aid of wind or special arrangements. 

 I was naturally desirous of instituting a comparison between the 

 quantities necessary to check camphor movements and the more 

 minute ones which could be rendered manifest by Plateau's 

 needle ; but the problem is of no ordinary difficulty. A direct 

 weighing of the contamination is out of the question, seeing 

 that the quantity of oil required in the well of the apparatus, 

 even to stop camphor, would be only -^-^ milligram. 



The method that I have employed depends upon the prepara- 

 tion of an ethereal solution of olive oil, with which clean 

 platinum surfaces are contaminated. It may be applied in two 

 ways. Either we may rely upon the composition of the solution 

 to calculate the weight of oil remaining upon the platinum after 

 evaporation of the solvent, or we may determine the relative 

 quantities of solution required to produce the two sorts of 

 effects. In the latter case we are independent of the precise 

 composition of the solution, and more especially of the question 

 whether the ether may be regarded as originally free from dis- 

 solved oil of an involatile character. In practice, both methods 

 have been used. 



The results were not quite so regular as had been hoped, the 

 difficulty appearing to be that the oil left by evaporation upon 

 platinum was not completely transferred to the water surface 

 when the platinum was immersed, even although the operation 

 was performed slowly, and repeated two or three times. On 

 the other hand, there was no difficulty in cleansing a large 

 surface of platinum by ignition in the flame of a spirit-lamp, so 

 that it was absolutely without perceptible effect upon the 

 movement of the needle over a purified water surface. 



The first solution that was used contained 7 milligrams of oil 

 in 50 c.c. of ether. The quantities of solution employed were 

 reckoned in drops, taken under conditions favourable to uni- 

 formity, and of such dimensions that 100 drops measured o "6 c.c. 

 The following is an example of the results obtained : — On April 

 25, the apparatus was rinsed out and recharged with distilled 

 water. Time =13. After purification of surface by wind and 

 heat, 5^ ; rising, after a considerable interval, to 6. After in- 

 sertion of a large plate of platinum, recently heated to redness, 

 time unchanged. A narrow strip of platinum, upon which, 

 after a previous ignition, three drops of the ethereal solution 

 had been evaporated, was then immersed, with the result that 

 the time was at once increased to 8|. In subsequent trials, two 

 drops never failed to produce a distinct effect. Special experi- 

 ments, in which the standard ether was tested after evaporation 

 upon platinum, showed that nearly the whole of the effect was 

 due to the oil purposely dissolved. 



The determination of the number of drops necessary to check 

 the movements of camphor upon the same surface seemed to be 

 subject to a greater irregularity. In some trials 20 drops sufficed ; 

 while in others 40 or 50 drops were barely enough. There seems 

 to be no doubt that the oil is left in a rather unfavourable con- 

 dition,^ very different from that of the compact drop upon the 

 small platinum surface of former experiments ; and the appear- 

 ance of the platinum on withdrawal from the water often indicates 

 that it is still greasy. Under these circumstances it is clearly 

 the smaller number that should be adopted ; but we are safe in 

 saying that xV of the oil required to check camphor produces a 

 perceptible effect upon the time in Plateau's experiment, and 

 still more upon the behaviour of the surface before the advancing 

 needle, as tested by observation of motes. At this rate the 

 thickness at which superficial viscosity becomes sensible in 

 Plateau's apparatus is about ^V of a micro-millimetre, or about 

 -^^s of the wave-length of yellow light, 



I It should be stated that the evaporation of the ether, and of the dew 

 which was often visible, was facilitated by the application of a gentle 

 warmth. 



NO. 108 1, VOL. 42] 



A tolerably concordant result is obtained from a direct 

 estimate of the smaller quantity of oil, combined with the former 

 results for camphor, which were arrived at under more favour- 

 able conditions. The amount of oil in two drops of the solu- 

 tion is about o'ooi7 milligram. This is the quantity which 

 suffices to produce a visible effect upon the needle. On the 

 large surface of water of the former experiments the oil required 

 to check camphor was about i milligram. In order to allow 

 for the difference in area, this must be reduced 64 times, or to 

 o'oi6 milligram. According to this estimate the ratio of thick- 

 nesses for the two classes of effects is about as 10 : i. 



Very similar results were obtained from experiments with an 

 ethereal solution of double strength, one drop of which, eva- 

 porated as before, upon platinum, produced a distinct effect 

 upon the time occupied by the needle in traversing the arc from 

 90° to 30°. 



I had expected to find a higher ratio than these observations 

 bring out between the thicknesse-; required for the two effects. 

 The ratio 15:1 does not give any too much room for the sur- 

 faces of ordinary tap water, such as were used in the bath 

 observations upon camphor, between the purified surfaces on 

 the one side and those oiled surfaces upon the other, which do 

 not permit the camphor movements. 



It thus became of interest to inquire in what proportion the 

 film originally present upon the water in the bath experiments 

 requires to be concentrated in order to check the motion of 

 camphor fragments. This information may be obtained, some- 

 what roughly it is true, by dusting over a patch of the water 

 surface in the centre of the bath. When a weighed drop of oil 

 is deposited in the patch, it drives the dust nearly to the edge, 

 and the width of the annulus is a measure of the original 

 impurity of the surface. When the deposited oil is about 

 sufficient to check the camphor movements, we may infer that 

 the original film bears to the camphor standard a ratio equal to 

 that of the area of the annulus to the whole area of the bath. 

 Observations of this kind indicated that a concentration of about 

 six times would convert the original film into one upon which 

 camphor would not freely rotate. 



Another method by which this problem may be attacked 

 depends upon the use of flexible solid boundary. This was 

 made of thin sheet brass, and is deposited upon the bath in its 

 expanded condition, so as to enclose a considerable area. Upon 

 this surface camphor rotates, but the movement may be stopped 

 by the approximation of the walls of the boundary. The results 

 obtained by this method were of the same order of magnitude. 



If these conclusions may be relied upon, it will follow that the 

 initial film upon the water in the bath experiments is not a large 

 multiple of that at which superficial viscosity tends to disappear. 

 At the same time, the estimate of the total quantity of oil which 

 must be placed upon a really pure surface in order to check the 

 movements of camphor must be somewhat raised, say from i '6 

 to I "9 micro-millimetre. It must be remembered, however, 

 that on account of the want of definiteness in the effects, these 

 estimates are necessarily somewhat vague. By a modification of 

 Plateau's apparatus, or even in the manner of taking the 

 observations, such as would increase the extent of surface from 

 which the film might be accumulated before the advancing edge 

 of the needle, it would doubtless be possible to render evident 

 still more minute contaminations than that estimated above at 

 one-tenth of a micro-millimetre. 



[P.S. June 4. — In order to interpret with safety the results 

 obtained by Plateau, I thought it necessary to follow closely his 

 experimental arrangements ; but the leading features of the phe- 

 nomenon may be well illustrated without any special apparatus. 

 For this purpose, the needle of the former experiments may be 

 mounted upon the surface of water contained to a depth of i or 

 2 inches in a large flat bath. Ordinary cleanliness being ob- 

 served, the motes lying in the area swept over by the needle are 

 found to behave much as described by Plateau. Moreover, the 

 motion of the needle under the action of the magnet used to dis- 

 place it is decidedly sluggish. In order to purify the surface, a 

 hoop of thin sheet brass is placed in the bath, so as to isolate a 

 part including the needle. The width of the hoop must, of 

 course, exceed the depth of the water, and that to an extent 

 sufficient to allow of manipulation without contact of the fingers 

 with the water. If the hoop be deposited in its contracted state, 

 and be then opened out, the surface contamination is diminished 

 in the ratio of the areas. By this simple device there is no diffi- 

 culty in obtaining a highly purified surface, upon which motes 

 lie quiescent, almost until struck by the oscillating needle. In 



