90 Lord Bayleigh [March 28, 



It is seen that a film of oil about 1^ millionth of a millimetre thick is 

 able to produce this change. I know that large numbers are not 

 readily appreciated, and I will therefore put the matter diflPerently. 

 The thickness of the oil film thus determined as sufficient to stop the 

 motions of the camphor is one 400th of the wave length of yellow light. 

 Another way of saying the same thing is that this thickness of oil 

 bears to one inch the same ratio that one second of time bears to half 

 a year. 



When the movement of the camphor has been stopped by the 

 addition of a minute quantity of oil, it is possible, by extending the 

 water surface enclosed within the boundary, without increasing the 

 quantity of oil, to revive the movements of the camphor ; or, again, by 

 contraction, to stop them. I can do this with the aid of a flexible 

 boundary of thin sheet brass, and you see that the camphor recovers 

 its activity, though a moment ago it was quite dead. It would be an 

 interesting subject for investigation to determine what is the actual 

 tension of an oily surface contaminated to an extent just sufficient to 

 stop the camphor movements ; but it is not an easy problem. Usually 

 we determine surface tensions by the height to which the liquids will 

 rise in very fine tubes. Here, however, that method is not available, 

 because if we introduce a tube into such a surface, there is no proof 

 that the contamination of the inner surface in the tube is the same as 

 that prevailing outside. Another method, however, may be employed 

 which is less open to the above objection, and that is to substitute for 

 the very fine or capillary tube, a combination of two parallel plates 

 open at their edges. We have here two such plates of glass, kept 

 from absolutely closing by four pieces of thin metal inserted at the 

 corners, the plates being held close against these distance-pieces by 

 suitable clamps. If such a combination be inserted in water, the 

 liquid will rise above the external level, and the amount of the rise is 

 a measure of the surface tension of the water. You see now the 

 image on the screen. A is the external water surface ; B is the height 

 of the liquid contained between the glass plates, so that the tension 

 may be said to be measured by the distance AB. If a little oil be 

 now deposited upon the surface, it will find its way between the 

 plates. The fall which you now see shows that the surface tension 

 has been diminished by the oil which has found its way in. A very 

 minute quantity will give a great eftect. When the height of the pure 

 water was measured by 62, a small quantity of oil changed the 62 into 

 48, and subsequent large additions of oil could only lower it to 38. 

 But after oil has done its worst, a further effect may be produced by 

 the addition of soap. If Mr. Gordon now adds some soap, we shall 

 find that there is a still further fall in the level, showing that the 

 whole tension now in operation is not much more than one-third of 

 what it was at first. This is an important point, because it is some- 

 times supposed that the effect of soap in diminishing the tension of 

 water is due to merely the formation upon the surface of a layer of 

 oil formed by decomposition of the soap. This experiment proves the 



