1918]	PROPERTIES  OF THIN  OILY  FILMS	•	5JJ7
The blotting-paper was of course applied to the foot and logs of the carriage, as well as to the slab.
In attempting to interpret these results, it is desirable to know what sort of thickness to attribute to the greasy til ma on handled surfaces. But this is not so easy a matter as when films are spread upon water. In an experiment made some years ago* I found that the mean thickness of the layer on a glass plate, heavily greased with fingers which had touched the hair, was about •I of the wave-length of visible light, viz. about 1()~^4 mm. The thickness of the layer necessary to induce slipperiness must be a small fraction of this, possibly /,-,, but perhaps much less. We may compare this with the thickness of olive oil required to stop tin* camphor-movements on water, which I found f to be. about 2 x 10~" mm. It may well be. that there is little difference in the, quantities re-quired for the two effects.
In view of the above estimate and of the probability that the point at which surface-tension begins to fall corresponds to a thickness of a single layer of molecules*, wo see that the phenomena here in question probably lie outside the Held of the. usual theory of lubrication, where the layer of lubricant is assumed to bo at least many molecules thick. We are. rather in the region of incipient neizt'iKj, as in perhaps not. surprising when we consider the small-ness of the surfaces actually in contact. And as regards seizing, there is difficulty in understanding why, when it actually occurs, rupture should ensue, at another place, rather than at the recently engaged HtirfaeeH.
It may perhaps be doubted whether tin; time in yet ripe for a full dis eussion of the behaviour of the thinnest, films, but I will take, this opportunity to put forward a lew remarks. Two recent French writers, l)ovanx§ and Marcel in I], who have made interesting contributions to the Hiibject, accept my suggestion that the (Jrop of tension in contaminated surfaces commences when the layer is one molecule thick; but HardyH points out a difficulty in the cose of pure oleie acid, where it appears that the drop commences at a thickness of I'M x 1()-* mm., while the thicknoHS of a molecule should be decidedly iohh. Many of Devattx* observations relate to the easo where the quantity of oil exceeds that required for the formation of the mono-molecular layer, and he formulates a conclusion, not accepted by Maroelin, that the thickness of the layer depends upon the existence and dimensions of the globules into which moHt of the superfluous oil is collected, inasmuch as experiment proves that when a layer with fine globules exists beside a layer with largo globules,
• * Mill. .Vit//. Vol. xix. p. 98 (1910) ; Scientific. Paper*, Vol. v. p. 5SH. t Pmc. fttnj. Sac. Vol. xtvn. p. 804 (1890); Hcltntlfic Payers, Vol. lit. p. 849. * Phil. Man. Vol. xlviii. p. mi (189«); Ncicnti,ftc Papers, Vol. iv. p. 480. § A Huuimary of Devatu' work, clftting from 11)08 onwards, will be found in the Revue Gtn. d. Science* for Feb. 28, 1918.
I  Aniifili'H il. Physique, t. I. p. 10 (1914).
i; I'ruc. Hoy. .S'oc. A, Vol. Lxxxvm. p. 819 (1918).