feet of about the same size and shape as the glass ones gave different results. It should, however, be noticed that the brass feet, though fairly polished, could not have been so smooth as the fire surfaces of the glass. The present carriage weighed (with its load) 6 oz., and on the wellhandled glass slide moved with oz. in pan. When the slide was breathed upon, the motion was as free as, perhaps more free than, before. And when the feet stood in a pool of water, there was equal freedom. A repetition gave confirmatory results. On another day paraffin oil was tried. At the beginning oz. in pan sufficed on the handled slab. With a pool of oil the carriage still moved with oz. in pan, but perhaps not quite so certainly. As the oil was removed with blotting-paper the motion became freer, and when the oil-film had visibly disappeared the oz. in pan could about be dispensed with. Doubtless a trace of oil remained. The blotting-paper was of course applied to the feet and legs 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 films on handled surfaces. But this 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 of the wave-length of visible light, viz. about 10-4 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 the camphormovements on water, which I found to be about 2 × 10-6 mm. It may well be that there is little difference in the quantities required 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 moleules ‡, we see that the phenomena here in question probably lie outside the field of the usual theory of lubrication, where the layer of lubricant is assumed to be at least many molecules thick. We are rather in the region of incipient seizing, as is perhaps not surprising when we consider the smallness of the surfaces Phil. Mag. vol. xix. p. 96 (1910); Scientific Papers, vol. v. p. 538. p. 349. Phil. Mag. vol. xlviii. p. 321 (1899); Scientific Papers, vol. iv. p. 430. 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 surfaces. It may perhaps be doubted whether the time is yet ripe for a full discussion of the behaviour of the thinnest films, but I will take this opportunity to put forward a few remarks. Two recent French writers, Devaux* and Marcelin †, who have made interesting contributions to the subject, accept my suggestion that the drop of tension in contaminated surfaces commences when the layer is one molecule thick; but Hardy points out a difficulty in the case of pure oleic acid, where it appears that the drop commences at a thickness of 1.3 x 10-6 mm., while the thickness of a molecule should be decidedly less. Many of Devaux' observations relate to the case where the quantity of oil exceeds that required for the formation of the mono-molecular laver, and he formulates a conclusion, not accepted by Marcelin, that the thickness of the layer depends upon the existence and dimensions of the globules into which most of the superfluous oil is collected, inasmuch as experiment proves that when a layer with fine globules exists beside a layer with large globules, the former always contracts at the expense of the latter. As to this, it may be worth notice that the tension T of the contaminated surface could not be expressed as a function merely of the volume of the drop and of the two other tensions, viz. T the tension of an air-oil surface and T2 that of a water-oil surface. It would be necessary to introduce other quantities, such as gravity, or molecular dimensions. I am still of the opinion formerly expressed that these complications are the result of impurity in the oil. If the oil were really homogeneous, Devaux' views would lead one to regard the continued existence of two sizes of globules on the same surface as impossible. What would there be to hinder the rapid growth of the smaller at the expense of the greater until equality was established? On the other hand, an impurity, present only in small proportion, would naturally experience more difficulty in finding its way about. The importance of impurities in influencing the transformations of oil-films was insisted on long ago by Tomlinson §; A summary of Devaux' work, dating from 1903 onwards, will be found in the Revue Gén. d. Sciences for Feb. 28, 1913. + Annales d. Physique, t. i. p. 19 (1914). Proc. Roy. Soc. A, vol. lxxxviii. p. 319 (1913). and as regards olive oil, Miss Pockels showed that the behaviour of purified oil is quite different from that of the common oil. She quotes Richter (Nature, vol. xlix. p. 488) as expressing the opinion that the tendency of oil to spread itself on water is only due to the free oleic acid contained in it, and that if it were possible to completely purify the oil from oleic acid, it would not spread at all*. Some confusion arises from the different meanings attached to the word "spreading." I suppose no one disputes the rapid spreading upon a clean surface which results in the formation of the invisible mono-molecular layer. Miss Pockels calls this a solution current-a rather misleading term, which has tended to obscure the meaning of her really valuable work. It is the second kind of spreading in a thicker layer, resulting in more or less rapid subsequent transformations, which is attributed to the presence of oleic acid. Miss Pockels says:-"The Provence oil used in my experiment was shaken up twice with pure alcohol, and the rest (residue) of the latter being carefully removed, a drop of the oil was placed upon the freshly formed water-surface in a small dish by means of a brass wire previously cleaned by ignition. The oil did not really spread, but after a momentary centrifugal movement, during which several small drops were separated from it, it contracted itself in the middle of the surface, and a second drop deposited on the same vessel remained absolutely motionless." I have repeated this experiment, using oil which is believed to have come direct from Italy. A drop of this placed upon a clean water-surface at once drives dust to the boundary in forming the mono-molecular layer, and in addition flattens. itself out into a disk of considerable size, which rapidly undergoes the transformations well described and figured by Devaux. The same oil, purified by means of alcohol on Miss Pockels' plan, behaves quite differently. The first spreading, driving dust to the boundary, takes place entirely as before. But the drop remains upon the water as a lens, and flattens itself out, if at all, only very slowly. Small admixtures of the original oil with the purified oil behave in an intermediate manner, flattening out slowly and allowing the beautiful transformations which follow to be observed at leisure. Another point of importance does not appear to have been noticed. Water-surfaces on which purified olive oil stands in drops still allow the camphor movements. Very small fragments spin merrily, while larger ones by their slower *Nature,' vol. 1. p. 223 (1894). movements testify to the presence of the oil. Perhaps this was the reason why in my experiments of 1890 I found the approximate, rather than the absolute, stoppage of the movements to give the sharpest results. The absolute stoppage, dependent upon the presence of impurity, might well be less defined. If, after the deposition of a drop of purified oil, the surface be again dusted over with sulphur or talc and then touched with a very small quantity of the original oil, the dust is driven away a second time and camphor movements cease. The manner in which impurity operates in these phenomena merits close attention. It seems pretty clear that from pure oil water will only take a layer one molecule thick. But when oleic acid is available, a further drop of tension ensues. The question arises how does this oleic acid distribute itself? Is it in substitution for the molecules of oil, or an addition to them constituting a second layer? The latter seems the more probable. Again, how does the impurity act when it leads the general mass into the unstable fattened-out form? In considering such questions Laplace's theory is of little service, its fundamental postulate of forces operating over distances large in comparison with molecular dimensions being plainly violated. Terling Place, Witham, Dec. 31, 1917. XVIII. On the Second Postulate of the Theory of Relativity: Experimental Demonstration of the Constancy of Velocity of the Light reflected from a Moving Mirror. By Q. MAJORANA, Professor of Physics at the Polytechnic School of Turin *. THE HE Theory of Relativity is based upon two well-known fundamental postulates. The first affirms the impossibility of discovering the movement of a system without referring this system to other systems; that is to say, it denies the physical reality of absolute motion. The second postulate affirms that the velocity c of propagation of light in vacuo is a universal constant. Both these postulates are generalizations of facts or principles already admitted by physicists. In fact, we may regard the first as the extension to optical or electrical phenomena of a classical principle of mechanics, * Communicated by the Author. an extension justified by the negative results of certain experiments (Michelson and Morley, Trouton and Noble) by which it was sought to discover the absolute motion of the earth, or the ethereal wind which must traverse all terrestrial objects. The second postulate is the generalization of a fundamental principle in the theory of æthereal or electromagnetic undulations. But if these two principles, derived from quite different chapters of physics, have been fully accepted severally by modern physicists, their origin has been almost forgotten; an ingenious structure arose upon their union: the theory of relativity. This theory, while repudiating according to Einstein and others a theoretical conception which had given occasion for the formulation of the second postulate (the æther), serves well to explain the insuccess of the abovecited experiments. Now our imagination, accustomed, as W. Ritz has said, to "substantialize" physical phenomena, if it easily grasps the essence of the first postulate, does not do so in the case of the second; and the more so since, as has already been said, some adherents to relativistic theories do not retain as necessary the existence of a medium of transmission (the æther) in order to explain the constancy of e. Moreover on the second postulate, or, more precisely, on a certain portion of this, depend the conclusions which appear artificial or extraordinary in the whole relativistic theory *. The second postulate must be understood in the sense that an observer who measures the velocity of light finds always the same value if both he and the source be at rest, relative or (if the possibility be admitted) absolute, or if the source or the observer, or both, have a uniform motion of translation. In short, the second postulate affirms the absolute independence of c of any contingency whatever of uniform motion of translation of the source or of the observer. It is known that an hypothesis of a mechanical character (emissive or ballistic), according to which to the ordinary velocity of light must be added that of the source, can explain, like the theory of relativity, the failure of the abovequoted experiments. But this hypothesis is radically in contrast with the electromagnetic theory, and consequently is not much favoured t. But in any case laboratory experiments can be conceived which should decide between the Carmichael, Phys. Rev. xxxv. p. 168 (1912). In this connexion should be recalled the important critical work of W. Ritz (Euvres, p. 317) which perhaps has not been taken into sufficient consideration by physicists. |