TRANSACTIONS OF THE SECTIONS. 29 VIII. This column shows that the difference between the corrected mean time and the calculated time in no case exceeds 0.01 second. IX. The approximate velocities, deduced by drawing tangents to the curve. Experiments on Vortex-rings in Liquids. By H. DEACON. On Units of Force and Energy. By Professor J. D. EVERETT, F.R.S.E. The object of the paper was to urge the necessity of giving names to absolute units of force and energy, that is, units not varying with locality, like the gravitation units vulgarly employed (pound, foot-pound, &c.), but defined by reference to specified units of length, mass, and time, according to the condition that unit force acting on unit mass produces unit acceleration. The author proposed that the units of force and of energy (or of work), thus related to the metre, gramme, and second, be called respectively the dyne and the pone (dúvaμus, óvos), and that the names kilodyne, megadyne, kilopone, megapone be employed to denote a thousand and a million dynes and pones. The megadyne and megapone will thus be the units of force and energy related to the metre, the tonne, and the second. He also proposed that the units of force and energy related to the foot, the pound, and the second be called respectively the kinit and the erg *. On the Corrosion of Copper Plates by Nitrate of Silver. In some recent experiments in chemical dynamics, the authors had occasion to study the action of nitrate of silver on copper plates in various positions. They observed that when the plate was vertical there was rather more corrosion at the bottom than at the top. This is easily accounted for by the upward current, which flows along the surface of the deposited crystals, and which necessitates a movement of the nitrate-of-silver solution towards the copper plate especially impinging on the lower part. It was also found that when the copper plate was varnished on one side it produced rather more than half the previous decomposition, and was most corroded at the edges of the varnish. By making patterns with the varnish, this edge action became very evident. This was explained by the fact that the long crystals of silver growing out from the copper at the borders can spread their branches into the open space at the side, and so draw their supply from a larger mass of solution than the crystals in the middle can do; and increased crystallization of silver means increased solution of copper. This was proved by making the varnish a perpendicular wall instead of a thin layer, when the greater corrosion was not obtained. In a plate completely surrounded with liquid, the greatest growth of crystals is also evidently from the angles. It was likewise observed that if a vertical plate be immersed, the lower part in nitrate of copper, and the upper part in nitrate of silver, there is greater corrosion about the point of junction. This was attributed to the greater conduction of the stronger liquid. Some Remarks on Physics. By M. JANSSEN. *Since the reading of the paper, a Committee has been appointed by the Association "to frame a nomenclature of absolute units of force and energy." On Democritus and Lucretius, a Question of Priority in the Kinetical Theory of Matter. By T. M. LINDSAY and W. R. SMITH. Physicists who have recently called attention to the anticipation of modern doctrines as to the ultimate nature of matter by the ancient atomists, have looked too exclusively to Epicurus and his expositor Lucretius, to the neglect of Democritus and Leucippus. Democritus had no such expositor as Lucretius, but his main views are accessible in the fragments collected by Mullach, and in the wellknown references of Aristotle, Simplicius, and Laertius. With the help of these sources, the paper sketches the main features of the earliest atomic theories. The following are leading points : Democritus and Leucippus trace the variety of phenomena to three primitive differences in the ultimate elements of nature, viz. differences (1) in Figure, σxpa, as between A and N; (2) in Order, ráĝis, as AN, NA; (3) in Position, déos, as Z, N [Arist. Met. A. 4]. From the motion in vacuo of atoms with these primary differences, the whole variety of nature is deduced, generation and corruption being merely syncretion and division (σvyкpiσis, diákpiois) [Ar. De Gen. et Cor. i. 8, i. 2, Phys. viii. 9]. All atoms have the same density and the same opun ris Popās (specific gravity ?) [Ar. De Cœlo, i. 7, Theophrastus De Sensu, 71]. Hence all tend to fall in one downward direction; but being ignorant of the law of inertia, Democritus supposes that the larger atoms fall faster, impinge on lighter particles, and produce a vortex motion (Sun). In this vortex similars come together and cohere, lighter particles go to the surface, and at length worlds (kóσuo) are generated [Diog. Laertius, ix. 31]. Epicurus differs from Democritus mainly by maintaining that all atoms have equal and invariable downward velocities, and come into collision only by fortuitous automatic deflection from the line of fall. The first half of this theory looks like the first law of motion, but is really as far from being in harmony with the laws of acceleration and other known truths as the earlier view. As physicists, therefore, Epicurus and Lucretius made no advance on Democritus, while by mixing up with legitimate physical speculation the incongruent metaphysical notion of chance (not the mathematical notion of chance, which plays a part in the modern kinetic theory of gases), they produced that hybrid of physics and metaphysics, a materialistic philosophy. It was by adopting the Epicurean doctrine of chance that Gassendi, the first of modern atomists, became also the father of modern materialism. Speculations on the Continuity of the Fluid State of Matter, and on Relations between the Gaseous, the Liquid, and the Solid States. By Prof. JAMES THOMSON, LL.D. Through the recent discovery of Dr. Andrews on the relations between different states of fluid matter, a difficulty in the application of our old ordinary language has arisen. He has shown the existence of continuity between what is ordinarily called the liquid state and what is ordinarily called the gaseous state of matter. He has shown that the ordinary gaseous and ordinary liquid states are only widely separated forms of the same condition of matter, and may be made to pass into one another by a course of continuous physical changes presenting nowhere any interruption or breach of continuity. If, now, there be no distinction between the liquid and gaseous states, is there any meaning still to be attributed to those two old names, or ought they to be abandoned, and the single name the fluid state to be substituted for them both? The answer must be that in speaking of the whole continuous state we have now to call it simply the fluid state; but that there are two regions or parts of it, meeting one another sharply in one way, and merging gradually into one another in a different way, to which the names liquid and gas are still to be applied. We can have a substance existing in two fluid states different in density and other properties, while the temperature and pressure are the same in both: and we may then find that an introduction or abstraction of heat without change of temperature or of pressure will effect the change from the one state to the other, and that the * Cf. the argument in Zeller, Phil. der Griechen, i. 913, ff. change either way is perfectly reversible. When we thus have two different states present together in contact with one another, we have a perfectly obvious distinction, and we can properly continue to call one of them a liquid state and the other a gaseous state of the same matter. The same two names may also reasonably be applied to regions or parts of the fluid state extending away on both sides of the sharp or definite boundary, wherever the merging of the one into the other is little or not at all apparent. If we denote geometrically all possible points of temperature and pressure jointly, by points spread continuously in a plane surface, each point in the plane being referred to two axes of rectangular coordinates, so that one of its ordinates shall represent the pressure and the other the temperature denoted by that point, and if we mark all the successive boiling- or condensingpoints of pressure and temperature as a continuous line on this plane, this line, which may be called the boiling-line, will be a separating boundary between the regions of the plane corresponding to the ordinary liquid state and those corresponding to the ordinary gaseous state. But by consideration of Dr. Andrews's experimental results (Phil. Trans. 1869), we may see that this separating boundary comes to an end at a point of temperature and pressure which, in conformity with his language, may be called the critical point of pressure and temperature jointly; and we may see that from any ordinary liquid state to any ordinary gaseous state the transition may be gradually effected by an infinite variety of courses passing round the extreme end of the boiling-line. Fig. I is a diagram to illustrate these considerations and some allied considerations to which they lead in reference to transitions between the three states, the gaseous, the liquid, and the solid. This figure is intended only as a sketch to illustrate principles, and is not drawn according to measurements for any particular substance, though the main features of the curves shown in it are meant to relate in a general way to the substance of water, steam, and ice. AX and AY are the axes of coordinates for pressures and temperatures respectively; A, the origin, being taken as the zero for pressures and as the zero for temperatures on the Centigrade scale. The curve L represents the boiling-line. This terminates towards one direction in the critical point E; it passes in the other direction to T, the point of pressure and temperature where solidification sets in. This point T is to be noticed as a remarkable point of pressure and temperature, as being the point at which alone the substance, pure from admixture with other substances, can exist in three states, solid, liquid, and gaseous, together in contact with one another. In making this statement, however, the author wishes to submit it subject to some reserve in respect to conditions not as yet known with perfect certainty. He observes that we might not be quite safe in assuming that the melting-point of ice solidified from the gaseous state is the same as the melting-point of ice frozen from the liquid state, and in making other suppositions, such as that the same quantity of heat would become latent in the melting of equal quantities of ice formed in these two ways. Such considerations as these into which we are forced if we attempt to sketch out the course of the boiling-line, and to examine along with it the corresponding boundary-lines between liquid and solid and between gas and solid, may be useful in suggesting questions for experimental and theoretical investigation which may have been generally overlooked before. Proceeding, however, upon assumptions such as usually are tacitly made, of identity in the thermal and dynamic conditions of pure ice solidified in different ways, the anthor points out that we must suppose the three curves (namely, the line between gas and liquid, the line between liquid and solid, and the line between gas and solid) to meet in one point, shown at T in the figure. This point of pressure and temperature for any substance may then be called the triple point for that substance. In the figure the line T M represents the line between liquid and solid. It is drawn showing in an exaggerated degree the lowering of the freezing temperature of water by pressure, the exaggeration being necessary in order to allow small changes of temperature to be perceptible in the diagram. The line TN represents the line between the gaseous and the solid states of water substance. The two curves T L and TN, one between gas and liquid and the other between gas and solid, have been constructed for water substance through a great range of temperatures and pressures by Regnault, from his experiments on the pressure of saturated aqueous gas at various temperatures above and below 0° Centigrade. He has represented and discussed his results above and below the temperature at which the water freezes (which in strictness is not 0° C., but is the freezing temperature of water in contact with no atmosphere except its own gas), as if one continuous curve could extend for both. As brought out experimentally, indeed, they present so little appearance of any discontinuity that the distinctness of the two curves from one another might readily escape notice in the consideration of the experimental results. Prof. Thomson points out, however, that the range from temperatures below to temperatures above freezing comprises what ought to be regarded as two essentially distinct curves meeting one another in the point T; and he further suggests that continuations of these curves, sketched in as dotted lines TP and TQ, may have some theoretical or practical significance not yet fully discovered. He thinks it likely that out of the three curves at least the one, MT, between liquid and solid may have a practically attainable extension past T, as shown by the dotted continuation TR. Various known experiments seem to render this supposition tenable, whether the condition supposed may have been actually realized in experiments hitherto or not. He thinks, too, that there is much reason to suppose that the curve LT between gas and liquid has a practically attainable extension past T, as shown by the dotted continuation T P. In reference to the continuity of the liquid and gaseous states, Prof. Thomson showed a model in which Dr. Andrews's curves for carbonic acid are combined in a curved surface, obtained from them, which is referred to three axes of rectangular coordinates, and is formed so that the three coordinates of each point in the curved surface shall represent, for any given mass of carbonic acid, a pressure, a temperature, and a volume, which can coexist in that mass. This curved surface shows in a clear light the abrupt change or breach of continuity at boiling or condensing, and the gradual transition round the extreme end of the boiling-line. Using this model and a diagram of curves represented here in fig. 2, the author explained a view which had occurred to him, according to which it appears probable that although there be a practical breach of continuity in crossing the line of boilingpoints from liquid to gas, or from gas to liquid, there may exist, in the nature of * Mémoires de l'Académie des Sciences, 1847, pl. viii. things, a theoretical continuity across this breach, having some real and true sigRSIT! nificance. The general character of this view may readily be seen by a glance at fig. 2, in which Dr. Andrews's curves are shown by continuous lines (not dotted), and curved reflex junctions are shown by dotted lines connecting those of Dr, An Fig. 2. drews's curves which are abruptly interrupted at their boiling- or condensing-points of pressure. It is to be understood that each curve relates to one constant temperature, and that pressures are represented by the horizontal ordinates, and corresponding volumes of one mass of carbonic acid constant throughout all the curves are represented by the vertical ordinates. The author points out that, by experiments of Donny, Dufour, and others, we have already proof that a continuation of the curve for the liquid state past the boiling stage for some distance, as shown dotted in fig. 2, from a to some point b towards f, would correspond to states already attained. He thinks we need not despair of practically realizing the physical conditions corresponding to some extension of the gaseous curve such as from c to d in the figure. The overhanging part of the curve from e to f he thinks may represent a state in which there would be some kind of unstable equilibrium; and so, although the curve there appears to have some important theoretical significance, yet the states represented by its various points would be unattainable throughou. any ordinary mass of the fluid. It seems to represent conditions of coexistent temperature, pressure, and volume, in which, if all parts of a mass of fluid were placed, it would be in equilibrium, but out of which it would be led to rush, partly into the rarer state of gas, and partly into the denser state of liquid, by the slightest inequality of temperature or of density in any part relatively to other parts. * Donny, Ann. de Chimie, 1846, 3rd series, vol. xvi. p. 167; Dufour, Bibliothèque Universelle, Archives, 1861, vol. xii. 1871. 3 |