276 



NATURE 



[July 23, 1891 



A liquid would no longer rise in a narrow capillary tube ; 

 it would stand at the same level outside and inside. 



It was suggested by Prof. James Thomson, and by Prof. 

 Clausius about the same time, that if the ideal state of 

 things were to exist, the passage from the liquid to the 

 gaseous state should be a continuous one, not merely at 

 and above the critical point, but below that temperature. 

 And it was suggested that the curves, shown in the figure, 

 instead of breaking into the straight line of vapour- 

 pressure, should continue s-inuously. Let us see what 

 this conception would involve. 



On decreasing the volume of a gas, it should not 

 liquefy at the point marked v, on the diagram, but should 



still decrease in volume on increase of pressure. This 

 decrease should continue until the point E is reached. 

 The anomalous state of matters should then occur, that a 

 decrease in volume should be accompanied by a decrease 

 of pressure. In order to lessen volume, the gas must be 

 exposed to a continually diminishing pressure. But such 

 a condition of matter is of its nature unstable, and has 

 never been realized. After volume has been decreased to 

 a certain point, F, decrease of volume is again attended 

 by increase of pressure, and the last part of the curve is 

 continuous with the realizable curve representing the 

 compression of the liquid, above D. 



Dr. Sydney Young and I succeeded, by a method which 

 I shall briefly describe, in mapping the actual position 

 of the unrealizable portions of the curve. They have the 



NO. I 134, VOL. 44] 



form pictured in this figure. The rise from the gaseous 

 state is a gradual one ; but the fall from the liquid state i& 

 abrupt. 



Consider the volume 14 cubic centimetres per gram on 

 the figure. The equi-volume vertical line cuts the iso- 

 thermal linesfor the temperatures 175°, 180°, 185°, 190^, and 

 so on, at certain definite pressures, which may be read 

 from a properly-constructed diagram. We can map the 

 course of lines of equal volume, of which the instance 

 given is one, using temperatures as ordinates and 

 pressures as abscissae. We can thus find the rela- 

 tions of temperature to pressure for certain definite 

 volumes, which we may select to suit our convenience — 

 say, 2 c.c. per gram ; 3, 4, 5, 6, and so on. Now, all such 

 lines are straight— that is, the relation of pressure to 

 temperature, at constant volume, is one of the simplest ; 

 pressure is a linear function of temperature. Expressed 

 mathematically — 



p = bt — a, 



where b and a are constants, depending on the volume 

 chosen, and varying with each volume. But a straight 

 line may be extrapolated without error ; and so, having 

 found values for a and b for such a volume as 6 c.c. per 

 gram, by help of experiments at temperatures higher 

 than 195°, it is possible by extrapolation to obtain the 

 pressures corresponding to temperatures below the critical 

 point 195° by simple means. But below that temperature 

 the substance at volume 6 is in practice partly liquid and 

 partly gas. Yet it is possible by such means to ascertain 

 the relations of pressure to temperature for the unrealiz- 

 able portion of the state of a liquid — that is, we can 

 deduce the pressure and temperature corresponding to a 

 continuous change from liquid to gas. And in this 

 manner the sinuous lines on the figure have been 

 constructed. 



It is possible to realize experimentally certain portions 

 of such continuous curves. If we condense all gaseous 

 ether, and, when the tube is completely filled with liquid,, 

 carefully reduce pressure, the pressure may be lowered 

 considerably below the vapour-pressure corresponding to 

 the temperature of ebullition, without any change further 

 than the slight expansion of the liquid resulting from the 

 reduction of pressure — an expansion too small to be seen 

 with this apparatus. But on still further reducing 

 pressure, sudden ebullition occurs, and a portion of the 

 liquid suddenly changes into gas, while the pressure rises 

 quickly to the vapour-pressure corresponding to the tem- 

 perature. If we are successful in expelling all air or gas 

 Irom the ether in filling the tube, a considerable portion 

 of this curve can be experimentally realized. 



The first notice of this appearance, or rather of one 

 owing its existence to a precisely similar cause, is due to 

 Hooke, the celebrated contemporary of Boyle. It is noted 

 in the account of the proceedings of the Royal Society 

 on November 6, 1672, that " Mr. Hooke read a discourse 

 of his, containing his thoughts of the experiment of the 

 quicksilver's standing top-full, and far above the height of 

 29 inches ; together with some experiments made by him,, 

 in order to determine the cause of this strange pheno- 

 menon. He was ordered to prepare those experiments 

 for the view of the Society." And on November 13 " the 

 experiment for the high suspension of quicksilver being 

 called for, it was found that it had failed. It was ordered 

 that thicker glasses should be provided for the next 

 meeting." 



There can be no doubt that this behaviour is caused 

 by the attraction of the molecules of the liquid for each 

 other. And if the temperature be sufficiently low, the 

 pressure may be so reduced that it becomes negative — 

 that is, until the liquid is exposed to a strain or pull, as is 

 the mercury. This has been experimentally realized by 

 M. Berthelot and by Mr. Worthington, the latter of whom 

 has succeeded in straining alcohol at the ordinary tem- 



