July 23, 1891] 



NATURE 



275 



stands at 373 divisions of the scale. The gas has thus 

 expanded from 290 to 373 divisions, i.e. its volume has 

 increased by 83 divisions ; and the temperature has risen 

 from 17' to 100°, i.e. through 83°. This law of the ex- 

 pansion of gases was discovered almost simultaneously 

 by Dalton and Gay-Lussac in 1801 ; it usually goes by 

 the name of Gay-Lussac's law. Now, if we do not allow 

 the volume of the gas to increase, we shall find that the 

 pressure will increase, in the same proportion that the 

 volume would have increased had the gas been allowed 

 to expand, the pressure having been kept constant. To 

 decrease the volume of the gas, then, according to Boyle's 

 law, will require a higher initial pressure ; and if we were 

 to represent the results by a curve, we should get an 

 hyperbola, as before, but one lying higher as regards 

 pressures. And so we should get a set of hyperbolas 

 for higher and higher temperatures. 



We have experimented up to the present with air — a 

 mixture of two gases, oxygen and nitrogen ; and the 

 boiling-points of both of these elements lie at very low 

 temperatures : -- 184^ and - I93'"i respectively. The 

 ordinary atmospheric temperature lies a long way above 

 the boiling-points of liquid oxygen and liquid nitrogen at 

 the ordinary atmospheric pressure. But it is open to us to 

 study a gas, which, at theordinaryatmospheric temperature 

 and pressure, exists in the liquid state; and for this purpose 

 I shall choose water-gas. In order that it may be a gas at 

 ordinary atmospheric pressure, however, we must heat it 

 to a temperature above 100' C., its boiling-point. This 

 tube contains water-gas at a temperature of 105° C. ; it 

 is under ordinary pressure, for the mercury columns are 

 at the same level in both the tubes and in this reservoir, 

 which communicates with the lower end of the tube by 

 means of the india-rubber tubing. The temperature 

 105° is maintained by the vapour of chlorobenzene, boiling 

 in the bulb sealed to the jacket, at a pressure lower than 

 that of the atmosphere. 



Let us now examine the effect of increasing pressure. On 

 raising the reservoir, the volume of the gas is diminished, 

 as usual ; and nearly in the ratio given by Boyle's law ; 

 that is, the volume decreases in the same proportion as 

 the pressure increases. But a change is soon observed ; 

 the pressure soon ceases to rise ; the distance between 

 the mercury in the reservoir and that in the tube remains 

 constant, and the gas is now condensing to liquid. The 

 pressure continues constant during this change ; and it is 

 only when all the water-gas has condensed to liquid 

 water that the pressure again rises. After all gas is con- 

 densed, an enormous increase of pressure is necessary to 

 cause any measurable decrease in volume, for liquid 

 water scarcely yields to pressure, and in such a tube as 

 this, no measurements could be attempted with success. 



Representing this diagrammatically, the right-hand 

 part of the curve represents the compression of the gas ; 

 and the curve is, as before, nearly a hyperbola. Then 

 comes a break, and great decrease in volume occurs 

 without rise of pressure, represented by a horizontal 

 line ; the substance in the tube here consists of water- 

 gas in presence of water ; the vertical, or nearly 

 vertical line represents the sudden and great rise 

 of pressure, where liquid water is being slightly com- 

 pressed. The pressure registered by the horizontal 

 line is termed the " vapour-pressure " of water. If, now, 

 the temperature were raised to no", we should have a 

 greater initial volume for the water-gas ; it is compres- 

 sible by rise of the mercurj' as before, the relation of 

 pressure to volume being, as before, represented on the 

 diagram as an approximate hyperbola ; and as before, 

 condensation occurs when volume is sufficiently reduced, 

 but this time at a higher pressure. We have again a 

 horizontal portion, representing the pressure of water-gas 

 at 110° in contact with liquid water ; again, a sharp angle 

 where all gaseous water is condensed, and again a very 

 steep curve, almost a straight line, representing the 



NO. 1134, VOL. 44] 



slight decrease of volume of water produced by a great 

 increase of pressure. And we should have similar lines 

 for 120^, 130°, 140", 150°, and for al' temperatures within 

 certain limits. Such lines are called isothermal lines, 

 or shortly " isothermals," or lines of equal temperature, 

 and represent the relations of pressure to volume for 

 different temperatures. 



Dr. Andrews made similar measurements of the rela- 

 tions between the pressures and volumes of carbon 

 dioxide, at pressures much higher than those I have 

 shown you for water. But 1 prefer to speak to you 

 about similar results obtained by Prof. Sydney Young 

 and myself with ether, because Dr. Andrews was unable 

 to work with carbon dioxide free from air, and that in- 

 fluenced his results. For example, you see that the 

 meeting-points of his hyperbolic curves with the straight 

 lines of vapour-pressures are curves, and not angles ; that 

 is caused by the presence of about i part of air in 500 

 parts of carbon dioxide ; also the condensition of ga.s 

 was not perfect, for he obtained curves at the points df 

 change from a mixture of liquid and gas to liquid. We, 

 however, were more easily able to fill a tube with etheir 

 free from air, and you will notice that the points I havie 

 referred to are angles, not curves. i 



Let me first direct your attention to the shapes of the 

 curves in the diagram. As the temperature rises,the vapouj- 

 pressure lines lie at higher and higher pressures, and thp 

 lines themselves become shorter and shorter. And 

 finally, at the temperature 31° for carbon dioxide, and ^t 

 195° for ether, there ceases to be a horizontal portion j^t 

 all ; or rather, the curve touches the horizontal at onje 

 point in its course. That point corresponds to a definitje 

 temperature, 195° for ether; to a definite pressure, 2j7 

 metres of mercury, or 35 6 atmospheres ; and to a definitte 

 volume, 4'o6 cubic centimetres per gram of ether. A(t 

 that point the ether is not liquid, and it is not gas; it is ft. 

 homogeneous substance. At that temperature ether ha!s 

 the appearance of a blue mist ; the striae mentioned h^ 

 Dr. Andrews, and by other observers, are the result of 

 unequal heating, one portion of the substance beint 

 liquid, and another gas. You see the appearance of this 

 state on the screen. 1 



When a gas is compressed, it is heated. Work is donie 

 on the gas, and its temperature rises. If I compress thje 

 air in this syringe forcibly, its temperature rises so high 

 that I can set a piece of tinder on fire, and by its helJD 

 explode a little gunpowder. If the ether at its critical 

 point be compressed by screwing in the screw, it is some- 

 what warmed, and the blue cloud disappears. Conversely, 

 if it is expanded a little by unscrewing the screw, and 

 increasing its volume, it is cooled, and a dense mist is 

 seen, accompanied by a shower of ether rain. This is 

 seen as a black fog on the screen. 



I wish also to direct your attention to what happens if 

 the volume given to the ether is greater than the criticcll 

 volume — on increasing the volume, you see that it boils 

 away and evaporates completely ; and also what happens 

 if the volume be somewhat less than the critical volume — 

 it then expands as liquid, and completely fills the tube. 

 It is only at the critical volume and temperature that the 

 ether exists in the state of blue cloud, and has its critical 

 pressure. If the volume be too great, the pressure is 

 below the critical pressure ; if too small, the pressure is 

 higher than the critical pressure. 



Still one more point before we dismiss this experiment. 

 At a temperature some degrees below the critical tem- 

 perature, the meniscus, i.e. the surface of the liquid, is 

 curved. It has a skin on its surface ; its molecules, as Lord 

 Rayleigh has recently explained in this room, attract one 

 another, and it exhibits surface-tension. Raise the tem- 

 perature, and the meniscus grows flatter ; raise it further, 

 and it is nearly flat, and almost invisible ; at the critical 

 temperature it disappears, having first become quite flat. 

 Surface-tension, therefore, disappears at the critical point. 



