1891.] on Liquids and Gases. 369 



sharp angle, where all gaseous water is condensed ; and again a very 

 steep curve, almost a straight line, representing the slight decrease of 

 volume of water, produced by a great increase of pressure. And 

 should we have similar lines for 120^ 130°, 140^ 150°, and for all 

 temperatures — 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 relations between the 

 pressure and volumes of carbon dioxide, at pressures much higher 

 than those I have shown you for water. But I prefer to speak to you 

 about similar results obtained by Professor Sydney Young and myself 

 with ether, because Dr. Andrews was unable to work with carbon 

 dioxide free from air, and that influenced 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 1 part of air in 5U0 parts of 

 carbon dioxide ; also, the condensation of gas was not perfect, for 

 he obtained curves at the points of change from a mixture of liquid 

 and gas to liquid. We, however, were more easily able to fill a tube 

 with ether, free from air, and you will notice that the points I have 

 referred to are angles, not curves. 



Let me first direct your attention to the shapes of the curves in 

 the figure, which represents such relations of volume, temperature, 

 and pressure in the case of ether. As the temperature rises, the 

 vapour-pressure lines lie at higher and higher pressures, and the 

 lines tliemselves become shorter and shorter. And finally, at the 

 temperature 31° for carbon dioxide, and at 125° for ether, there 

 ceases to be a horizontal portion at all ; or, rather, the curve 

 touches the horizontal at one point in its course. That point 

 corresponds to a definite temperature, 195° for ether; to a definite 

 pressure, 27 metres of mercury, or 35*6 atmospheres ; and to a 

 definite volume, 4 • 06 cubic centimetres per gram of ether. At that 

 point the ether is not liquid, and it is not gas ; it is a homogeneous 

 substance. At that temperature ether has the appearance of a blue 

 mist. The strias mentioned by Dr. Andrews and by other observers 

 are the result of unequal heating, one portion of the substance being 

 liquid and another gas. You see the appearance of this state on the 

 screen. 



When a gas is compressed, it is heated. Work is done on the 

 gas, and its temperature rises. If I compress the air in this syringe 

 forcibly, its temperature rises so high that I can set a j)iece of tinder 

 on fire, and by its help 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 



