LIQUIDS AND GASES. 307 



bly its teinperatiue rises so liigli that 1 can set a })iece of tinder on 

 tire and by its help explode a little gunpowder. If the ether at its 

 critical point be compressed by screwing^ in the screw, it is somewhat 

 warmed and the blue cloud disapi)ears. Conversely, if it is expanded 

 a little by unscrewing- the seicw 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 log on the screen. 



I "wish also to direct your attention to wluit ha|)pens if the vohime 

 given to the ethtn- is greater than the critical volume — on increasing 

 the volume you see that it boils away and evaporates completely; and 

 also what hajipens if the vobime be somewhat less than the critical 

 volume — it then expainls as ]i(piid and completely tills the tube. It is 

 only at the critical volume and temperature that the ether exists in the 

 state of bbie ch)Tnl, and has its critical ])ressure. If the volume be 

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

 pressure is higher than the critical pressure. 



Still one more x^oii't betbre we dismiss this experiment. At a tem- 

 perature some degrees below the critical temperature, the meniscus, 

 /. c, the surface of the liquid, is curved. It has a skin on its surface; 

 its molecules, as Lord Kayleigli has recently explained in this room, 

 attract one another, and it exliibits surface tension. IJaise the tem- 

 perature and tlie meniscus grows liatter; raise it further, aud it is 

 nearly flat and almost invisible; atihe (-ritical temperature it disap- 

 pears, having first become quite tlat. Surface tensicm therefore dis- 

 appears at the critical i)oint. A liipiid would no longer rise in a nar- 

 rowcapillary tube; it would stand at the same level outside and inside. 



It A\ as suggested by I'rof. Janu's Thomson, ami by Prof. Clausius 

 about the same time, that il'lhe i(l<'al state of things weie to exist, the 

 ]»assage from the liqnid to the gaseous state shcmld be a conlinuoas 

 one, not nuMcly at ami above the critical i>oint, but below that t<'mper- 

 ature. And it was suggested that the curves, shown in the (igure, in- 

 stead of breaking into the straight line of \apor ])iessure, should con- 

 tinue sinuously. Let us see what this conce})tion would involve. 



On decreasing the volume of a gas, it should not liquefy at the point 

 marked B on the diagram (Fig. 2), but should still decrease in volume 

 on increase of pressure. This decrease should continue until the pohit 

 E is reached. The anomalous state of matters should tiien occur, that a 

 decrease in vohime should be accompanied by a decrease of pressure. 

 In order to lessen volume, the gas must be exposed to a continually di- 

 minishing ]uessure. P>ut such a comlition of matter is of its nature 

 unstable, an<l Inis never been realized. After volume has been de- 

 creased to a certain point, F, decrease of volume is again attended by 

 iiH-rease of pressure, ami the last i)art of the curve is continu<ms with 

 the realizable curve re])r(^seuting the compression of the li(iuid, above D. 



J)r. Sydney Young and I succeeded, by a method which I shall 



