64 



NATURE 



{Nov. 21, 1878 



now diffused through a wider space, continue to move with the 

 same velocity and to exert the same pressure as before. In this 

 case the forci of cohesion of the hquid particles exactly balances 

 the expansive force or kinetic energy, and serves to a certain extent 

 as a measure of its amount. 



Now let the vapour be heated, after it has been withdrawn 

 from the action of the liquid ; its expansive force will then in- 

 crease ; it will dilate, and may then be compressed, imtil, 

 by the approximation of its particles, it is again brought 

 within the sphere of action of the cohesive force, that is to 

 say, to the point of saturation corresponding with the 

 temperature to which it has been raised. With the increase 

 of temperature, the expansive force or kinetic energy of the 

 vapour likewise increases, whereas the cohesion of the liquid 

 becomes less : hence the necessity of further diminishing the 

 distances between the particles by increase of pressm-e. But 

 this double effect of increased kinetic energy of the gaseous 

 molecules, and diminished cohesion of the liquid molecules, 

 going on progressively as the temperature rises, a point will at 

 length be attained at which the energy of the molecular move- 

 ment will finally gain the victory over the force of cohesion, 

 whatever be the pressure to which the vapour is subjected. The 

 minimum temperature at which this effect is produced, and at 

 which, therefore, a vapour can no longer co-exist with its liquid 

 under any pressure whatever, has been called by my friend. Dr. 

 Andrews, the critical point, and by M. Mendelejeff, the 

 absolute boiling point. Above this temperature, what- 

 ever may be the pressure, the gas, whether dilated or com- 

 pressed, will maintain the same physical state, characterised 

 by freedom of molecular or calorific movement. 



I can show you by an experiment this peculiar phenomenon 

 of the sudden passage of a liquid mass to the state of gas, by 

 heating liquid carbonic acid in a closed vessel, just as Cagniard 

 de Latour formerly heated ether. Here is a tube, half filled 

 with liquid carbonic acid, which we are about to immerse in 

 water at 35* ; you observe that the liquid first rises quickly in the 

 tube, its coefficient of expansion being greater than that of gases ; 

 at the same time the meniscus flattens more and more, indicating 

 a diminution of cohesion in the liquid (Andrews), and finally 

 disappears altogether ; in fact the liquid itself has disappeared, 

 having been entirely and suddenly transformed into gas. What 

 now must we do to cause it to reappear? We must lower 

 the temperature, so as to diminish tlae kinetic energy of the 

 gas, and increase the cohesion of the liquid. A moment will 

 then arrive when the cohesive force will again be able to 

 resume the contest, and the liquid will be reconstituted. 



We are now in a position to understand why certain gases, 

 hitherto called permanent^ cannot be liquefied except by the 

 combined action of very strong pressure and a very great degree 

 of cold. The critical points of these gases are situated at very 

 low temperatures. They have quite recently been liquefied, this 

 great discovery having been made by MM. Cailletet and Raoul 

 Pictet. 



The principle of Cailletet's apparatus is the following : — The 

 gas to be liquefied is introduced into a cylindrical glass vessel 

 and transferred by means of mercury to a very strong glass 

 tube sealed into the reservoir. This latter is firmly fixed in a 

 cylindrical cavity hollowed out of a block of iron, and serving 

 as a kind of closed mercurial trough. The cylindrical cavity 

 communicates with a hydraulic press which injects water on to 

 the surface of the mercury, driving it into the gas reservoir, 

 which is ultimately quite filled with that liquid, the gas being 

 thereby driven into the tube, where it is liquefied. 



In this manner we shall be able by a few strokes of the 

 piston of the hydraulic press to liquefy carbonic acid. Other 

 gases less easily condensable may be liquefied in a similar 

 manner, if the tube be cooled to - 20° or - 30°. But these temper- 

 atures do not suffice for the liquefaction of the so-called permanent 

 gases. To cool these gases to lower temperatures, M. Cailletet 

 avails himself of sudden expansion {detcjtte). The gas, com- 

 pressed to several hundreds of atmospheres, when allowed to 

 expand suddenly and drive the air before it, consumes a certain 

 quantity of heat, and is thereby reduced to a kind of mist, which 

 will appear on the screen, and pass away like a cloud, if 

 we suddenly expand the strongly compressed carbonic acid gas, 

 which we have here, in default of oxygen or hydrogen, 



M. Raoul Pictet has succeeded in condensing oxygen and 

 hydrogen in the form of liquids, properly so called, and 

 even in obtaining the latter of these gases in the solid 

 state. To produce this effect, he employs condensing 



apparatus of incomparable power, combining the action 

 of a cold of 120° to 140° below zero with that of enormous 

 pressures amounting to 550 and even 650 atmospheres. The 

 pressure is produced by the accumulation of the gases in a 

 closed space consisting of a long copper tube of very thick metal. 

 The oxygen was produced by heating potassium chlorate in a 

 howitzer shell, having a copper tube soldered into its orifice. 

 The hydrogen was prepared in a similar apparatus, by decompo- 

 sition of a dry mixture of potassium formate and potassium 

 hydrate. 



To produce very low temperatures of 120° or even 140° below 

 zero, M. Pictet resorts to a very ingenious artifice. Over the 

 reservoir-tube which surrounds the copper tube, and in which 

 these low temperatures are intended to be produced, he super- 

 poses another system of concentric tubes, intended to produce a 

 first fall of temperature amounting to - 65°, by the volatilisation of 

 liquid sulphurous acid. By means of this first depression of tem- 

 perature it has been found possible to liquefy carbonic acid gas in 

 the inner tube of the system just mentioned, by a pressure of only 

 a few atmospheres. The carbonic acid thus liquefied being intro- 

 duced into the lower reservoir-tube of the apparatus, produces by 

 its volatilisation, a second fall of temperature round the copper 

 tube containing the compressed oxygen which is to be liquefied. 

 M. Pictet has in fact established a double circulation, one of 

 sulphurous acid, the other of carbonic acid. I will describe the 

 former. Sulphurous acid gas is liquefied by a pressure of three 

 atmospheres and collects in a strong vessel, from which it passes 

 through a tube into the upper reservoir. The pressure is exerted 

 by means of a force-pump. A suction-pump connected with the 

 force-pump, and acting in concert with it withdraws the liquid 

 sulphurous acid from the reservoir-tube, and transfers it to the 

 force-pump, which brings it back to the vessel, and thence to the 

 upper reservoir- tube. 



The circulation of the carbonic acid is established in the same 

 manner, by means of two pumps, one of which condenses the 

 gas by forcing it into tubes cooled to - 61°, while the other, which 

 is a suction-pump, sends it back to the force-pump. The vola- 

 tilisation of the carbonic acid produces round the copper tube 

 the low temperatures above-mentioned. The copper tube is in 

 fact surrounded by solid carbonic acid. 



In this manner M. Pictet has liquefied oxygen, and has 

 approximately calculated its density. He has also liquefied and 

 even solidified hydrogen, which he has seen to issue from the 

 tube in the form of a steel-blue liquid jet, which partly solidi- 

 fied. The solid hydrogen, in falling on the floor, produced the 

 shrill noise of a metallic hail, thus confirming the bold and 

 ingenious idea of Faraday, who first suggested that hydrogen is 

 a metal. 



The experiments of MM. Raoul Pictet and Cailletet have 

 then removed from science the distinction between permanent 

 and condensable gases. Permanent gases exist no longer. All 

 aeriform fluids may be liquefied with a facility greater in pro- 

 portion as their critical points are situated at higher tempera- 

 tures. From a physical point of view, therefore, gases and 

 vapours have the same constitution, being formed of molecules 

 which move freely in space. In what, then, do they differ? 

 They differ by the nature and constitution of these molecules ; 

 and here we enter on the domain of Chemistry. 



It is supposed, in chemistry, that the molecules of each spe- 

 cies of gas or vapour are formed of a definite number of atoms. 

 The simplest molecules, like those of mercury-vapour, are 

 formed of single atoms. Others include several atoms of the 

 same or of different kinds : and these latter molecules may be 

 very complex, that is to say, formed of a large number of atoms 

 held together by affinity, and vibrating in concert in a system to 

 which they are attached, viz., the molecule. In this system, 

 which has a definite form, extent, and centre of gravity, the 

 molecules execute their own proper movements, and are at the 

 same time carried forward with the entire system in the molecular 

 paths. 



I cannot here dilate on the nature and chemical properties of 

 the several gases and vapours. I wish merely to throw light on 

 a single point, which is of great importance, inasmuch as it 

 constitutes one of the foundations of chemical science. 



The proposition which I am about to enunciate is generally 

 adopted by chemists, resting as it does on an imposing array of 

 facts : Equal volumes of gases or vapours, under the same condi- 

 tions of pressure and Jemperature, contain equal numbers of 

 molecules. 



