Nov. 2 1, 1878] 



NATURE: 



63 



veen certain aeriform fluids. He it was who first spoke of Gas 



vestre, formed by the combustion of charcoal, and given off 



■ring the fermentation of beer. To him, also, we owe the dis- 



:-:Ction — which kept its ground for two centuries — between 



ises and vapours. He regarded gases as aeriform fluids, inca- 



.ble of reduction to the liquid state by cooling, whereas vapours 



: c juire the aid of heat to maintain them in the gaseous state. 



\\\ important difference of constitution seemed, therefore, to 



jexist between these two kinds of aeriform fluid. This difference, 



however, is not fundamental, and the distinction between gases 



and vapours has disappeared, in a theoretical point of view, 



Leiiig, in fact, reduced to a simple question of temperature and 



pressure. 



On March 13, 1823, Faraday, then a young "man engaged 

 z.% chemical assistant at the Royal Institution, read before the 

 Royal Society a note entitled "On Fluid Chlorine." He had 

 i ucceeded in condensing this gas to a liquid by a process which 

 has become classical. This process consists in heating in a 

 closed vessel placed in a water-bath crystals of chlorine hydrate. 

 This compound, very rich in chlorine, is resolved at a gentle 

 heat into chlorine and liquid water, the quantity of which is not 

 sufficient to dissolve the whole of the chlorine. The latter is 

 iherefore disengaged in great part in the state of gas, which 

 accumulates in the small space remaining to it, and is liquefied 

 by the pressure which it exerts upon itself. 



On the same day Sir Humphry Da\'y read a note " On the 

 Liquefaction of Hydrochloric Acid Gas," which he effected 

 by decomposing sal-ammoniac with sulphuric acid in a closed 

 vessel, The.-:e researches were completed by Faraday, who, 

 on April 10 of the same year, described the liquefaction of a 

 large number of gases, directing his efforts, by Davy's advice, 

 chiefly to those which are dense, or very soluble in water, 

 such as sulphurous acid, ammonia, sulphuretted hydrogen, 

 carbonic acid, and protoxide of nitrogen. 



To enumerate the special processes adopted in each particular 

 case would occupy too much time. We shall therefore merely 

 observe that the chief, if not the only, means of condensation 

 -adopted in these experiments was compression, that is to say, 

 the reduction of the gas to a small volume, and that this com- 

 pression was exerted by the gas upon itself, as it accumulated in 

 the very strong sealed glass tubes in which it was disengaged. 

 Sir Humphry Davy, in the note above cited, had remarked 

 that pressure appeared to be a more efficacious method of con- 

 xiensation than cooling, inasmuch as a double pressure reduces 

 the volume of the gas to one-half, whereas a depression of tem- 

 perature of 1° F. reduces the volume by only -^-;^, the lowering 

 of temperature, moreover, soon attaining an impassable limit. 

 It must, however, be especially observed that, even in his first 

 experiments, Faraday made use of differences of temperature, if 

 not to liquefy the gases, at all events to distil and isolate the 

 liquids. Thus it was in the case of chlorine, for example, and 

 in that of ammonia, which he liquefied by heating ammoniacal 

 -silver chloride in a bent tube sealed at both ends, the liquid 

 -ammonia then distilling over and collecting in the empty branch 

 of the tube, which was cooled to a low temperature. 



Similar phenomena will be exhibited in the experiment which 

 I am about to show you, consisting in the liquefaction of 

 cyanogen gas by heating cyanide of mercury in a small glass 

 tube terminated by a long capillary tube bent in the form of the 

 -letter U, The figure of this curved portion will be projected on 

 a screen by the electric light, and in a few seconds you will see 

 the liquid cyanogen collect in the bend. 



Before leaving this part of my subject, I would recall to your 

 attention two of Faraday's discoveries resulting from the appli- 

 cation of the principles just explained. Having compressed coal- 

 .gas to twenty-fivi atmospheres, Faraday in 1825 discovered two 

 important bodies, namely, butylene, a compound of great 

 importance in a theoretical point of view — and benzene — 

 so named by Mitscherlich several years afterwards — which in 

 our own time has become the object of numerous and important 

 applications, and the pivot of an entire department of chemistry. 



Another instance is afforded by sulphurous acid gas 

 4S0j), which was liquefied by Bussy in 1824, at the ordinary 

 atmospheric pressure, by the effect of a cold of 12° to 15° below 

 zero. 



\Vhether we condense gases by pressure or reduce them to the 

 liquid state by diminution of temperature, the result of either 

 method is to bring their particles closer together. It would seem 

 tlien, in accordance with Davy's view, that pressure ought to be 

 more efficacious, as a- means of condensation, than cooling. 



Nevertheless it is not so. The mere approximation of the 

 particles of certain gases does not suffice to effect their liquefac- 

 tion, and moreover, the distances between the particles cannot 

 be diminished indefinitely by pressure alone, M, Natterer, of 

 Vienna, has compressed oxygen, hydrogen, and nitrogen to 3,000 

 atmospheres without effecting their liquefaction. These gases, 

 hitherto called permanent, cannot be liquefied by pressure alone, 

 and their liquefaction, which has quite recently been effected, is 

 the joint effect of strong pressure and a great degree of cold. 

 This is the important point, and I request your permission to 

 offer in this place a few explanations which will serve to place it 



. in its true light. 



j The impossibility of liquefying certain gases by "pressure alone 



: is in accordance with the ideas which are current at the present 

 day respecting the nature of aeriform fluids, and likewise with a 

 discovery made in England within the last few years, on the 



! continuity of the gaseous and liquid states. I will explain my- 



I self briefly on these two points. 



! Daniel Bernouilli first enunciated the idea that gases are formed 



j of material particles, free in space, and animated by very rapid 

 rectilinear movements, and that the tension of elastic fluids 

 results from the shock of their particles against the sides of the 

 containing vessel^. Such is the origin of the kinetic 

 theory of gases, which has been revived since 1824 by 



i Herapath, Joule, and Kronig, and developed chiefly by Clausitis 

 and Clerk Maxwell. 



The law of Boyle and of Mariotte follows as a natural con- 



; sequence of this idea. [Suppose a gas occupying a certain 

 volume, and composed of a definite number of material particles 



I — or molecules so-called — to be contained in a closed vessel, 

 such as the cylinder of an air-pimap ; the pressure on the piston 

 will be determined by the number of shocks of the molecules 

 diffused through the neighbouring stratum of gas. If, then, the 

 volume of the gas be reduced, the number of particles in this 

 layer will be increased, as well as the sum of the shocks, and 

 the pressure will be increased in proportion thereto. 



The velocities with which these molecules move are enormous. 



i Clausius supposes that the molecules of air move with a mean 



i velocity of 485 metres per second, and those of hydrogen with a 

 mean velocity of 1844 metres per second, I say mean velocity, 

 for all the particles of a gas do not move at the same rate. But 

 can the particles freely traverse these wide spaces? By no 

 means ; their number is so immense, that at every instant they 



j enter into collision with one another, and rebound in such a 

 manner that their motion is altered bctii in velocity and in direc- 

 tion. It follows, therefore, that the molecules of a gaseous 

 mass are continually moving in all directions with variable velo- 

 cities, their motion in the intervals between the collisions being 



\ sensibly rectilinear. The distribution of the velocities has been 

 made the subject of important researches by Clerk Maxwell. 



These movements of gaseous molecules determine a very 

 important physical condition, namely, temperature. In 

 fact,' the energy of the rectilinear movements, that is to 

 say, the mass of the gaseous molecules multiplied by the 

 square of their velocity, gives the measmre of the tempe- 

 rature, which consequently increases proportionally to the 

 energy of the rectilinear movement, or, for the same gas — since 

 the masses remain constant — it increases as the square of the 

 velocity. If the velocity were reduced to nothing, the calorific 

 motion would be annihilated, that is to say, the gas would be 

 entirely deprived of heat. This state corresponds with the 

 absolute zero. 



The gaseous molecules moving in all directions and coming 

 into colhsion with one another m space, are very nearly emanci- 

 pated from cohesion. Nevertheless this attractive force makes 

 itself felt for the infinitely short time during which the molecules 

 actually touch one another, or are on the point of doing so. 

 This influence of cohesion is one of the causes of deviation from 

 the law of Boyle or of Mariotte, 



In liquids the influence of cohesion is manifest, preventing 

 the molecules from separating, though it allows them to glide 

 one over the other. This molecular cohesion, or attraction, is in 

 continual strife with the force of expansion, or kinetic energy, 

 which, if unopposed, would launch the molecules into space. 



To understand the antagonism between these two forces, con- 

 sider for a moment a saturated vapour in contact with the liquid 

 from which it has been formed. When it is reduced to a smaller 

 volume, a certain number of its molecules are brought within the 

 sphere of action of cohesion ; they are consequently aggregated 

 together and precipitated in the liquid state, while the rest, being 



