150 



KNOWLEDGE. 



[July 2, 1894. 



readily changed into a fluid and then into a vaporous state. 

 Perhaps the best known of all is water, though even tliis 

 in hot climates is rarely seen in the there uncommon solid 

 form ; so that the African chief, who had accepted many 

 statements from his European visitor, utterly refused to 

 believe him when he was told that, owing to the intense 

 cold in some countries, rivers got hard enough to be walked 

 over. 



Lately, in many text-books of science, gases were divided 

 into two classes — those which could be liquefied by the 

 application of cold, or pressure, or both, and those which 

 TfieTe permanent, or were known to us only in the form of 

 gases. It was suspected by scientific men, especially after 

 the extensive experiments by Faraday, who succeeded in 

 liquefying many of the "permanent" gases, that all gases 

 without exception could be changed into the liquid form 

 if they were exposed to a sufficiently great pressure and at 

 the same time cooled down far enough. This surmise has 

 been proved correct only within comparatively late years, 

 and now all gases, including the air we breathe, must be 

 considered as differing from vapours, such as steam, only 

 because at the usual temperatures at which we generally 

 deal with them they are in a condition similar to that 

 which other substances attain when heated to a very high 

 temperature. 



I propose to give a short account of the work hitherto 

 done in the liquefaction of gases, commencing with that 

 so ably carried out by that unsurpassed experimenter 

 Michael Faraday, and going on to refer to the researches 

 commenced by Eaoul Pictet and by Caillelet in France, 

 who, about the same time (1877), succeeded in liquefying 

 oxygen and even hydrogen. These researches were pur- 

 sued by Wroblewski and Olzewski in Russia, and have 

 been continued lately in our own country by Prof. Pewar, 

 who, with such striking success, has liquefied air in large 

 quantities, and has even handed it about in pint bottles 

 for inspection by a large audience. 



There are two means open to us of liquefying a vapour. 

 Let us increase the pressure upon it, or lower its tempera- 

 ture, and if we proceed far enough in these operations the 

 vapour will become liquid. A further condition is neces- 

 sary with the " permanent " gases — we must cool them 

 down below their critical temperature. This critical tem- 

 perature is that above which no amount of pressure applied 

 to the gas will be capable of changing its state into that 

 of a liquid. 



Faraday, in his series of experiments, applied the simple 

 but effective means of generating the gas, in a strong glass 

 tube, from those compounds which evolved the required 

 gas on heating. In this way the gas, being produced in 

 a limited space, produced a great pressure, under which 

 pressure of its own vapour the gas became a liquid. On 

 breaking the tube the gas, compelled by pressure to exist 

 as a liquid, would revert to the form natural to it at the 

 temperature of the experiment, and would do this with 

 explosive violence. There was thus always a tendency for 

 the gas to burst the tube, and this sometimes occurred 

 during Faraday's experiments. Hence he was careful to wear 

 a mask made from wire gauze or thick glass, but even thus 

 he did not entirely escape injury. Faraday immersed one 

 end of his tube in a freezing mixture while the other was 

 exposed to heat. In this way he succeeded in liquefying a 

 large number of gases, and examined their properties while 

 in this unusual state. Amongst the gases so treated were 

 the following : — carbonic acid, hydrochloric acid, sulphur 

 dioxide, cyanogen, ammonia, and chlorine. He carried 

 out his first set of experiments on this subject m the year 

 1823. Later, in the year 1845, after Thilorier had shown 

 bow carbonic acid gas could be obtained in the liquid form 



on a large scale, and also as a solid, Faraday used the 

 solid carbonic acid mixed with ether, and by means of 

 the cold produced by the evaporation of the mixture 

 he reduced the temperature to about —100° Cent., and 

 obtained most of the above-mentioned gases, and also 

 nitrous oxide, not only in a liquid but also Ln a solid 

 state. Hydrochloric acid, hydrogen arsenide, ethylene, 

 silicon fluoride, boron fluoride, and chlorine he at this time 

 managed to get in the liquid form, but was unable to 

 solidity these substances. The gases hydrogen, oxygen, 

 nitrogen, nitric acid, carbon monoxide, and marsh gas 

 resisted all Faraday's attempts to liquefy them, and it was 

 not until more than thirty years later that these substances 

 were condensed. 



Andrews, by his classical researches on the critical point 

 of gases and vapours, and especially by his thorough 

 investigation of the behaviour of carbon dioxide when 

 exposed to great pressure at varying temperatures, paved 

 the way for future work on the liquefaction of gases, and 

 showed that great pressure of itself was not sufficient to 

 cause a gas to turn into a liquid, but that a certain 

 limiting temperature must be passed in cooling down the 

 gas before it can by any amount of pressure be liquefied. 

 Thus above this limiting or critical temperature, which is 

 different for each gas, it may be called truly permanent, 

 while below this temperature the gas is liquefiable if only 

 enough pressure is applied to it, and the gas may then be 

 described as a vapour. 



When substances evaporate or change from the liquid to 

 the gaseous state, a large amount of heat disappears or 

 becomes latent, and is used up in separating the molecules 

 of the liquid farther apart and giving them increased 

 energy of motion. On account of this disappearance 

 of heat during evaporation great degrees of cold may 

 be produced, and it was by this means that Pictet in 

 1877 obtained a temperature of — 140° Cent., and obtained 

 oxygen in the liquid state. He cooled liquid carbon dioxide 

 by surrounding it with liquid sulphurous acid, kept boiling 

 in a vacuum, and got a still greater degree of cold by then 

 allowing the liquid carbonic acid to evaporate rapidly in 

 an exhausted space. The oxygen was generated in the 

 usual way from potassium chlorate, a salt which splits up 

 and gives off' oxygen gas when it is heated ; but the gas 

 was produced in a strong iron retort, so that by means of 

 its own pressure alone the gas was compressed by a force 

 several hundred times greater than that of the ordinary 

 pressure of the atmosphere. 



The method adopted by M. Pictet is the same in prin- 

 ciple as that employed by Faraday. The gas is generated 

 in a closed vessel capable of standing a great pressure, and 

 it is condensed by being simultaneously exposed to great 

 cold and to the pressure of the gas itself, forced to occupy 

 a very small space. In Pictet's original experiment he 

 obtained a temperature of —130° Cent., at which tempera- 

 ture oxygen is liquefied, when the pressure is raised to 

 two hundred and seventy-three atmospheres. 



While Pictet was continuing his experiments and 

 endeavouring to liquefy the hitherto permanent gases, the 

 same subject was being investigated by Cailletet, and it 

 was on the same day, the 24th December, 1877, that the 

 French Academy was informed of the success of both these 

 experimenters in liquefying oxygen. Cailletet attained 

 his object by exposing the gas to enormous pressure, 

 produced by means of a hydraulic press, while at the same 

 time the temperature was lowered by suddenly allowing 

 the gas to expand. In this way a sudden disappearance of 

 heat takes place, the heat energy becoming transformed 

 into mechanical motion of the particles of the expanding 

 gas. In Cailletet's apparatus the pressure was produced 



