C. \KTSII KKKIK 



GAS AND OASES 



93 



Ciartslirrrh'. See COATHI:II>I;K. 

 lias and <^as>s. Ua-. ;i term applied by Von 

 Helmoni ( ).">77 1044) to vapour not \et shown 

 in In- conden.-ahlc, uiiil poMluly suggested ly the 

 |)iilfli '/".v/, spirit,' 'ghoul.' It now signifies 

 either ( 1 ) a vaporous substance not condensed into 

 a liquid at ordinary terrestrial temperatures ami 

 |>re-Mire>. or ( _' I one which at ordinary temperatures 

 i> not condensable into a liquid by pressure alone. 

 In both tlie^e seu-rs, air under ordinary atmospheric 

 conditions i> a gas ; when cold enough it is not a 

 gas liut a vapour, and pressure alone can then con- 

 den-e it. Sulphurous acid gas is ordinarily gaseous, 

 1'iit it is a ' vapour' because pre>sure alone will con- 

 dense it at ordinary temperatures. Above 30-92 C. 

 '>7 F. ) carbonic acid is a true gas; no pressure 

 will then liquefy it ; but at 30-92 C. a pressure of 

 77 atmospheres, and lielow 30'92 C. progressively 

 .-mailer pressures will condense it; at and below 

 that temperature ( Andrews's Critical Temperature) 

 gaseous carbonic acid is a ' vapour,' condensable by 

 pressure alone. Saturated steam is, in the same 

 sense, a permanent "as at all temperatures above 

 , 'J<M J C. ; it cannot be liquefied by pressure unless 

 it- temperature be below that limit. The critical 

 temperature for hydrogen is - 240-4 C. ; but the 

 lowest temperature that has been actually pro- 

 duced (by the evaporation of liquid oxygen into a 

 vacuum) is- 223 C. ( Wroblewski ) ; hydrogen alone 

 among gases has not yet l>een condensed. It was 

 believed that Messrs Cailletet of Paris and Raoul 

 Pictet of Geneva had, in 1877, succeeded in con- 

 densing hydrogen as well as all the other gases 

 then believed to be non-condensable ; but as to 

 hydrogen this is now considered doubtful. Hydro- 

 gen conducts itself under varying pressures and 

 temperatures in such a way as to show that, if it 

 could be exposed to - 240;4 C., 13-3 atmospheres' 

 pressure would condense it (Wroblewski). 



Gases have small densities : hydrogen has, com- 

 pared with water, a density, at C. and 760 mm. 

 barometric pressure (32 F. and 29-922 in.), of 

 0-6000895682, and air a density of '0012932. 

 Taking hydrogen as a standard, oxygen is very 

 nearly 16 times, nitrogen 14, air 14'47, carbonic 

 acid 22 times as heavy. 



Gases have no free surface-boundary, but occupy 

 any space within which they may be confined. Tne 

 smaller the space within which a given quantity of 

 gas is confined, the greater is the expansive pressure 

 which it exerts on the walls of the containing 

 vessel ; approximately, for a given quantity at a 

 given temperature, the pressure varies inversely as 

 the volume (Boyle's Law, Mariotte's Law), or the 

 pressure multiplied by the volume gives a constant 

 product : j> = c. This law is fairly well obeyed 

 by such gases as air ; but in all gases, other than 



(hydrogen, it is observed that there is with pro- 

 gressively increasing pressures a fall in the value of 

 the product p v, which attains a minimum and then 

 rim ; and even with hydrogen the apparent excep- 

 tion has been removed by the labours of Wroblewski, 

 who found that at very low temperatures the same 

 phenomena were observed in that gas ; and that, in 

 general, if we draw curves representing, for a series 

 of gases, the respective pressures at which the 

 minimal values of p v occur at various temperatures, 

 then if our diagrams are so plotted out as to re- 

 present the respective temperatures and pressures 

 in terms not of degrees or millimetres, but as 

 multiples of the critical temperature (measured 

 troin - 273 C. as absolute zero) and of the corre- 

 sponding critical pressure of each gas, the curves 

 are, for all gases, the same. Under circumstances 

 which are similar with respect to the critical 

 temperature and pressure, therefore, all gases 

 behave similarly in this respect ; and hydrogen acts 

 at - 183 C. (the temperature of boiling oxygen), 



but not at - 103-5 C. (the temperature of boiling 

 ethyh-ne), like air and other gases at ordinary 

 terrestrial temperatures. Carbonic acid gas, in 

 order in art like li\drogeri at - 103'5 C., niUKt be 

 at a temperature of about 1287" C. ; both are then 

 at a tempera! ure about five time* their respective 

 critical temperatures, measured from absolute zero. 

 When the temperature of a given quantity of gas 

 is altered, the product/? v is altered so ax, to a fina 

 approximation, to lie proportional to the almolute 

 temperature (- 273 C. =0 Ah*.). There are, 

 however, some abnormalities : keep the pressure 

 constant and let the volume increase, and we have 

 a certain coefficient of expansion under constant 

 pressure, which is approximately ,$* of the bulk at 

 C. for each ( '. degree of increase in temperature ; 

 keep the volume constant and let the pressure in- 

 crease, and we have a coefficient of increase in ex- 

 pansive pressure, which ought to be the same and is 

 very nearly the same as the previous coefficient ; but 

 not exactly so. The former coefficient is, except 

 in hydrogen, a very little larger than the latter ; in 

 the readfly condensable gases the product p v rises 

 more rapidly than the absolute temperature ; and 

 with progressively ascending pressures, the rate 

 of increase of p v itself rises more markedly in 

 the easily condensable gases than in air. These 



Ehenoniena indicate the existence of inter-molecu- 

 ir forces between the particles of a gas, which 

 manifest themselves the more clearly the nearer 

 is the approach towards liquefaction ; when the 

 liquid state has been reached there is cohesion 

 within the liquid. That gases are compressible by 

 increase of pressure above the atmospheric, as well 

 as dilatable by diminution of pressure, follows from 

 what has been said ; if the pressure be doubled the 

 volume will be halved, and vice versft. When gases 

 are compressed, work is done upon them, and the 

 compressed gas tends to expand ; when the preasure 

 is wnolly or partly relieved, the gas expand- ami 

 does work, as in the air-gun or in compressed-air 

 machines. The pressure at all points in the same 

 horizontal level is, or soon becomes, the same ; 

 whence, if pressure be applied to one part of a mass 

 of gas, the pressure is soon transmitted throughout 

 the whole, and thus energy may l>e conveyed, even 

 to considerable distances. The restitution-pressure 

 tending to cause expansion is equal to the external 

 pressure applied, and the coefficient of elasticity 

 is at all temperatures, provided there is no change 

 of temperature during the compression, numerically 

 equal to the pressure ; while if the compression 

 could be so conducted as to allow absolutely MO 

 heat to escape, the elasticity, in air, would be 

 numerically 1 406 times as great as the pressure. 

 Through this elasticity of gases, local displace- 

 ments set up wave-motions, which, mostly in air, 

 are the usual cause of sound. The speed of pro- 

 pagation of such waves (unhampered by boundary 

 walls) is equal to the square root of the quotient of 

 the coefficient of elasticity divided by the density; 

 and thus the velocity of sound is, within the same 

 gas, independent of the pressure (for the pressure 

 and the density are directly proportional to one 

 another). It is, however, directly proportional to 

 the square root of the absolute temperature. 



According to Dal ton's Law, when a number 

 of gases are mixed, each exerts its own pressure 

 according to the quantity in which it is present ; 

 this law is the less perfectly obeyed the nearer 

 the gases are to their condensing temperatures, 

 and the greater their mutual solvent action. 

 When a gas is greatly rarefied, a small mass 

 holds possession or a relatively great space ; such 

 a space is called a vacuum, which in fact it 

 is not, for two reasons that the ether of space 

 is not eliminated, and that traces of the gas 

 (one hundred-millionth of an atmosphere in the 



