94 



GAS AND GASES 



best vacua ) are always retained. If two gases be 

 placed at different levels in a vessel, even with the 

 lighter gas uppermost, they will rapidly diffuse into 

 one another, and even if connected only by a long 

 glass tube they will soon mix, and will not there- 

 after separate. This is due to molecular move- 

 ment, and dust-particles are not appreciably trans- 

 ferred ; thus the dust of a closet is not removed, 

 though the air is renewed, by opening the door. 

 If, however, the two gases to be exchanged be of 

 notably different densities, there may be a pressure 

 resulting from the tendency of the lighter gas to 

 pass more rapidly into the heavier than the heavier 

 one travels into it. The rate of mixing by diffusion 

 between two gases is measured by their coefficient 

 of diffusivity, which is to be experimentally found. 

 The significance of this coefficient is that where 

 we, adopting a consistent system of units, say 

 centimetre, gramme, and second, state in the shape 

 of a formula the known laws of gaseous diffusion 

 viz. that ( 1 ) the quantity of matter transferred 

 across any layer is inversely proportional to the 

 thickness of that layer, (2) that it is directly pro- 

 portional to the area exposed, (3) directly pro- 

 portional to the time taken, and also (4) to the 

 difference of densities on either side of the layer 

 we may convert this formal statement of proportions 

 into a numerical identity by inserting the proper 

 numerical factor or coefficient ; thus if M be the 

 number of grammes transferred, ab the area ex- 

 posed in sq. cm., c the thickness of the layer, t the 

 time, and d the difference of densities, M is pro- 



,. , , ab.t.d , ab.t.d 



portional to , or equal to k . , where 



C C 



k is the coefficient of diffusivity. But k becomes 

 a different number when we change our units of 

 length or time ; it varies numerically according to 

 the square of the unit of length, and inversely 

 according to the unit of time adopted, and hence 

 the coefficient of diffusivity is usually stated as 

 being so many square centimetres per second. 

 Some numerical values for this coefficient will be 

 found in Clerk-Maxwell's Theory of Heat (appendix). 



Diffusion in gases has also been measured in 

 another way. Hydrogen separated from the outer 

 air by a plaster-of-Paris plug, escapes into the air 

 about four times as fast as air traverses the plug in 

 order to get into the hydrogen. The law is that 

 the rate of traversing the plug is inversely pro- 

 portional to the square root of the density of the 

 gas ; or, in terms of the kinetic theory of gases, it is 

 directly proportional to the average velocity of the 

 molecules of each gas. The rates at which gases 

 will traverse a single small aperture ( ' effusion ' ) 

 are within the limits of experimental error, in 

 accordance with the same law. The rates at which 

 gases slowly pass under pressure through extremely 

 fine long tubes, or are 'transpired,' have no rela- 

 tion to the diffusion or effusion rates ; the mass of 

 gas passing per second varies as the motive pressure, 

 as the density, and inversely as the length of the 

 tube, aiid also as a coefficient of transpiration 

 special to each gas, and presenting from gas to 

 gas certain coincidences as yet unexplained (see 

 Graham's Collected Works, or Miller's Chemical 

 Physics). The rate is slower the higher the 

 temperature, but is independent of the material of 

 the tube. 



When gases are separated by membranes, in 

 which they are unequally soluble, or for which 

 they have unequal affinities, the diffusion-rates are 

 interfered with and become abnormal e.g. benzol- 

 vapour and air separated by a thin india-rubber 

 membrane ; the benzol traverses, the air does not. 

 Thus also carbonic oxide, an extremely poisonous 

 gas, may traverse red-hot cast-iron, a fact to be kept 

 in mind in reference to overheated stoves. This is 

 due to solution of the gas in the solid, which 



behaves like a liquid film in reference to it. Gases 

 are also condensed on the surface of solids ; every 

 solid object bears a condensed film of air on its 

 surface ; some substances have enormous power of 

 condensation, notably cocoa-nut charcoal (Hunter), 

 which absorbs 170 times its own volume of am- 

 monia, 69 of carbonic acid, 44 of water- vapour. 

 This power is beneficially utilised in charcoal 

 respirators, in which oxygen and oxidisable gases 

 are condensed together and combine ; and in 

 Dobereiner's hydrogen lamp, in which hydrogen 

 plays upon platinum black, and is condensed so 

 rapidly ( perhaps being oxidised at the same time ) 

 that the platinum becomes incandescent and 

 ignites the hydrogen jet. 



The superficial film of air on solids plays a part 

 in friction in air ; a pendulum has the amplitude 

 of its swing slightly diminished by this friction : a 

 waterfall drags air down and is retarded by this 

 frictional action ; and the examples of railway 

 trains and cannon-balls will readily occur. The 

 slide-valve of a steam-engine is pressed upon by the 

 steam, and this gives rise to friction. 



Gases are in many cases soluble in liquids ; some 

 are greatly so (ammonia in water at C. , 1049'6 

 volumes; at 20 C., 654 volumes), some slightly 

 (hydrogen in water at C.,' 0'0193 volume). The 

 general rule is (Henry's Law) that, at any given 

 temperature, the volume of gas dissolved is con- 

 stant at all pressures, so that the quantity of gas 

 dissolved is proportional to the pressure ; and on 

 liberation from pressure some of the gas escapes. 

 This law is interfered with in most cases by the 

 formation of chemical compounds (hydrates) be- 

 tween the water and the gas dissolved. Again, 

 when a mixture of gases is presented to a liquid, 

 the general rule is that each is dissolved in pro- 

 portion to the partial pressure exerted by it, com- 

 bined with its own specific solubility in the liquid : 

 thus the small quantity of air dissolved in water, 

 which subserves the respiration of aquatic life, 

 contains 34 '82 per cent, of oxygen instead of 20 '9 

 per cent., as air does, because oxygen is more 

 soluble in water than nitrogen is. Where, how- 

 ever, the gases have a mutual chemical action, this 

 rule is completely departed from. One effect of 

 the formation of hydrates may be that the gas is 

 not expellable by boiling : hydrochloric acid gas is 

 an example : a certain excess of gas may be driven 

 off by heat, but beyond that the aqueous solution 

 of hydrochloric acid distils over as a whole : am- 

 monia gas or carbonic acid, on the other hand, may 

 be completely driven off from water, any feeble 

 hydrates formed being decomposed. Gases may, 

 it appears, dissolve gases ; oxygen evolved from 

 chlorate of potash may (Schutzenberger) contain 

 chlorine unrecognisable by any chemical test vintil 

 a red heat has been applied ; and it seems that 

 there is no case of evaporation without the vapour 

 carrying off some of the solids dissolved in the 

 evaporating liquid, a phenomenon specially ob- 

 served in the case of boracic acid solutions, and 

 also in the case of coal-gas, which may, especially 

 when rich in the vapour of liquid hydrocarbons, 

 carry much solid naphthaline in a state of invisible 

 suspension approximating to true solution. 



Gases are to a certain extent viscous ; air or 

 steam in motion will drag the surrounding air 

 along with it, and will thereby have its own motion 

 checked. Wave-motion set up in air may travel 

 far, but has at length its energy worn down into 

 heat through the viscosity of the air. Air is at 

 0'6 C. about a hundred times less viscous than 

 water is, and at 90 C. it is only about twelve 

 times less viscous than water at that temperature. 

 The viscosity of any given gas, dynamically 

 measured, does not vary with its density. 



Gases also possess a 'feeble power of conducting 



