204 



NATURE 



{Jan. lo, 1878 



kind, or, more accurately speaking, of the same mass. 

 This latter case is evidently exceptional, and if a case be 

 exceptional the term " law " becomes no longer applicable 

 to it. 



11. The rates of diffusion of hydrogen and oxygen 

 across the porous diaphragm are known to be as four to 

 one, i.e. as the molecular velocities. The above illustra- 

 tion of the spheres may serve to exhibit the physical basis 

 or cause of this fact in a clear light. The mere statement 

 that the rates of diffusion are inversely as the square roots 

 of the molecular weights of the gases, evidently throws 

 no light on the cause or physical basis of the action, 

 which is always the main thing to realise in physical 

 science. The fact that diffusion is in the above ratio to 

 the molecular weight, evidently only happens to be true 

 because the molecular velocity is in that same ratio to the 

 molecular weight, otherwise molecular weight has nothing 

 whatever to do with the rate of diffusion. So it will be 

 equally apparent, from the above illustration, that the rate 

 of diffusion of a gas through a porous diaphragm has 

 nothing whatever to do with the pressure of the gas, but 

 depends, cceteris paribus, on the number of molecules of 

 the gas in unit volume. An increase of the number of 

 molecules in unit volume (by adding to the number of 

 impacts of the molecules against the vessel) increases the 

 pressure, and this is why diffusion appears to be depen- 

 dent on pressure, though evidently physically it has 

 nothing to do with it. This serves to explain how, pro- 

 vided the molecular velocities of the gases are consider- 

 ably diverse, such enormous differences of pressure can 

 take place by diffusion through a porous diaphragm, the 

 pressure having no power whatever to adjust itself 

 through the diaphragm ; for the passage of a molecule 

 through the diaphragm simply depends whether, in its 

 normal motion, it happens to encounter a pore or not. 

 The above illustration may also serve to show that the 

 velocity of propagation of any impulse (" wave ") by a 

 system of bodies in free collision can only be dependent 

 on the normal velocity of the bodies, just as a system of 

 couriers interchanging motion among themselves convey 

 a message at their own rate. So the molecules of a gas 

 interchanging motion among themselves convey an im- 

 pulse at their own rate, and thus the velocity of sound in 

 a gas can be solely dependent on, and proportional to, 

 the velocity of the fnolectiles of the gas, and on nothing 

 else. This must evidently be true On the basis of the 

 kinetic theory, and this theory being now accepted, it 

 would be not unreasonable to expect that in so funda- 

 mental a matter as the propagation of sound, an explana- 

 tion of it on the basis of this theory would be looked to, 

 for a statical theory of the propagation of sound appears 

 scarcely to harmonise with the dynamical theory of gases. 

 We have alluded to this fact as briefly as possible, having 

 the illustration of the spheres at hand. There may be a 

 liability to lose sight of facts like the above unless due 

 care be taken to realise molecular phenomena by picturing 

 them on a larger scale. The velocity of sound in hydro- 

 gen is four times greater than in oxygen, solely because 

 the velocity of the molecules of hydrogen is four times 

 greater than the velocity of the molecules of oxygen — 

 nothing conceivably to do with the molecular weight of 

 the gas, excepting in so far as a less molecular weight 

 determines a higher molecular velocity.^ The rate of pro- 

 pagation of the wave is affected by temperature in so far 

 as the velocity of the molecules of the gas (in whose 

 motion the heat of the gas consists) is affected by 

 temperature. 



12. As an illustration of a simple form of apparatus 

 adapted for converting normal temperature heat into 



» It is evident that though the velocity of the wave is proportional to the 

 velocity of the molecules, the absolutt velocity of the wave must be to a 

 certain fixed degree less than that of the molecules ; for the molecules in 

 their normal motions arc moving more or less obliquely to the path of the 

 wave. This I have pointed out in a paper, published in the Philosophical 

 Magazine for June, 1877, where the true matheraatica! relation for the 

 velocity has been detenmaed by Prof. Maxwell, and is there given. 



work, and admitting of continuous actuation, the following 

 rough sketch may serve : — Let the annexed diagram 

 represent a cylinder containing three pistons, B, D, c, the 



central one, D, of which is furnished with any porous 

 diaphragm (such as of plumbago, or porous earthenware). 

 Let any light gas (hydrogen being the most effective) be 

 supposed introduced into one-half of the cylinder, some 

 heavier gas (or air) filling the other half. All three pistons 

 are supposed (first) fixed. Then, as is known, diffusion 

 commences through the porous diaphragm, everything 

 remaining necessarily at normal temperature so long as 

 the pistons are fixed and no work is done. The rapidly 

 moving molecules of the light gas pass in greater 

 numbers through the pores of the diaphragm than those 

 of the heavy gas (or air), so that the pressure rises in the 

 compartment originally filled with air. As soon as the 

 pressure has attained a maximum, the central piston is 

 automatically released, and is thus driven by the excess 

 of pressure towards the opposite end of the cylinder, the 

 portion of gas whick does the work being chilled and 

 the heat transferred in the form of work to the outside 

 machinery with which the central piston is connected. A 

 certain part of the heat goes to the portion of gas towards 

 which the piston is driven, heat thus passing from a 

 colder to a hotter body (for as soon as the portion of gas 

 commences to be chilled, it is already the colder). Simul- 

 taneously with the stroke of the central piston, a con- 

 venient automatic arrangement connected with the ma- 

 chinery oscillates the two end pistons inwards and 

 outwards, expelling in the inward stroke (through con- 

 venient openings) the diffused mixture of gas and air, 

 and by the outward stroke drawing in a fresh supply. Of 

 course the valves suitable for this are not given, as it is 

 only our purpose to sketch the principle of such an appa- 

 ratus as a scientific point, and having no regard to any 

 question of commercial value or not. Clearly the power 

 derived would depend on the specific gravity of the gas 

 used, and would be proportional {cceteris paribus) to the 

 area of the piston. Coal gas would give a less power 

 than hydrogen. A diffused mixture of gas and air is 

 necessary for gas engines, the mixture being exploded in 

 them. It is clear that it would be possible, by means of 

 an apparatus of the above character, to derive power in 

 the act of mixing the gas and air previous to exploding the 

 mixture. The gaseous mixture, after passing through the 

 apparatus, could be stored in some reservoir or receptacle, 

 so as to recover (before combustion in the gas engine) 

 from surrounding objects the heat which it lost by con- 

 version into work in the diffusion engine. By this pro- 

 cedure it may be observed that the heat converted into 

 work is derived from the normal store of heat possessed 

 by surrounding objects, and their store is finally made 

 good by the sun, which latter may therefore be regarded 

 as the ultimate source of the energy derived. 



13. In view of the numerous porous structures existing 

 in the animal and vegetable world {porosity being a dis- 

 tinguishing characteristic of animal and vegetable or- 

 ganisms), also taking into consideration the prevalence of 

 gases of different molecular weights, notably oxygen and 

 carbonic acid (which are known to be intimately con- 

 nected with animal and vegetable processes) ; the con- 

 clusion would seem warranted, and even necessary, that 

 work on the above principle must take place widely in 

 nature, and thus part of the store of energy accumulated 

 in materials on the earth's surface by the sun, is made to 

 fulfil a useful end, instead of being dissipated uselessly in 

 space. S, ToLVER Preston 



