40 



NATURE 



[May 9, 1878 



In the mutual inter-diffusion without change of temperature of 

 two gases of different densities through a fixed porous diaphragm, 

 although no energy is withdrawn from or communicated to either 

 of the gases, the rate at which single molecules take their equal 

 measures of gas volume through the partition being very different, 

 like their velocities, the measure of gas-volume which accumulates 

 on the side of the denser gas soon raises the pressure there, in- 

 creasing the intensity of the mechanical tendency 3/ z^ on that 

 side of the partition, while the same kind of mechanical tension 

 diminishes on the other, and the t«mperature on each side of the 

 partition is at the same time unaffected . As the porous diaphragm 

 by its immobility (which prevents one of the sets of molecules 

 from doing any work upon the other) resists the resulting force 

 upon it, its counter-tendency is entirely derived from the increase 

 of its own external virial, which has sprung up (at no expense of 

 work, supposing the_diaphragm to be perfectly rigid) in main- 

 taining everywhere in spite of their impacts the common tem- 

 perature or mean energy of bath sets of molecules. Were the 

 diaphragm away, it is evident that the rapid flow of rare gas- 

 volume across the confines between the two gases towards the 

 denser side would cause the centre of mass of the gas-layer, in 

 which the mixture begins, to move bodily away from the denser 

 gas, just as the diaphragm would do if it were free to move ; and 

 the bodily motion so given to the medial gas-layer will, as a form 

 of external radiality in the layer, arising from the heterogeneity, 

 require, in order to be constantly neutralised in the whole body 

 of the mixing gases, such a redistribution of their temperature 

 and density to be taking place at every instant throughout the 

 two bodies of gas placed in communication, that their centre of 

 mass as a connected (but otherwise isolated) system may never 

 undergo any change of place during the mutual diffusion. The 

 space originally occupied by the rarer gas will accordingly become 

 the hotter, and that by the denser the colder portion of the whole 

 volume which the gases continue to occupy when they are mixed. 

 It is in the same way that we can explain the action of re- 

 generators in such air-engines as Stirling's and Ericson's, in 

 passing through which gases change their temperature and 

 volume (and therefore their tendency or "radiality," ^ - 3 pv), at 

 constant pressure, the counter-tendency being at the same time 

 lodged or relaxed during the process in the regenerator, where it 

 must be kept by non-conduction in the tense state (of actual heat- 

 energy and virial combined) of "radiality "^corresponding to the 

 similar "heat- tension" of the gas by which its heat- energy exchanges 

 are secuied. The prpperty of the usual non-conductivity required 

 in the regenerator, is one of indifference of the molecules of a 

 substance to the radiality (or to the sum of the sensible heat and 

 the virial), of neighbouring molecules, or in which different values 

 of the quantity E -^pvoi small neighbouring parts of the sub- 

 stance equalise themselves with difficulty through the mass. But 

 perhaps it is not the inter- but only the jw/rc'-molecular forces that 

 furnish the radiality (and "virial") that determines the trans- 

 mission of heat? If the former forces balance each other, which 

 they do when the body is not vibrating by its elasticity, the 

 virial of the intro-molecular forces only, together with the vis 

 viz0'^fi\z.y be "conservative" with regard to heat-energy, and 

 may be employed in its transmission? Since radiancy of momentum 

 is not heat-energy, we see that this natural effect of force radiality, 

 or of virial and vis viva combined, can only be converted finally 

 into actual heat-energy by some mechanism peculiar to the 

 molecular structure of the solid and liquid bodies in which the 

 heat-energy transmission takes place. Some kind of heat-engine 

 apparently effects this process, for example, at the confines 

 between the vapour and the liquid, when steam is condensed into 

 water, but it is certainly a non-reversible one when the water- 

 spray is colder than the steam which it condenses ; and in the 

 conduction of heat by solid bodies the process is also a non- 

 reversible one ; we only know the part which sensible heat, as 

 temperature, or vis viva, acts in promoting heat conduction ; and 

 the virial by which it is perhaps also carried on, and which with 

 vis viva conserves radiancy of momentum, may also be a fellow- 

 regulator of the operation of which we have no certain knowledge, 

 and over which we certainly have no direct control. But that 

 it should invariably tend to lower the availability of heat, by 

 heat conduction among the comparatively fixed molecules of 

 liquid and solid bodies, will not, perhaps, when the internal 

 motions of molecules are better understood, be more difficult to 

 demonstrate from some theory of Its action, than that it should 

 sometimes serve to raise the availability of thermal energy by 

 its action on heterogeneous gas masses. A. S._Herschel 



College of Science, Newcastle-on-Tyne 

 P.S. — Maturer reflection, since the first impression of 



surprise which Mr. S. Tolver Preston's announcement of 

 the new experiment to which it relates caused me to ex- 

 press at some length, and perhaps unguardedly, in this letter 

 has shown me that the properties of the virial, easily as they 

 may conduct to some Important results, do not. In this case, 

 supply a complete solution of the problem of the final state of 

 two gas masses at the same pressure but of two dissimilar densi- 

 ties on temperatures left to diffuse into each other in a confined 

 space. The equal pressure on all parts of the Inclosure pre- 

 determines the fixity of the centre of mass during the process, 

 and consequently an unequal distribution of density, and there- 

 fore of temperature finally, when the mixture is complete ; but 

 the equation of the virial or the principle of conservation of the 

 radiancy of momentum supplies no certain Information what 

 must be the law of this final distribvition, one of Its terms, 3/z', 

 or the virial of the Inclosing pressiure, being capable of under- 

 going unknown variations during the progress of the diffusion ; 

 and although the stationary condition of the mass at last implies 

 that this term will not be permanently changed, yet both its 

 value and that of the system's total moment of inertia round its 

 centre of gravity (the acceleration of whose magnitude is the 

 rate of change of the radiancy of momentum) may vary in the 

 interval, with the result of leaving the latter moment of inertia 

 permanently altered to an extent and in a way which cannot 

 be defined. When In the simple case of a perfect gas the con- 

 dition of the virial fails to afford positive information regarding, 

 the law of conduction and transference, or of rest and repose of. 

 heat in them under various disti-ibutlons of temperature, it can 

 hardly be expected that the same principle will furnish useful^ 

 and definite results regarding heat-transmission through solids, 

 and other kinds of bodies of which the modes of molecular 

 aggregation are almost totally unknown. A. S. H. 



Time and Longitude 



There is an old and instructive problem which I have lately 

 propounded to several people, and have been struck by the great 

 variety of answers given to it. 



Although we often lose sight of the fact, it is nevertheless 

 true that any given day or year does not begin all over the 

 world at the same moment, but, commencing first at some point 

 in the east, it travels round westward with the sun, so that two 

 different years are often coexistent at the same moment, and it 

 is easily possible for two events to occm- a few hours apart, and 

 yet that which happened first to occur in 1878, and the later* 

 event In 1877. In the same way each day of the week starts 

 somewhere to the eastward of us and dies somewhere in the 

 west. Taking, then, any given day of the week as Monday, 

 the problem is — When and where did last Monday first com- 

 mence, where did it end, and how long did it exist ? Or, to 

 put a similar question, Where did the year 1878 first commence, 

 and at what Greenwich time ? 



I will simply state my belief that last Monday commenced iii 

 New Zealand somewhere about noon on Sunday, but not at 

 noon, its commencement at that time and place being In no 

 way connected with its position as our antipodes, but being a 

 mere accident of civilisation. If the whole northern hemisphere 

 should become civilised and inhabited, the day would then 

 almost certainly commence at Behring's Straits, and would last 

 forty-eight hours. A person crossing Behring's Straits east or 

 west would gain or lose a whole day just as he now does by 

 sailing round the globe ; so that he might easily cross over arid 

 spend a few hours of to-morrow with his friends and return in 

 time for dinner, or might enjoy the New Year's Eve on two 

 successive days. 



If the Pacific Ocean became Inhabited land, a merldiaii 

 would have to be chosen as a starting point for the day, and a 

 person stepping across this imaginary line would gain or lose a 

 day. At the same moment that Sunday morning was com- 

 mencing on the one side of this line, Monday morning would 

 be commencing on the other, and there would be con- 

 stantly two different days going on side by side with twenty-fouf 

 hours' difference of time between them, though only a few yards 

 apart. It would be possible for a person standing astride this 

 line to have for an instant one foot in Monday morning, the 

 other foot In Monday night, and his body in the previous 

 Sunday. 



I purposely avoid giving any reasons, and do not assert that 

 air my views are correct, but I throw out the problem as an 

 amusinT one for aroTiment and discussion, as it abounds In 



