8o 



NA TURE 



[Mav 24, 1906 



ation of the molecules into high and low velocity groups 

 very nearly perfect. A demon might be slaving with the 

 most commendable energy, but all his exertions would be 

 rendered inoperative by the imperfections of our apparatus. 

 To my mind, the evidence for the second law, even applied 

 to the best actual heat engines, is extremely slight. 



But even if the evidence were overwhelming, there would 

 be no justification for applying the law to a process of such 

 an entirely different nature as osmosis, where, moreover, 

 there is some presumption that it is not true. No actual 

 membrane is perfectly semi-permeable ; some molecules of 

 the solute pass through ; it is not wildly improbable that 

 these molecules possess velocities within some narrow range. 

 But if this is so. Maxwell's demon is at work, the second 

 law is not applicable, and thermodynamic reasoning is 

 absurd. Definite experimental proof must be offered before 

 the validity of the law for osmosis can be considered even 

 probable. Some progress might be made by examining the 

 same membrane at different temperatures; if its "degree 

 of imperfection " did not vary rapidly with the tempera- 

 ture, the existence of such a separation as has been sug- 

 gested would be rendered less probable. 



Mr. VVhetham has offered some proof already. He points 

 out that there are five assumptions involved, and asserts 

 that the truth of all of them is proved by the agreement 

 between theory and experiment. But he ignores the possi- 

 bility that two or more of the assumptions may be incorrect 

 and that the errors thus introduced may cancel each other. 

 He offers a particular solution of an equation containing 

 five variables, and assumes that it is the only solution 

 possible. 



It must be remembered that there is not perfect agree- 

 ment between theory and experiment. The errors are 

 larger than those involved in the direct measurement of 

 the pressure and the other quantities involved : there is a 

 systematic error. But this is due, say the thermo- 

 dynamicists, to the imperfection of the membrane. Exactlv 

 so ; but that imperfection may invalidate the whole proof ; 

 in order to support their proof they may be denying one 

 of their fundamental assumptions. 



Mr. Whetham says that to reject the theory because 

 there is no perfect membrane would be as absurd as to 

 reject all thermodynamics because there is no reversible 

 engine. I agree; but then I am such a heretic that I 

 reject both. Our inability to construct a nerfectlv re- 

 versible engine is connected with the impossibilitv of hand- 

 ling individual molecules ; friction and the rest would 

 vanish if we could replace the material cylinder bv a 

 swarm of trained demons. When we have constructed a 

 perfectly reversible engine we shall be possessed of the 

 powers of those demons, and we shall be no longer bound 

 by the second law, which merely asserts that we do not 

 possess those powers. .So far as physicists are concerned, 

 reversible thermodynamics is " a vain thing." 



Neither am I convinced of the perfection of Mr. 

 Whetham 's two perfect membranes. Thev are doubtless 

 perfect so far as the solute is concerned, but his assump- 

 tion (2) may be violated by the molecules of the solvent. 

 It is quite possible that it is the swifter molecules which 

 escaoe in the vapour and the slower which escape into the 

 solid, and that, if our experimental devices were sufficiently 

 delicate, we could use the separation thus effected to per- 

 form useful work. At any rate, proof is required to the 

 contrary before thermodynamic deductions can be made 

 with accuracy. 



So far as I can see, thermodynamic reasoning aoolied to 

 osmotic phenomena, as to most others, proves nothing but 

 that the sum of the errors introduced by the various 

 rather doubtful assumptions is not very different from zero 

 — a result that does not seem to me worth the labour that 

 has been expended in obtaining it 



Norman R. CAHrcEi^L. 



Trinity College, Cambridge, Mav 20. 



The Oscillation of Flame Cones. 

 Prof. Galloway (N.ature, April ig, p. 584) considers 

 that my explanation of the phenomenon described by Mr. 

 Temple in his letter (March 2q, p. i;i2) is inadequate, and 

 he offers a different explanation. With the view of deciding 

 the question some experiments have been made here bv 

 NO T908, VOT. 74] 



Mr. C. E. W'hitelev. I may perhaps repeat that the pheno- 

 menon in question is the continued descent and re-ascent 

 of the inner cone of a coal-gas and air flame when a 

 suitable mi.xture of the two is ignited at the end of a 

 glass tube fixed so as to form a prolongation of the metal 

 tube of a Bunsen burner. 



The following results were obtained by Mr. Whiteley : — 

 (i) The continued oscillation of the inner cone could not 

 be established with a forced supply of both gas and air, 

 but only when the air was sucked in by the injector action 

 of a gas jet, as in the ordinary Bunsen burner. (Mr. 

 Temple informs me that this was also his method of work- 

 ing.) (2) The continued oscillation of the inner cone could 

 be maintained when the apparatus was tilted even to 

 horizontality or beyond. (3) When the inner cone began to 

 descend a back pressure was immediately produced in the 

 ascending current of gas and air. 



I think the determining influence is clear from these 

 observations. When the cone begins to descend and causes 

 a back pressure this will momentarily check the indraught 

 of air without materially checking the supply of gas. A 

 stratum of mixture containing less air is thus produced ; 

 its rate of inflammation is less than its upward velocity, 

 and so the cone is carried to the top of the tube. Soon 

 the normal air supply is re-established, a mixture with 

 a higher rate of inflammation is restored, and the cone 

 again descends. 



A confirmation of this explanation is afforded by two 

 further observations : — (4) a shortening of the glass tube 

 increases the rapidity of oscillation in conformity with the 

 shorter distance to be traversed by the altered stratum ; 

 (5) a " capacity " in the form of a globe at the bottom 

 of the glass tube stops the oscillation. Such an arrange- 

 ment would both damp the back-pressure impulse and 

 obliterate stratification. 



Observations (2) and (5) show, I think, that the chimney- 

 like action suggested by Prof. Galloway cannot be the 

 determining cause, and indeed this could hardly be ex- 

 pected, inasmuch as such action would increase the 

 aspiration of air and produce a mixture having a higher 

 rate of inflammation, a condition which would oppose the 

 other effect, viz. the increased upward velocity of the 

 mixture to which alone Prof. Galloway alludes. 



Mv own nrevious explanation was inadequate to explain 

 the continued oscillation, and only important in relation 

 to the lighting back of Bunsen flames. 



Arthur Smithells. 



The University, Leeds, May 10. 



Ancient Fire Festivals. 



In reference to your series of articles which have recently 

 appeared in N.xture on Stonehenge and the ancient festivals, 

 I send vou the following notes on a \\'iltshire celebration 

 of the August fire customs. Tan Hill Fair is held on 

 August 6, and the coincidence of the name Tan (Celtic for 

 fire) and the date point to a time long prior to our era, 

 when the fire festivals were annually held. 



This fair, the origin of which is lost in antiquity, is 

 held in the very last place likely to be chosen for such a 

 purpose, and must have had its beginning at a time when 

 men assembled there for some purpose very different to 

 what brings them there now, for neither roads nor water- 

 wavs (conditions essential to most fairs) lead to Tan Hill. 



Tan Hill is on the highest part of the downs (near 

 Devizes, north Wiltshire), q:;8 feet above sea-level, looking 

 down on Avebury and dominating the whole country, and 

 crossed only by British trackways which lead to the fair. 



Sacred fires lit of old on this Tan Hill would have been 

 seen from Martin.soU (near Marlborough), Hackpen, Old- 

 bury, and for miles around, and were probably eagerly 

 watched for by the people taught to expect the blessing on 

 the crops of the ensuing year consequent on these fires ; 

 and it is on this bleak, desolate down that one of the 

 largest fairs of the countrv is held. 



Fairs in Ireland and in Wales carry on the same tradition 

 of the ancient fire festival held in August, as well as this 

 one at Tan Hill. 



In ancient Ireland this August celebration was called 

 " the Lugiiassad." the feast of Lug (a sun god), and 

 according to Prof. Rhys " this festival was the great event 



