8o 



NATURE 



\Nov. 29, 1877 



their mean value according to the well-known exponential law, 

 and are alike in all directions. But the gas of a compressed 

 Crookes's layer is not in the ordinary state ; it is under con- 

 straint, as I have elsewhere shown, oiving to the proximity of the 

 heater and cooler between which it is confined. In consequence 

 of this constraint there are what I have described as processions 

 going on in the layer of gas : in other words, the velocities of (he 

 molecules at any situation within the layer are not alike in all 

 directions, but are greatest in the direction of the cooler, least in the 

 direction of the heater, and of intermediate values in lateral dire :- 

 tions. The heat in crossing the layer from the heater to the 

 cooler maintains this polarised molecular structure, and if the 

 How of heat is increased it does not simply increase the mean 

 velocity of the molecules, but also augments the disparity of the 

 velocities in different directions. 



Now the ordinary laws of the communication of heat to and 

 through gas are bastd on the opposite supposition that when heat 

 reaches any portion of the gas all the molecules of that portion are 

 equally affected, that though their mean velocity is increased the 

 laws of the dibtribution of the velocities about that mean, and in 

 different directions, are not changed. Hence Prof. Osborne 

 Reynolds has fallen into an error in applying the ordinary '* law 

 of the diffusion of heat in gases " to the case of compressed 

 Crookes's layers. The law employed by Prof. Reynolds does 

 not prevail unless there is sufficient room in front of the heater for 

 the development of a complete unrestricted Crookes's layer ; 

 Crookes's force only presents itself when the thickness of that 

 layer is restricted by a cooler. 



The transmission of heat across Crookes's layers is made the 

 subject of investigation in a memoir which I laid before the Royal 

 Dublin Society last May, which has recently been printed in the 

 Transactions of that body, and of which a copy will shortly 

 appear in the Philosophical Ma:;azine. The law proves to be 

 enurely different from any of the laws for the propagation of heat 

 hitherto known, and I have therefore called this mode of trans- 

 ferring heat by a new name — the penetration of heat. Moreover, 

 the results of theory had been verified by anticipation more than 

 thirty years before by MM. De la Provostaye and Desains, in 

 two elaborate experimental investigations into what we now 

 know to have been the penetration of heat ; so that our know- 

 ledge of its laws, which are entirely different from the laws of the 

 diffusion of heat, quoted by Prof. Reynolds, already stands on 

 both a deductive and experimental basis. 



2. Prof. Osborne Reynolds further states that with each gas 

 the force depends only on one variable, viz., the rate at which 

 heat is communicated by the heater to the aijacent gas, and that 

 it is proportional to this rate. Probably o .', ing to a mere slip 

 on Prof. Reynolds's part, he has here omitted a second variable, 

 viz., the temperature of the gas, which is implicitly contained 

 in the equation of his first paper to which he refers. With this, 

 however, I have no concern ; what I have to point out is that 

 in making the statement, whether in an amended or in its actual 

 form. Prof. Osborne Reynolds has overlooked the fact that the 

 machinery of Crookes's stress consists of a cooler as well as of the 

 heater and intermediate gas, and thit a sufficient proximity of the 

 cooler is essential. Accordingly, the true expression for the force 

 (of which I hope to publish an investigation made some time 

 ago, as soon as my health will allow) is not so simple as Prof. 

 Reynoldi supposes, but is a function of the temperatures of the 

 heater and cooler, and of the rate at which heat reaches the 

 . ooler by penetration, in addition to the single variable which 

 one Prof. Osborne Reynolds admits. The vice of the mathe- 

 matical reasoning, on which Prof. Reynolds bases his statement, 

 is that it starts from a kinetic expresbion for the pressure of gas, 

 which is only true when the mean of the squares of the velocities 

 of the molecules is the same in all directions. Accordingly, his 

 discussion does not reach the phenomenon it professes to 

 explain; ''it is irrelevant to the case of compressed Crookes's 

 layers, in which the gas is polarised, and where the degree of 

 polarisation is itself a lunction of Prof. Reynolds's variable along 

 with other thermal variables. 



Thus, in all parts of his inquiry. Prof. Osborne Reynolds has 

 been led into error by having regarded the gas of compressed 

 Crookes's layers as gas in its ordinary state ; in other words, 

 because he has not had a glimpse of that peculiar molecular 

 structure in the gas, which is the real source of Crookes's stress. 

 From a review of the whole subject I think myself justified in 

 submitting that the only discovery which brought with it any 

 knowledge of the cause of Crookes's stress and of penetration, 

 was the discovery that gas could assume this polarised con- 

 dition ; and I must say that it does not appear to me that 



to this discovery Prof. Ooborue Reynolds has in any degree 

 contributed. 

 Dublin, November 15 G. Johnstone Stoney 



Postscript, November 23, — Pi of. Osborne Reynolds has 

 written a further letter to Nature (vol. xvii. p. 61), in which he 

 says : — "The fact that Mr. Stoney has in no way referred to my 

 work, although I preceded him by some two year?, has relieved 

 me from all obligation to discuss Mr. Stoney 's theory." I am 

 sorry Prof. Osborne Reynolds should have thought me capable 

 of discourtesy or inattention to the claims of a fellow-worker, and 

 fortunately I am not conscious of being liable to the imputation. 

 I became acquainted with Prof. Reynolds's paper in the interval 

 between the publication of my first and second paper?, but did 

 not refer to it in my second paper because 1 found on reading it 

 that Prof. Reynolds's explmations of Crookes's force were all 

 erroneous (viz., the evaporation of mercury or other vapour, 

 and heat communicated to diffused particles of ga=;, or to gas 

 brought by convection currents) ; because the mathematical 

 analysis widi which he supports his [hypotheses is irrelevant 

 to the problem with which he is dealing ; and finally, because for 

 the purposes of my inves igation I had no occasion to point out 

 these mistakes, inasmuch as Prof. Reynolds had not approached 

 the subject of polarised layers of gas and their mechanical pro- 

 perties, which was the subject matter of my papers. 



I ought to add a word in reference to the criticism of my 

 memoir on penetration, which is contained in Prof. Osborne 

 Reynolds's last letter. He seems to overlook a condition laid 

 down in the second paragraph of my memoir, which diipo-:es of 

 the criticism, viz. : "Let us further res^ard this gas as a perfect 

 non-conductor of heat." Your mathematical readers will at once 

 perceive that this condition is a legitimate simplification of the 

 problem, because the diffusion or conduction of heat in gases is 

 very sluggish compared with penetration, the phen jmtnjn with 

 which I was dealing. 



It appears from Prof. Osborne Reynolds's last letter that my 

 wish to make my note to Nature (vol. xvii. p. 43) a fortnight 

 ago short, led me to make it obscure. I will ttierefore, with 

 your permission, try to state the matter more clearly. 



As I understand the scientific question in discussion before us, 

 it is this : — Assuming (l) that, when heat is communicated from 

 a solid surface to a gas in contact with it, a force arises (equiva- 

 lent to a pressure against the surface) which is proportional to 

 the rate of communication of heat, and (2) that the conducting 

 power of a gas for heat is independent of its density, Prof. 

 Reynolds concludes thit the driving-force on the vanes of a 

 radiometer does not increase with the rarefaction of the air, but 

 that rarefaction favours the motion only in so far as it lesssni the 

 opposing force due to convection- currents. I, on the other 

 hand, while admitting Prof. Reynolds's premisses, do not admk 

 his conclusion. On the contrary, I believe that, in the radio- 

 meter, rarefaction must increase the rate of communication of 

 heat, and hence also the force. To see how this may be, 1 t A B 

 represent the thickness of a stratum of gas contained between 

 two parallel solid surfaces, whose temperatures, measured from 

 any zero, are represented respectively by A c and B D. Then, 

 I imagine, the flow of heat through the gas will take place as 

 though there were, in contact with each solid surface, a layer of 

 gas whose temperature is throughout the same as that of ths 

 contiguous solid, and whose thickne;s is equal (or at leist pro- 

 portional) to the mean length of path of the molecules. The 

 virtual thickness of the stratum of gas, whose conductivity comes 

 into account in determining the rate of transmission of heat, is 

 then the actual thickness diminished by the aggregate thicknesses 

 of these two layeis. For example, if Aa and B^ represent the 

 thicknesses of the hot and cold layers respectively, the virtual 



A CO ccl 



I HB 



thickness of the stratum across whic'i conduction takes place is 

 ad, and the distribution of temperature from side to side of 

 the whole quantity of gas is given by the ordinate^ of the 



