September 12, 1907] 



NA rURE 



49: 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Regnault's Experiments on the Joule-Thomson Effect. 



That Regnault made a number of experiments similar 

 to some of those described by Joule and Kelvin in theii 

 papers on the thermal effects of fluids in motion does 

 not seem to be commonly known ; at all events, the 

 numerical results of his work on this subject are not often 

 mentioned. After experimenting on the flow of gases 

 through small holes in thin plates, and through long 

 capillary tubes, he followed the example of Joule and 

 Kelvin in using porous bodies, especially discs and tubes 

 of soft, unglazed porcelain. He found great difficulty in 

 getting definite results, especially in the regard that the 

 temperature observed in the stream of gas issuing from 

 the porous wall depended on the position of the thermo- 

 meter. He concludes the account of his experiments in 

 the following words : — " Apr&s ces tentatives infruc- 

 tueuses et beaucoup d'autres dont je ne parlerai pas, j'ai 

 renonc^ k I'espoir d'obtenir quelque chose de precis, 

 d'exp^riences fondles sur la mesure de temperatures dans 

 les courants gazeux, et il ne m'est rest^ que le regret 

 d'avoir consacr^ beaucoup de temps k des recherches 

 st^riles " (Regnault, " Relation des Experiences," vol. iv., 

 p. 707, 1870). 



At the present day we can see that Regnault's experi- 

 mental method Was not in this case the best that might 

 have been devised, but in spite of his dissatisfaction with 

 the results of the work, his most definite numerical values 

 are not very different from those of Joule and Kelvin. Air 

 under pressure was forced through the wall of a porous 

 porcelain tube from the outside. The tube was 20 cm. 

 long and 2-8 cm. in diameter, the thickness of the wall 

 not being stated. The fall of temperature At was found 

 in two ways. In the first method, one mercury thermo- 

 meter was placed in the axis of the porous tube, while a 

 second thermometer was immersed in the large water bath 

 containing the worm for leading in the compressed air. 

 In the second method an iron-copper thermocouple was 

 used, one junction being in the axis of the porous tube 

 and the other in the compressed air just outside. The 

 results of the thermoelectric measurements are somewhat 

 irregular, and it seems probable that the fault was mainly 

 with the imperfect electrical instruments of the time. 

 The two series with the mercury thermometers are more 

 satisfactory. 



In one of these, the mean of eleven measurements in 

 which the fall of pressure, Ap, at the porous wall was 

 between 4-9 metres and 3.Q metres of mercury, gave 

 At/A/) = o-293, the separate values ranging from 0-285 to 

 0-303. Nine subsequent measurements in the same series 

 gave values ranging from Af/A/> = o-4o7 for A^ = 3-i metres 

 up to 0-685 f""" A/) = 0-65 metre and then down to 0-312 

 for A/) = o-30 metre. In the other series, made with a 

 somewhat thinner-walled tube, the mean of the twenty- 

 two measurements, in which the fall of pressure was 

 between 6-6 metres and 3-4 metres, gave A(/A/> = o-255, 

 the extreme values being 0-234 3"<J 0-313, while twelve of 

 the twenty-two values are between 0-250 and 0-260. 



It is interesting to compare the mean of these values, 

 At/A^ = o-27° C. per metre of mercury, with the value 

 obtained by Joule and Kelvin. Their results may be re- 

 presented sufficiently well, in the same units, by the 

 equation 



A//A/> = o-36 — 00017?, 



which gives for t = 2o° C. A(/A/) = o-33. Regnault does 

 not state the temperature at which his e.xperiments were 

 made, but it was probably not far from 20°, so that in 

 reality his work agreed fairly well with that of Joule and 

 Kelvin. 



Edg.^r Buckingham. 

 Bureau of Standards, Washington, August 27. 



NO. 1976, VOL. 76] 



Genetics. 



In a recent review of the report of the third Inter- 

 national Conference, 1906, on Genetics (Nature, August 22, 

 p. 417), the following sentence occurs : — " Mendel's peas 

 have already been called classical ; and it is a very re- 

 markable fact that no one has repeated Mendel's experi- 

 ments with the deliberate intention of testing the 

 Mendelian interpretation of the results." This statement 

 is misleading, for in the following five instances many of 

 -Mendel's experiments on the pea have been repeated and 

 confirmed : — 



Correns, C, Deutsch. Bot. Gesellsch., 1900. 



Tschermak, E., Zts. f. d. landw. Versuchsw. in Osterr., 

 1900 (and later). 



Hurst, C. C, Journ. R. Hort. Soc, 1904. 



Lock, R. H., Ann. R. Bot. Gard., Peradeniya, 1904. 



Bateson, W., and Miss Killby, Rep. II. to Evolution 

 Committee of Roy. Soc, 1905. 



In each case the e.xperimental results have led the author 

 to accept Mendel's interpretation of the phenomena. 



R. C. PUNNETT. 



Gonville and Caius College, Cambridge, August 30. 



The sentence which Mr. Punnett quotes from my review 

 is not in the least misleading. I did not say that Mendel's 

 experiments had not been "repeated and confirmed"; I 

 said that they had not been repeated with the deliberate 

 intention of testing the interpretation which Mendel put 

 upon them. This statement I repeat, and shall shortly prove 

 to be correct. I am familiar with the work of the five 

 authors in Mr. Punnett's list (which surely should have in- 

 cluded a reference to de V'ries's papers), and, what is more, 

 I have a first-hand acquaintance with the facts themselves. 



I do not doubt for one moment the correctness of 

 Mendel's results, as my critic implies in his sentence 

 which follows the quotation from my review. I do not 

 doubt, as Weldon did, the validity of the generalisation 

 that yellowness of cotyledons in peas is dominant over 

 greenness, or that these characters segregate in Mendelian 

 fashion in P.. I do not doubt the reality of the phenomena 

 in any of the cases described by the authors whom Mr. 

 Punnett cites any more than I doubt the reality of the 

 results which I have myself obtained ; but does that mean 

 that I must accept the Mendelian interpretation of them? 

 Certainly not. The Mendelian phenomenon is a fact ; but 

 the Mendelian interpretation is an inference, and it is 

 dangerously misleading not to lay stress on the distinc- 

 tion between the two. 



What I mean by " testing the interpretation " is the 

 carrying out, by one who is actively sceptical of Mendel's 

 hypothesis, of an experiment of such a kind that the 

 result can leave no doubt in the mind of the experimenter 

 as to the validity, or otherwise, of the hypothesis tested. 

 I will suggest an experiment. The proportion 75 per cent, 

 yellow, 25 per cent, green, holds good for F, from crosses 

 between pure strains of yellow-seeded and pure strains of 

 green-seeded peas ; but does it hold good for F„ from 

 crosses between a pure yellow and an extracted green in 

 Fj, i.e. a green with all its parents yellow, and with all 

 its grandparents yellow, and with all its great-grand- 

 parents yellow, and with all its great-great-grandparents 

 vellow. and with half its great-great-great-grandparents 

 vellow? If Mendel's interpretation of the proportion he 

 obtained in F, (75 per cent, yellow, 25 per cent, green) is 

 correct, that proportion must hold good for F, from such 

 a cross as I have suggested. 



When I say that " no one has repeated Mendel's experi- 

 ments with the deliberate intention of testing the 

 Mendelian interpretation," I mean that crucial experiments 

 on the lines of that which I have suggested have not 

 been carried out by sceptical observers. I know perfectly 

 well that Mendel's experiments have been repeated on a 

 large scale, that alleged exceptions have been satisfactorily 

 disposed of, and that a number of new Mendelian 

 characters have been discovered, not only in peas them- 

 selves, but in a host of other plants and animals. But 

 that is not testing the interpretation ; it is witnessing the 

 phenomenon. " In each case," says Mr. Punnett. " the 

 experimental results have led the author to accept Mendel's 

 interpretation of the phenomena." -Surely he must follow 

 willingly who can be led by so slender a thread. 



The Reviewer. 



