NA TURE 



[July 4. 1907 



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.] 



Absorption of the Air for Light of Short Wavelengths. 



In his well-lcnown work on the absorption of the air 

 for light of very short wave-lengths, Schumann has con- 

 cluded that the opacity is due to o.xygen, and that this 

 gas, in thickness of a few millimetres, absorbs completely 

 all wave-lengths shorter than about 1800 tenth-metres. 



I have recently been investigating the question of gas 

 absorption in this region by means of the vacuum spectro- 

 scope which I employed in measuring the lines in the 

 spectrum of hydrogen. 



In part my results agree wilh those of Schumann, for 

 I find that hydrogen, nitrogen, helium, and argon are all 

 quite transparent to very short wave-lengths. In one very 

 important respect, however, I cannot agree with his con- 

 clusions, for I find that oxygen is not opaque for all wave- 

 lengths below a certain value, but that its absorption is 

 represented by a band with definite limits. With a gas 

 path of nine millimetres and a pressure of one-half an atmo- 

 sphere, this band extends from 1750 to 1275 tenth-metres. 



Though the investigation of the behaviour of oxvgen 

 below wave-lenglh 1230 is hindered at present by' the 

 opacity of the fluorite windows which enclose the absorp- 

 tion chamber, yet it appears possible that light of even 

 the shortest known wave-lengths may be able to penetrate 

 air paths of more than a centimetre. The application of 

 this result to the behaviour of ether vibrations of extremely 

 high frequency seems important. Theodore Lvm.4N. 



Jefferson Physical Laboratory, Harvard University, 

 June 21. 



The Structure of the iEther. 



In the issue of Nature of June 13 (p. 150) Dr. C. V. 

 Burton raises an objection, raised elsewhere by Prof. Hicks 

 and Sir Oliver Lodge, to the correlation of the magnetic 

 vector with the velocity of the a:ther, on the ground that 

 the motion of an observer relative to the ajther would 

 alter the relative velocity of the a;ther, and thus produce 

 a change in the magnetic vector in the direction of the 

 change in the observer's motion. 



If it were stated definitely that the magnetic force in the 

 free a;ther was proportional to the velocity of the aether 

 relative to the observer, the objection would be valid ; but 

 this is reading into the scheme of the aether more than can 

 legitimately be done. In the discussion the fact has been 

 apparently overlooked that the correlation of the two 

 vectors extends only to their rates of change in space and 

 time, so that if identified with one another at any one 

 point at any one instant, they will be identical at all 

 points at every instant ; otherwise thev mav differ bv any 

 constant, corresponding to a unitorrn but undetermined 

 constant drift of the ather as a whole, or, what is the 

 -same thing, to a uniform unknown velocity of the observer 

 through the tether. 



But it should be noticed that it is not permissible to 

 speak of the velocity of an observer relative to the jether, 

 as though the tether were a material medium given in 

 advance. Even if it were possible to isolate a fi.xed frame 

 of reference in such an aether — which appears questionable 

 in an infinite continuum — there is no physical means of 

 determining the velocity of a system relative to it. The 

 jether, as we know it, is defined by its electromagnetic 

 properties, and one property is that a' uniform drift of the 

 a;ther as a whole has no effect on electromagnetic pheno- 

 mena. It is known that the correlation between a stationary 

 and a_ moving system as regards the clectrodvnamic equa- 

 tions is exact, and Viot only correct to the second order. 

 The objection to Prof. Larmor's scheme of the a;ther does 



not apply if that scheme is stated accurately as follows : 



" The propagation of electromagnetic effects through space, 

 relative to a given frame of reference, mav be illustrated 

 by the propagation of disturbances in a rotationally elastic 

 medium, it being possible for a given frame of reference 

 to construct such a medium, in which the rotational dis- 

 NO. 1966, VOL. 76] 



placement at any point is proportional to the electric force 

 at that point, and the velocity relative to the frame of a 

 point of the medium is proportional to the magnetic force." 

 .Since the velocity of a point depends on the frame of 

 reference, it follows that the media constructed for two 

 frames of reference moving relatively to one another with 

 constant velocity will not be identical. The tuther is, in 

 fact, not a medium with an objective reality, but a mental 

 image which is only unique under certain limitations (cf. 

 footnote, p. 334, " ^Elher and Matlrr "|. I wo frames of 

 reference imply two aethers ; so long as we restrict our- 

 selves to a single frame, the objection to Larinor's»scheme 

 does not arise. E. Cunningham. 



St. John's College, Cambridge, June 28. 



Root Action and Bacteria. 



I DO not think that there is necessarily any antagonism 

 between the interesting results which Ur. E. J. Russell 

 has for some time past been obtaining and our observ- 

 ations on the behaviour of trees in heated soil. His deai 

 with the growth of plants, ours with the passage of the 

 plant from the dormant to the active condition, a process 

 analogous to that of germination. Nevertheless, it must 

 be freely acknowledged that, until further work on the 

 subject has been done, the view that bacteria are concerned 

 in the matter is a mere suggestion, and Dr. Russell's 

 opinion that the results may be the consequence of chemical 

 changes produced by the heating is somewhat strengthened 

 by the fact that the soil used was poor in lime, contain- 

 ing only about 1 per cent. CaO. There is, however, one 

 strong objection to accepting an explanation based on 

 chemical change, for two of the nine trees, as I men- 

 tioned, behaved exceptionally, and showed practically no 

 retardation in starting. These were two which had been 

 planted in earth heated to the highest temperature, 250°, 

 and were two out of three planted in the same batch of 

 heated earth. It seems impossible to explain these two 

 exceptions if the general results are due to chemical 

 change, but they are easily explained if these results are 

 due to bacterial action, for re-inoculation of the soil might 

 readily occur in one case and not in another. 



It may be added that, so far, the trees are behaving 

 normally as to their growth, now that they have once 

 started. Spencer Pickering. 



I HAVE always carefully looked for, but never found, 

 any retardation of germination in our experiments. The 

 young plants all come up at about the same time, and 

 make equal progress for some weeks ; then the plants on 

 the heated soil take on a greener colour, become larger 

 in the leaf and thicker in the stem, and ultimately make 

 about 100 per cent, more dry matter than the control 

 plants in unheated soil. There is no doubt, I think, that 

 bacterial action is involved, because the yield is depressed 

 when I inoculate the heated soils either by watering with 

 unsterilised well water or by adding small quantities of 

 unheated soil. .\ chemical change in the soil com- 

 pounds must also be involved, because of the increased 

 " availability " of the nitrogen and phosphorus compounds 

 indicated by the analyses quoted in my earlier letter. All 

 non-leguminous plants we have tried so far have shown 

 similar behaviour. 



It is, I think, quite possible to explain Mr. Pickering's 

 results on a chemical hypothesis. In certain circumstances 

 — deficiency of lime among others — organic substances 

 which retard germination and growth may be formed in 

 the soil ; in other circumstances, e.g. admission of air, they 

 are decomposed by soil organisms. If we assume that 

 some of these substances were formed during the heating of 

 Mr. Pickering's soil, and further "assume, with him, that 

 in two pots re-inoculation took place, the poisonous bodies 

 would be destroyed and growth would no longer be 

 inhibited. These assumptions are all based on well-known 

 facts ; on this view a soil rich in calcium carbonate should 

 behave as our soils have done and cause no retardation. 



However, as Mr. Pickering says, more work is wanted 

 before we can get much further. In the meantime, he has 

 established the very important point that growth may be 

 retarded in a heated soil, and the further development 

 of his experiments will be awaited with much interest. 



Edward J. Russell. 



