June 24, 1880] 



NA TURE 



169 



ments on loose contacts, that I thought of the experiment in 

 question. My idea was to replace his heated point of metal by 

 a continuous portion of the circuit which should act in the same 

 manner. It was obvious that this part must be of small diameter 

 to secure resistance enough to make the extension of the wire due 

 to the variation of the heating effect sensible ; and to insure that 

 the cooling should be rapid enough to allow a musical note to be 

 produced, 



I had reason beforehand to believe that the second condition 

 could be fulfilled in practice ; because I had found in my experi- 

 ments on Ohm's Law (Reports, British Association, 1S76, p. 58, 

 et scq.) that, when currents of different strengths alternated very 

 rapidly in a fine wire, the cooling was so rapid that the resistance 

 was sensibly greater during the passage of the strong current than 

 it was during the passage of the weaker. 



The accompanying figure represents the apparatus with which 

 I worked. I generally used a microphone attached to a 

 violin, belonging to Mr. Blyth, for a transmitter. In this way 

 music was reproduced so as to be audible all over Prof. Tait's 

 large class-room. For reproducing articulate speech I have 

 found a small mica diaphragm, like that used by Edison, Blyth, 

 and others, best ; words spoken to the violin are reproduced in 

 this way quite intelligibly, but as I do not possess a good 

 microphone transmitter, I have not experimented much in this 

 direction. 



«', fine wire ; 1 1, thicker copper terminals : tn jn, mercury cups over which 

 i i pas,s : ^^, string kept tight hy small load ; / /, line \vires to micro* 

 phone • (i, drum head which distributes the sound. 



So far as the limited supply of fine wire permitted, I had 

 arrived substantially at the same results as Mr. Freece regarding 

 the length and tension of the wire. My best results were got 

 with a very fine silver-palladium wire given' me by Prof. Tait ; 

 with it I could see the string s s move in time with the swell of 

 the music. I also used a thicker and shorter platinum wire, 

 heated by the current to a dull red, during silence ; it w^as 

 beautiful to see this wire burst into a bright glow when there 

 came a prolonged note, more especially a high one. 



Dr. Ferguson, in the paper above referred to, regards these 

 effects as not due to heating (although he seems to think they 

 follow the same law), but as being molecular in some other sense 

 not very clearly defined. Except in the case of iron,' I do not 

 see at present any necessity for so regarding them. The fact 

 that induction-currents of small heating effect will produce them 

 is not conclusive ; for it must be remembered that it is not the 

 w/iole heating effect, but the z'arialion of it during a very short 

 interval that is in question, and some calculations from rough 

 data in my possession have led me to think that there would be 

 no inconsistency in explaining Dr. Ferguson's ticks and the 

 speech and music in Mr. Preece's experiment and my own as 

 due to the same cause. I think also that it is quite possible that 



* Iron may be anomalous on account of its powerful magnetic properties ; 

 and Prof. Tait has suggested that, at a certain high temperature, it may, for 

 well-known reasons, be incapable of producing these effects altogether. 



we might be able to hear ticks in a wire far too thick for the 

 reproduction of music. 



I have begun some experiments with a view to throw some 

 light on questions of this kind. Possibly in the meantime Dr. 

 Ferguson, Mr. Preece, or some other experimenter may be able 

 to produce facts that settle the matter beyond the possibility of 

 doubt. G. Chrystal 



15, Chalmers Street, Edinburgh, June 12 



The Aurora Borealis and its Colours 

 In reference to Mr. Backhouse's letter last week (p. 145) 

 we have n where stated that "similarity in colour in electric 

 discharges is sufficient to indicate similarity of constitution, even 

 when their spectra are quite unlike " If Mr. Backhouse will 

 again read our paper on the Aurora:, he will see that we say that 

 in the same gas the colour of the discharge varies with the 

 degree of exhaustion. In the P/iil. Trans., 1S7S, part I, we 

 have shown also that in the same gas at the same pressure the 

 colour of the discharge varies with the amount of current ; at 

 pp. iSo-lSl, for example, the strata in hydrogen at a pressure 

 of I 2 mm., 1579 M, were of a cobalt blue with a current of 

 o'oi994o W, and pink with a current of o'oo8504 W. In the 

 index at p. 239 are given several references to the same pheno- 

 mena ; and at p. 240 references are given to a number of 

 observations on the "spectra of strata and of glow on terminal v" 

 It is very difficult to say for certain whether the red is below 

 or above the green in aurora displays, by eye estimation ; for 

 although apparently above it may be really below. We adhere 

 to our opinion that the red is at the lower level. 



Warren De La Rue 

 73, Portland Place, W. Hugo Mijller 



On some Points Connected with Terrestrial Magnetism 



In Nature, vol. xxii. p. 147, our friend Prof. Balfour 

 Stewart makes the following statement in paragraph 7 :— 



" Above this again we have the lower strata of the atmosphere, 

 which are non-conductors, while above this we have the upper 

 strata, which are conductors." 



We venture to think that our researches prove that air cannot, 

 at any degree of attenuation, be considered as a conductor of 

 electricity in the ordinaiy acceptation of the term. 

 1,1 In the P/iil. Trans., Part I. p. 168, after citing a number of 

 experiments bearing on this' point, we say: — "These observa- 

 tions show clearly that the discharge through rarefied gases 

 cannot be at all analogous to conduction through metals ; for a 

 wire having a given difference of potential between its ends can 

 permit one — and only one — current to pass ; whereas we see that 

 with a given difference of potential between the terminals of a 

 vacuum tube currents of strengths vai7ing from I to 135 can 

 flow. We are therefore led to the conclusion that the discharge 

 in a vacuum tube does not differ materially from that in air and 

 other gases at ordinary atmospheric pressure, that it is, in fact, 

 a disruptive discharge." 



In Part III. of our researches (an abstract of part of which 

 appears in the same number of Nature), Phil. Trans., vol. 

 clxxi. p. 82, we sum up a series of experiments on the electric 

 discharge in various gases by stating that the same law holds 

 good for a constant pressure and varying distance between flat 

 terminals as for a constant distance and varying pressure, the 

 obstacle in the way of a discharge being, up to a certain point, 

 as the number of molecules intervening between the terminals. 



The hyperbolic curves laid down to represent the observations 

 in each case are perfectly continuous, and show no break from 

 the condition of a non-conductor to that of a conductor. 



Warren De La Rue 

 Hugo Muller 



June 21 



Calcareous Concretions in Timber 



In Nature, vol. x.xi. p. 376, 1 observe that in a remark of the 

 reviewer, and also in an extract from Mr. Ball's "Jungle Life in 

 India," the occun-ence of concretions of lime in trees is spoken 

 of as a rare and novel phenomenon. That Tcrminalia tomeiiiosa 

 contains calcareous matter has long been known to natives, and 

 a reference to Tennent's " Ceylon," i. 99, will show that they 

 make a practical use of their knowledge by using the ashes of 

 the bark as a substitute for lime, to chew with betel. Another 

 southern tree which contains an alkali in its bark is ATicenitia 

 tomentosa. It generally grows along the margins of backwaters, 



