« PreviousContinue »
observed successions of overtones may not be devoid of interest. There are four notes of the arrangement which I shall call I., and three notes of II. The pitch in semitones is appended for comparison. In both cases the point of attachment was nearly, though not exactly, in the middle of the string.
Comparing these numbers with the overtones indicated in column 3 of the calculated Table, it will be seen that they follow, as far as they go, the general course indicated by theory in the hypothetical case assumed; and it may be inferred that this case furnishes a rough representation of the circumstances of the two experiments examined.
The above results are the only experimental ones which I know of.
The calculation of the length of the middle segment in the hypothetical case follows easily from the numbers in column 1 of the calculated Table. The fundamental, of course, has for its segment the whole string. In the other cases, expressing
7 in terms of, which is the length of each segment except the middle one, we get the middle segment at once, since we know the actual number of segments. (It is hardly necessary to remark that the numbers of segments in the successive overtones are the odd integers, by the symmetry.)
So that, as the pitch of the note sounded rises, the reed diminishes more and more the segment to which it is attached, as compared with the others. Of course this remark is confined for the present to cases resembling the hypothetical case.
The note employed may be either the fundamental or any one of the overtones. As these are in general all inharmonious to each other, only one can be used at a time. But it is probable that, in particular cases, some two or more may become harmonious; and they would then be capable of combining in a true periodic motion.
XIII. Carbon and Hydrocarbon in the Modern Spectroscope. By W. MARSHALL WATTS, D.Sc., Physical-Science Master in the Giggleswick Grammar School*.
N the January Number of the Philosophical Magazine appears a paper with the above title by Professor Piazzi Smyth, which calls for a few words from me by way of explanation.
1. Professor Smyth inquires "why, since for cometary work the reference-spectrum should be of feeble intensity, I do not examine it in that shape, viz. as given by the blue base of the flame of a small alcohol lamp, or the all but vanishing globule of flame when a common gas-light is on the point of going out from inanition?" The answer is simple, that with a spectroscope of six prisms the loss of light is so great that in the spectrum of a blowpipe-flame there would not be more than one line (5165.5) bright enough to be measured, and it was my object to employ as large a dispersive power as possible in order to secure as great accuracy in the determinations as I could.
The same reason explains why, "although the spectrum consists notably and notoriously of five bands, viz. the orange, citron, green, blue, and violet," I only give measurements for three of the bands: the orange and violet bands were not bright enough to be measured accurately.
2. An equally simple explanation solves the "strange problem" why the lines 5165.5 and 5585.5 are the best-determined.
5165.5 happens to fall close to the magnesium-line b, whose wave-length we know with great accuracy from the labours of Ångström.
5585-5 happens to be exactly coincident with an iron line in the solar spectrum. The first band of the citron group, although brighter than the second, does not fall near to any marked line in the solar spectrum which could be used as a reference-line; and its determination is therefore not quite so exact.
* Communicated by the Author.
3. Chemical Parentage of the Spectrum under discussion.-I freely admit the force of Professor Piazzi Smyth's remarks on the difficulty of volatilizing carbon; but that does not appear to me to affect the experimental evidence for my assertion that "this spectrum is the spectrum of carbon, and not of a hydrocarbon or any other compound of carbon." That evidence is very simple; this spectrum can be obtained alike from compounds of carbon with hydrogen, with nitrogen, with oxygen, with sulphur, and with chlorine.
Whether or not the spectrum is produced by the vapour of carbon is another question; but if this spectrum is, as Professor Piazzi Smyth asserts, that of a hydrocarbon, will Professor Piazzi Smyth explain how it is possible to obtain it from cyanogen, a compound of carbon and nitrogen, when no hydrogen is pre sent? I have just repeated the experiment with cyanogen for perhaps the fiftieth time. Dry mercuric cyanide was heated in a test-tube, and the gas evolved was dried by passing through a tube containing phosphoric anhydride; it then passed through a tube provided with platinum wires, the end of which dipped below warm and dry mercury. On passing the discharge from an induction-coil between the platinum wires a spark was obtained which gave the spectrum in question brilliantly, the gas being decomposed and carbon being deposited.
Professor Piazzi Smyth says that in May 1871, in a paper sent to the Royal Astronomical Society, he "gave such extracts from the authorities on either side as showed that the spectroscopists declaring for pure carbon, in opposition to those pronouncing for carbohydrogen, were blundering little less than the perpetual-motion men of last century." Permit me to quote from a paper communicated by myself to the Journal of Science for January 1871:
"At first sight it would appear that carbon is an element unlikely to yield a discontinuous spectrum, inasmuch as it is not known in the gaseous condition; and that if we obtain discontinuous spectra from carbon compounds, they must be due to some compound of carbon. Thus the bright blue lines observed by Swan (1856) in the spectrum of the Bunsen-flame might be supposed to be more probably due to carbonic oxide or carbonic acid than to carbon itself. But we find that these same lines occur not only in the spectrum of the flame, but also in the spectra obtained by passing the electric spark either through carbonic oxide, or olefiant gas, or cyanogen, and the lines thus found to be common to compounds of carbon with different elements must of course be due to carbon itself. Whether they are really produced by carbon in the gaseous state is a question which cannot yet be certainly decided. If the carbon is in the solid
state, we shall then have an exception to the law that incandes-
The difference to which Professor Piazzi Smyth calls attention between the spectra of compounds and elements (the difference, namely, between Plücker's "spectra of the first order" and "spectra of the second order") is important. It is perfectly true that the spectrum of carbon is a spectrum of the first order, and would, from that evidence, be inferred to be the spectrum of a compound. If, however, this spectrum be caused by a compound, it can only be a compound of carbon with carbon.
XIV. Note on the Spectrum of Carbon. By Dr. ATTFIELD,
To the Editors of the Philosophical Magazine and Journal.
N chemistry, when compounds of an element with dissimilar radicals yield similar reactions with a reagent, the reactions are held to be evidence of the presence of the element, even though that element in its free state be a massive metal and those compounds be liquid or even vaporous or gaseous. At least, to the question "who gainsays the deduction?" the answer is, "at present, no one."
In 1862 I showed that gaseous or vaporous compounds of carbon with dissimilar radicals, when ignited by the aid of the chemical force in flames, or by the electric force in tubes, or in certain cases by either force, yielded identical spectra; and therefrom I inferred that the spectrum was that of carbon, though I could not say whether the carbon was free or combined in the gases and vapours. And who gainsays the deduction? Mr. Piazzi Smyth, Astronomer Royal for Scotland, in the current Number of the Philosophical Magazine (January 1875).
Mr. Piazzi Smyth regards my deduction as a delusion, a blunder, an egregious error, a myth, a mistake patronized, he says, by the Royal Society through secret committees, which he designates accursed things, acting occasionally "as very dragons to keep out any salutary doubt expressed on a favoured topic."
These are strong terms, especially when penned by an Astronomer Royal, and with the deliberation involved in the creamstances of serial publication. Very strong experimental evidence obtained by himself would surely scarcely justify an Astrooster Royal in the employment of such terms. Yet will it be believed?) not a tittle of such evidence is forthcoming. Nay, the spectrum which I stated to be that of carbon, a statement confirmed over and over again by eminent chemists and physicists (Plücker, Morren, Marshall Watts), Mr. Piazzi Smyth asserts is not only not that of carbon, but solely that of a hydrocarbonagain an assertion unsupported by any experimental evidence whatever. It is true that Mr. Smyth quotes Lielegg and Crookes against me. But Lielegg supports me, and Crookes is cited because of an editorial footnote in the 'Chemical News' appended to a notice of Morren's paper, asking experimentalists what they meant by the "vapour of carbon" existing in a flame. As for Lielegg, I will quote without comment the last sentence but one of his paper (Eng. trans. in Phil. Mag. March 1869, p. 216:"Therefore tubes filled with combinations of carbon and hydrogen show the lines of carbon and those of hydrogen; tubes filled with carbonic oxide or carbonic acid gas show those of carbon and oxygen, giving, in fact, a spectrum of carbon, because the extremely small pressure and the high temperature cooperate in reducing the carbon to a gaseous condition." Plücker, who, with Faraday, General Sabine, and Geissler, spent two or three hours with me at the Royal Institution minutely examining my spectra-Plücker afterwards writes, on Nov. 12, 1862: "Je suis d'accord avec vous sur l'existence du spectre de la vapeur de carbone." Morren says, on page 6 of the paper already mentioned, "Je me mis done au travail avec la pensée préconçue de combattre l'assertion émise par le savant anglais; mais il résulte, au contraire, des expériences auxquelles je me suis livré, que M. Attfield a raison, et que c'est bien la vapeur du carbone qui donne le spectre indiqué plus haut.”.
I might just refer to some minor statements made by Mr. Piazzi Smyth. He says, in a paper to which he draws attention as not having been accepted, as he desired, by the Roysl Astronomical Society, but afterwards printed in the Observations' of his own observatory, that the question put by Crookes was never answered. I answered it at once, and the reply was inserted in the Chemical News' a fortnight after the question was asked. I did not work "in a rich London laboratory." With ordinary induction-coils, borrowed, the one from a captain in the army, the other from Mr. Gassiot; with a spectroscope which Dr. Frankland would scarcely now own; with tubes and apparatus made by my own bands, and made, I believe chemically clean; and by the aid of well-fitting shutters in an ill-fur