NOTE Q. space is not elective ir its action on light, but is of the character of a general absorption acting equally, or nearly so, on light of every degree of refrangibility. Whatever may be the true state of the case, the result of this re-examination of the spectrum of this nebula appears to give increased probability to the suggestion that followed from my former observations, namely, that the substances hydrogen and nitrogen are the principal constituents of the nebulæ of the class under consideration.' "—(Pp. 543, 544-) To this Dr. Frankland and myself sent the following reply: "GASEOUS SPECTRA.-To the Editor of the Chemical News "SIR,--Our attention has been drawn to a letter from Mr. W. Huggins, in your impression of the 9th instant, stating that, in a recent note to the Royal Society (in which we announced that, by varying the conditions of pressure and temperature, we had reduced the spectrum of hydrogen, as seen in our instrument, to one line absolutely, and the spectrum of nitrogen to one line nearly), we did not notice similar observations of his own presented to the Royal Society in 1868. In reply, we beg to state-first, that we did notice Mr. Huggins' observations, and, secondly, that they were not similar to our own. "With regard to the first point. The account you were good enough to publish contained the following:-The bearing of these latter observations on those made on the nebule by Mr. Huggins, Father Secchi, and Lord Rosse is at once obvious. The visibility of a single line of nitrogen has been taken by Mr. Huggins to indicate possibly-first, 'a form of matter more elementary than nitrogen, and which our analysis has not yet enabled us to detect;' and then, secondly, ‘a power of extinction existing in cosmical space.' "Our experiments on the gases themselves show not only that such assumptions are unnecessary, but that spectrum analysis here presen's us with a means of largely increasing our knowledge of the physical constitution of these heavenly bodies.' "We thus gave briefly the results of Mr. Huggins' inquiries, and references to where a complete account of his observations were to be found. This we considered to be sufficient, seeing that our communication was a preliminary note only. In the complete account of our researches, to be subsequently presented to the Royal Society, we shall, of course, dwell upon them at greater length. "With regard to the second point-the similarity of the observations. In 1864, Mr. Huggins first observed the spectrum of the nebulæ, and determined that one of the lines was coincident with one of the air lines due to nitrogen, the spark being taken close to the spectroscope. On this Mr. Huggins wrote:-'The speculation presents itself, whether the occurrence of this one line only in the nebula may not indicate a form of matter more elementary than nitrogen, and which our analysis has not yet enabled us to detect.' Some time 1 Chemical News, July 30th, 1869. afterwards, Mr. Huggins, instead of taking the air-spark close to the spectroscope, placed it outside the object-glass of his telescope, and then only saw one line in the spectrum-the line in question. He next got similar results close to the spark by interposing a dark glass to cut off the light, still using the spark in air at the ordinary pressure, and remarked that it was obvious that the hydrogen spectrum would give similar results. Then, after stating we cannot suppose that any lines have been extinguished by the effect of the distance of these objects from us,' he refers to the possibility of a 'power of extinction residing in cosmical space,' and the hypothesis of the more simple form of nitrogen appears to be given up. "Now, with regard to our own observations, we have not taken the spark in air; we have not used a dark glass; we have not worked at atmospheric pressure; nor have we disturbed the normal distance of the experimental tube from the slit of our instrument until we have reduced the complicated spectrum of nitrogen to one bright line and three or four other extremely faint ones. Further, we have not rested content with any assumption with regard to hydrogen, nor have we regarded anything as obvious. "What we have done is to carefully watch both spectra changing as the pressure becomes less, trying the effect of increased and decreased temperature at every stage under various conditions, which we shall state in our detailed paper. "It may be as well to refer, in conclusion, to Plücker's important work on nitrogen, which shows that, under certain conditions, the green line of nitrogen vanishes altogether from the spectrum. This fact alone shows at once the importance of our method of inquiry. "We are, &c., "E. FRANKLAND. "Royal College of Chemistry, Oxford Street, "July 23, 1869." In this case, as in the former one, Mr. Huggins has neither substantiated his statement nor withdrawn it. Instead of this, he has reiterated it,' taking no notice of our reply! After the notes on the function of pressure had appeared in the Proceedings of the Royal Society, Herr Wüllner published a memoir on the subject, in which he ascribed the changes observed to temperature. He has since, however, changed his opinion, and now supports the view put forward by Dr. Frankland and myself. M. Salet, however, ascribes these changes to temperature merely.2 1 See Proc. R.S., vol. xx. p. 383, Note. Thesis read before the Faculty of the University of Paris, p. 18. NOTE Q. NOTE Q. More recently, Schuster has discussed the action of electrical resistance in these terms: "We cannot alter the pressure of a gas without altering its electric resistance, and, therefore, also the strength of the electric current and the heat developed. We can only decide the question by subjecting the gas at the same temperature to different pressure. Now have there ever been any such experiments made? I think there have, and even very decisive ones. Frankland and Lockyer have found that if we increase the pressure of hydrogen while an electric current is passing through it, the lines begin to expand till the spectrum becomes continuous, and finally the resistance becomes so large that the electric current will not pass at all. On the other hand, Gassiot and Plucker have observed that, if we diminish the pressure of hydrogen, its electric resistance force diminishes, attains a minimum, then increases again, and if we keep on exhausting the tube, it becomes again so great that the current cannot pass. Plücker says that a tube exhausted to its utmost limits shows the lines of hydrogen and silica. He mentions at one place, I think, that the lines are very fine and distinct. If there would have been any widening he would have been sure to mention it. Now it is not too much to assume that the resistance of the gas at the moment when the discharge just ceases to take place is the same whether the increase of resistance is produced by too great a pressure or too great an exhaustion. At this moment, therefore, the current is the same, and the same energy must be converted into heat by resistance. But in the case in which the current does not pass on account of the excessive diminution of pressure, only a much smaller quantity of gas has to be heated than in the other case. It must, therefore, be heated up to a much higher temperature, and yet the spectrum is not continuous and the lines are not even widened. We are, therefore, compelled to accept Frankland and Lockyer's original conclusion, that pressure and not heat is the cause of the widening of the lines. "The question is one of considerable importance. If temperature would widen the lines, the widening ought always to begin at the same temperature, and the hydrogen in the solar protuberances which show only narrow lines could not be at a higher temperature than the hydrogen in our vacuum tubes, the moment the lines begin to widen. If our conclusion, however, is correct, the breadth of the lines will give us no indication whatever as to the temperature of the gas." |