XII. On the Spectra of Carbon. By W. MARSHALL WATTS, D.Sc.. Physical-Science Master in the Manchester Grammar School*.

IN

N a paper published in the Philosophical Magazine for October 1869, I showed that carbon, like hydrogen and nitrogen, is capable of giving two or more distinct spectra, and I endeavoured to explain these differences as dependent solely on differences in the temperature to which the carbon-vapour was heated. Leaving the spectrum of the Bessemer flame out of consideration, the following estimations of temperature were given for the carbon spectra I., II., and IV. :

The spectrum No. II. is that obtained by the electric discharge in carbonic oxide or olefiant gas at a few millimetres pressure. The only evidence for the temperature assigned to it was the fact that the electric spark in either carbonic oxide or olefiant gas at the ordinary pressure gives the spectrum No. I., that on increasing the density of the gas the temperature rises, as is shown by the addition of new lines (groups and ), but on diminishing the pressure the spectrum No. I. gives place to spectrum No. II. The lowest temperature at which the spectrum No. I. is produced having been shown to be about 1500° C., it was concluded that spectrum No. II. belonged to temperatures below 1500° C.

It appeared probable that more decisive evidence as to the temperature of the discharge in a Geissler's tube might be obtained by means of certain sodium- and lithium-lines, which become visible only at a high temperature. If a bead of sodium chlorate be brought into a Bunsen-flame, the spectroscope shows during the final deflagration of the salt, besides the D lines, four other groups of lines, whose wave-lengths are given in Angström's map as follows:

:

The readings of these lines on the scale of my spectroscope are

Na ß, 56; Nay, 75.5; Na 8, 83.2; Na e, 43;

the Fraunhofer lines C, D, and F reading 34-5, 50, and 90 respectively.

The same lines were observed by Huggins* in the spark taken between sodium poles, and were shown to be due to sodium itself, since they were obtained by the use of pure sodium-amalgam. Diacon + obtained them from the flame of hydrogen which had been passed through an iron tube in which sodium was volatilized. The sodium-spectrum obtained by means of a Bunsen burner gives only the D lines; but if the temperature of the flame be increased, these other lines become visible and in the order given above.

I find that Naß becomes visible almost precisely at the temperature at which platinum melts. This, according to the experiments of Deville, is 2000° C.

The flame of hydrogen in air (of which the temperature, according to the experiments of Bunsen, is 2024° C.) gives the D lines only. The flame is incapable of fusing platinum except at one point.

The Bunsen flame of coal-gas and air gives only D. The flame is incapable of melting platinum.

The flame of coal-gas fed by a jet of air gives a spectrum in which, besides D, the line Na ẞ is faintly visible. This flame is just capable of fusing a fine platinum wire.

The flame of coal-gas fed by a mixture of oxygen and air (containing about 30 per cent. oxygen and 70 per cent. nitrogen) gives Na ß distinctly and Na & faintly. Nay and e are not seen. The flame fused platinum tolerably easily.

The flame of coal-gas fed by pure oxygen gives a spectrum containing Na B, y, 8, and e, but e is only faint. The flame fuses platinum easily.

The flame of carbonic oxide in air (temperature 1997°, Bunsen) gives only the D line. It is incapable of melting platinum. Carbonic oxide fed by a jet of air still gives only D. The flame just melts platinum.

Carbonic oxide fed by oxygen (temperature of flame 3033° C., Bunsen) gives Na ß and y brilliantly. Na S and e were not seen. The flame melts platinum easily.

The flame of sulphur in air (calculated temperature 1900° C.) gives the D lines only. It melts gold, but not platinum.

The flame of sulphuretted hydrogen in air (calculated tempe

*Pogg. Ann. vol. cxxiv. p. 275. [Phil. Mag. 1864, vol. xxvii. p. 542.] + Comptes Rendus, vol. lv. p. 334.

rature 2250° C.) gives the D lines only. It melts gold but not platinum.

We may therefore employ the line Na ẞ as a test of temperature indicating a temperature at least 2000° C.

Certain lines of the lithium spectrum may be employed for the same purpose. In the Bunsen burner, a bead of lithiumchloride gives a spectrum of one red line whose wave-length is about 6684. The flame of coal-gas fed by a jet of air shows, besides the red line, an orange line whose wave-length is about 6107. In the flame of coal-gas and oxygen a blue line (4605) is added; and in the electric arc a fourth line (4921) becomes visible. All these lines can be obtained by means of lithiumchlorate in the Bunsen-flame.

A vacuum-tube containing coal-gas gives the same spectrum as if carbonic oxide or olefiant gas were employed, viz. the spectrum CII. This experiment was repeated with a coal-gas tube containing pieces of metallic sodium. At first the spectrum was that of carbon, as previously described; but as the tube became heated by the continued discharge, the line Na ß came out followed by the lines 7, 8, and e, and the carbon-lines faded away till ultimately the sodium-spectrum of five lines alone remained. During this change the carbon-lines and the sodium-lines were seen together; and as the temperature to which the sodium-vapour was heated cannot be supposed to be different from that to which the carbon-vapour was heated, it follows that the spectrum C II. may be produced by carbon heated above 2000° C.

It is to be observed also that this spectrum may be produced by carbon heated not much above 2000° C., since it was obtained together with CB and without Cy, which comes out under 3000° C.

Carbon-spectrum No. I. is given by the blue cone of the Bunsen flame, the temperature of which cannot be much above 1500° C., and is certainly less than 2000° C.; and the same spectrum (with the addition of two new groups of lines, and 6) is obtained at all temperatures up to that of the cyanogen-flame in oxygen, or probably 10,000° C. We have thus two quite different spectra, each of which has been shown to be due to carbon itself, and not to any compound of carbon, which are proved to be obtainable at the same temperature. In the case of the six different spectra of hydrogen described by Wüllner*, which are all obtained by the electric discharge in gas at different pressures, we may suppose the differences to be due to difference of temperature; but in the case of the carbon-spectra we are forced to some other explanation. It is worthy of remark that, while the spectrum CII. is obtained only by the use of electricity, the

* Pogg. Ann. vol. cxxxvii. p. 337. [Phil. Mag. May 1870, p. 366.]

spectrum CI. can be obtained both from flame and by the use of the electric spark.

The sodium-lines ß, y, 8, and a are seen in the spectrum of the electric light, of the spark of an induction-coil between sodium poles in air, both with and without the Leyden jar, and are obtained simultaneously with the hydrogen-lines a, ß, and y in a hydrogen vacuum-tube. The simplest mode of obtaining them is to heat the narrow part of the vacuum-tube with a Bunsen flame; the discharge inside the hot part of the tube becomes yellow and exhibits the sodium-lines brilliantly.

I have confirmed the results obtained by Wüllner in his experiments on hydrogen under pressure, and have pushed the pressure to nine atmospheres. The spectrum of the spark of an induction-coil (without condenser) in the gas at nine atmospheres' pressure is still far from being continuous. Ha is still a very bright and distinct line; Hẞ and y are merely maxima of light.

α

XIII. On Thermodynamics. By W. J. MACQUORN Rankine, C.E., LL.D., F.R.SS.L. & E., &c.

To the Editors of the Philosophical Magazine and Journal. GENTLEMEN,

IF

F I rightly understand the paper of the Rev. J. M. Heath, published in the Philosophical Magazine for July 1870, p. 51, he lays down a principle which may be virtually expressed by saying that the work done by a force in overcoming attractions or repulsions cannot take effect in producing heat,—that is, in accelerating molecular motions. That principle is perfectly correct, and is an obvious consequence of the laws of motion; and every one who knows those laws must agree with Mr. Heath when he states it. But from the remarks with which his statement is accompanied, he seems not to be aware that this very principle has been most carefully kept in view by every author of original researches in thermodynamics, and by every writer on the subject who has understood those researches. In fact the problem which is solved by the general equation of thermodynamics may be stated as follows:-A certain quantity of work being done by the action of external forces on a body in a certain way, to distinguish that quantity of work into two parts, one of which is expended in overcoming molecular attractions and repulsions, and the other in accelerating molecular motions. A system of particles contained within a vessel and in a state of rest, being kept in æquilibrio by their mutual attractions and repulsions, exerts a pressure or a tension against the internal surface of that vessel according as repulsions or attractions predo

minate; and work done in altering the capacity or the figure of the vessel does not produce heat, but only stored up energy, like that possessed by a bent spring.

A system of particles confined within a vessel, and not sensibly attracting or repelling each other, but in a state of motion, exerts outward pressure against the internal surface of that vessel through the reactions of the particles that tend to escape, but are prevented by the vessel from doing so; and work done in diminishing the capacity of this vessel wholly takes effect in accelerating the motions of the confined particles, that is, in the language of thermodynamics, producing heat.

The condition of actual bodies is compounded of those two; and it is by means of an equation deduced from what has been called the "Second Law" of thermodynamics, that the force exerted by a substance against the internal surface of a vessel containing it (in other words, the elastic force of the substance) is distinguished into two components, due respectively to molecular attractions and repulsions, and to the reactions of moving particles (of the nature of centrifugal force). It is the latter component of the force only that is taken into account in calculating how much heat is produced by a given alteration of the dimensions or figure of the containing vessel.

It has been proved by experiment that very nearly the whole of the work done in compressing a gas takes effect in producing heat; and hence it has been concluded that the elasticity of gases is almost wholly due to the motion of their particles, the component due to attractions and repulsions being small in comparison.

The detailed exposition of the principles to which I have briefly referred, and the comparison of their results with those of experiment, have been made so often, by so many authors, and in so many ways, that it would be a waste of time and space for me to explain them further here; and I shall therefore, in conclusion, merely refer to Professor Tait's work on Thermodynamics as the best source of information regarding the history and present condition of that science; for he gives a summary, in very moderate compass, of the different methods of demonstration followed by the various original authors. In most of the popular writings on the subject, the second law of thermodynamics, together with its proofs and consequences, is omitted, as requiring too much mental exertion for its comprehension.

I am, Gentlemen,

Your most obedient Servant,

Glasgow, July 5, 1870.

W. J. MACQUorn Rankine.