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that the spectrum appears to be totally changed when the mass of vapour is altered." I suppose, too, we may now add magnetism as capable of effecting a change in certain spectra—though this subject is at present new and only partially explored.

Thus far I have given the results of some observations made with a view to test experimentally Professor Ångström's conclusions on the aurora-spectrum. It will no doubt be remarked by some who look at my drawings, that I have obtained a number of negatives, but that there is no positive result anywhere. I quite admit this, and am disappointed in so doing; but then it must be remembered that one way of finding out what a thing is, is to find out what it is not. I venture to think, however, we may not consider the time entirely thrown away, and that, although no one examined spectrum complies with all the auroral conditions, we may still usefully examine the set in regard to their relative proximity thereto.

Tested by coincidence or close proximity of lines to those of the aurora we may arrange the spectra in the following order :— 1, iron; 2, air-spark; 3, hydrogen; 4, air-tube spectrum; 5, phosphoretted hydrogen; 6, carbon and oxygen.

The air-tube spectrum might perhaps stand higher in the scale but for its broad bands, which make comparison doubtful. Lines of oxygen possibly escape detection in the aurora from the faint character of its spectrum.

The phosphorus and iron spectra are specially interesting in connexion with Professor Nordenskiöld's "metallic and magnetic cosmic dust in the polar regions" (see Phil. Mag. vol. xlviii. No. 321, page 546).

If asked to give an opinion in the present state of our knowledge of the aurora question, I should say:

1st. That the yellow-green line, and possibly also the red, are due to phosphorescence or fluorescence. 2nd. That the fainter lines are partly due to the air-spectrum (but not specially the violet-pole), in which H lines probably play a prominent part, and the spark-spectrum appears nearer the mark than the tubespectrum; and that the remaining bands or lines may be due to phosphorus and iron (the close coincidences in this latter spectrum with the aurora-lines being certainly very striking). All these spectra seem to me to claim special and further attention; and to them I would add that of OH, in relation to aurore of a dense and misty character.

For measuring auroral and other lines, a cheap and very effective micrometer is constructed by making the whole slit-plate of the spectroscope (and consequently the spectrum itself) traverse the field with a fine micrometer-screw, a pointer or pointers being fixed in the eyepiece. My early observations were made with Phil. Mag. S. 4. Vol. 49. No. 325. April 1875.


one of Browning's larger direct-vision spectroscopes fitted in t way. Large aperture, however, both of prisms and lenses, almost indispensable in observations of faint auroral spectra. close this by adding a few remarks to Professor Herschel's list auroral lines (appended to his letter), which may be useful other observers.

Remarks by way of addition to Professor Herschel's list of auroral lines*.

1. Red line.-On my scale coincident with telluric group in solar spectrum. I found at the same time a faint tellu band or group coincident with the yellow-green line.

2. Yellow- or citron-green line. I always see this more p green than yellow; sometimes flickering and changing in tensity (Professor Herschel has also noted this on one occasio Not always sharp. Procter has recorded it nebulous; might its appearance in regard to width, intensity, and sharpness made a test of height of the aurora?

3. Greenish-blue or blue lines.-One cannot help suspecti difference of position as well as of intensity of lines in the cent portions of aurora. We know how varying aurora are in gene character and appearance; and although no doubt much must allowed for hasty observation, often made with imperfect inst ments so far as measuring is concerned, there still seems a re duum of considerable difference in the position of lines to accounted for. Lines Nos. 1 and 2 are closely fixed; but would counsel special attention to the places of the fainter line and it might be useful to put No. 2 out of the field, or hide by a bar while observing these.

Indigo and violet lines, Nos. 5 and 6.-As mentioned befor No. 5 seems most indefinitely positioned; No 6 I have new been fortunate enough to see.

XXX. On Salt Solutions and Attached Water.


[Concluded from p. 218.]

Chlorides of the Alkaline Earths.

§ 78. CHLORIDE of Barium.-The chloride of barium saturated solution becomes a solid cryohydrate -8° C. Of the portion last to solidify, 6-6790 grms. gave, d evaporation and heating to 350° C., 1-5490 Ba Čl. This co responds to 23.2 per cent. Of a previous crop of crystals, 6·135

*See also English Mechanic,' January 16th, 1874, No. 460.

grms. gave 1.4735 Ba Cl2, or 24.0 per cent. The first of these shows the formula

Ba Cl2+37.8H, O.

§ 79. Chloride of Strontium.-This salt forms a cryohydrate at -17°. Of the final portion, 5.8325 grms. contained 1.6085 of SrCl2, or 27.57 per cent. The crop before this showed 1.3655 grm. of salt in 4.9675 solution, or 27.5 of Sr Cl2. We may therefore deduce the formula

Sr Cl2+22.9H2O.

Nitrates of the Alkaline Earths.

§ 80. Nitrate of Barium.-The cryohydrate of this salt solidifies from a saturated solution at -0°.8. I have only one determination of its composition; namely, 3.0370 grms. gave 0.1610 of anhydrous salt, which corresponds to 5.30 per cent., and the molecular ratio

Ba 2 NO+259.0 H2O.

§ 81. Nitrate of Strontium.-On cooling a saturated solution to -6° the cryohydrate solidifies. Of the last portion, 6.8010 grms. gave 1.7675, or 25.99 per cent. of Sr2 NO. Of the previous crop of the cryohydrate, 46715 grms. contained 1.2105 of the anhydrous salt, or 25.91 per cent. These determinations concur towards the relationship

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§ 82. Sulphate of Iron.-Green vitriol separates from a saturated solution at -2°.2. The next to the last portion of the cryohydrate showed the following composition:-4-7000 grms. gave 0-8155 FeSO4, or 17.35 per cent. Of the last portion, 4.8140 grms. gave 0.8155, corresponding to 16.94 per cent., and showing the ratio

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Although the solution of this salt was quite clear when hermetically sealed for analysis in a tube containing not more than 5 cubic centim. of air, a considerable deposit of sesquioxide took place. The analysis, therefore, is probably not quite exact.

$83. Chromate of Potassium.-The cryohydrate solidifies at -11°. Of the last crop of crystals, 3.0690 grms. gave 1·1145 anhydrous salt, or 36.27 per cent. Of a previous crop, 5.4775 grms. gave 1.9945, or 36 41 per cent. The first indicates the formula

K2 CrO4+18.8 H2O.

From § 28 the bichromate has a cryohydrate solid at -1° and

is the brightest of the violet-pole group, and which represents a medium-intensity band in the capillary spectrum. After this is a faint band near a, representing two rather bright ones in the capillary spectrum, this last being succeeded by other bands in the violet. a, B, and y in the violet-pole were examined carefully for relative brightness, and are, I believe, correctly marked.

The red- or positive pole was next examined, but presented no peculiar features. It appeared as a fainter representation of the capillary air-spectrum with some few lines or bands absent; and, as will be seen after, it is also a fair representation of a diffused air-spectrum (see Plate V. spectrum 3).

Examined for comparative intensity, at 24 inches from the slit, the whole capillary air-spectrum showed faintly-the marked lines in the centre of the spectrum generally retaining their prominence; but after a I judged e next in brightness. On examining the violet pole at 12 inches from the slit, the whole spectrum was faint, and the bands a and B were alone distinctly seen. Next to the Geissler air-tube I examined an 66 aurora "-tube, about 15 inches long and 14 inch across, with platinum terminals and of the same diameter throughout. The discharge was of a rosy red colour; and the long flickering stream from pole to pole certainly much reminded me optically of an auroral streamer. Spectroscopically examined, the discharge presented a faint banded air-spectrum similar to that of the positive pole (see Plate V. spectrum 4); but the relative intensity of the lines was somewhat altered, while a very bright line in the green (seen also in the tube next described) was characteristic of the spectrum, and in this respect distinguished it from the ordinary airspectrum.

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Following this last tube I examined one purchased as "phosphorescent. It was rather short (6 inches), of equal calibre, and about the size of the bulb of a Geissler tube. It was filled with a white powder (probably one of the Becquerel compounds). On passing the current between the electrodes a bright rosecoloured stream appeared; and wherever this was in contact with the powder, the tube glowed with a brilliant green light. On stopping the current the tube still continued to shine, but with a fainter green glow, which gave only a continuous spectrum. When examined in full glow, the tube-spectrum was also in the main eontinuous and of a green tinge; but upon it were lines or bands in the blue and violet portions of the spectrum (which I have not yet worked out), while in the red, yellow, and green a faint but distinct air-spectrum was seen; and with this was also found the same bright line in the green which distinguished the << aurora "-tube.

I next took a 1-inch spark in air between platinum terminals

(see Plate V. spectrum 6). The principal lines in this spectrum were the line a (by far the brightest) corresponding to y in the violet pole; next was ẞ, a line in the yellow not appearing in the tube-spectrum, and then other lines of less intensity. In the "aurora" and "phosphorescent" tubes was found, as before mentioned, a line prominent for its brightness, and, indeed, in the “aurora "-tube the only one which survived when it was moved away from the slit. This line also appears in the spark-spectrum, but there only of an average brightness. I examined it carefully for position in the respective tubes, and on comparing them found it coincident with the ridge or centre of the wedge-like bright green broad band which is so conspicuous in the air-tube spectrum.

I think this edge-like centre has actually a line coincident with the line I refer to ; but if so, its intensity little exceeds that of the band itself.

To complete the set of air-experiments, I examined the same spark taken from the surface of a small meniscus of water placed upon the lower platinum wire. In this case the air-spectrum was plainly, but not brightly, seen at the violet end of the spectrum; the red, yellow, green and blue being filled with a continuous spectrum through which some of the air-lines faintly showed (see Plate V. spectrum 7). I reserve the remarks on the position of the air-lines as compared with those of the aurora till later.

Phosphoretted-Hydrogen Flame.

This was obtained from an ordinary hydrogen-bottle fitted with glass tubing, two or three minute pieces of phosphorus being placed with the zinc. The flame was of a bright yellow colour with a vivid cone of green light in its centre.

The spectrum was found to consist mainly of three bright bands in the yellow, green, and green-blue respectively (see Plate VI. spectrum 1).

The centre band was very striking in its emerald-green colour, while all the bands were remarkable as being very broad in proportion to the slit (which, however, was not fine). The yellow band had also a rich glow of colour. My spectrum was mapped out at ordinary temperature, and I found the bands sufficiently bright; but Mons. Lecoq de Boisbaudran, in his 'Spectres lumineux' (Texte, p. 188), has described how the brilliancy of these bands is increased when the flame is artificially cooled (refroidie). He also makes the important remark that the relative intensities of the bands are in such case altered, adding :"La plus importante de ces modifications consiste en un ren

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