Page images
PDF
EPUB

But what is the state of the flame in the interval between two images? The flame of common gas, or of olefiant gas, owes its luminousness to the solid particles of carbon discharged into it. If we blow against a luminous gas-flame, a sound is heard, a small explosion in fact, and by such a puff the luminousness may be caused to disappear. During a windy night the exposed gasjets in the shops are often deprived of their light and burn blue. In like manner the common blowpipe-jet deprives burning coal-gas of its brilliant light. I hence concluded, that the explosions, the repetition of which produces the musical sound in the case before us, rendered, at the moment they occurred, the combustion so perfect as to extinguish the solid carbon particles, but I imagined that the images on the screen would, on closer examination, be found united by spaces of blue, which, owing to their dimness, were not seen by the method of projection. This in many instances was found to be the case.

I was not, however, prepared for the following result :— -A flame of olefiant gas, rendered almost as small as it could be, was procured. The 3-foot 2-inch tube was placed over it; the flame on singing became elongated, and lost some of its light, still it was bright at its top; looked at in the moving mirror, a beaded line of great beauty was observed; in front of each bead was a little luminous star, after it, and continuous with it, a spot of rich blue light, which terminated, and left, as far as I could judge, a perfectly dark space between it and the next following luminous star. I shall examine this further when time permits me, but as far as I can at present judge, the flame was actually extinguished and relighted in accordance with the sonorous pulsations.

When a silent flame, capable, however, of being excited by the voice in the manner already described, is placed within a tube, and the continuous line of light produced by it in the moving mirror is observed, I know no experiment more pretty than the resolution of this line into a string of richly luminous pearls at the instant when the voice is pitched to the proper note. This may be done at a considerable distance from the jet, and with the back turned towards it.

The change produced in the line of beads, when a tuning-fork capable of giving beats with the flame is brought over the tube or over a resonant jar near it, is also extremely interesting to observe. I will not at present enter into a more minute description of these results. Sufficient, I trust, has been said to induce experimenters to produce the effects for themselves; the sight of them will give more pleasure than any description of mine could possibly do.

and indicate alternate contractions and dilatations of the flame corresponding with the sonorous vibrations of the column of air."-Phil. Trans., 1834, p. 586.

LXIV. On the Natural Groupings of the Elements. By WILLIAM ODLING, M.B. Lond., Professor of Practical Chemistry at Guy's Hospital; Secretary to the Chemical Society*.

IN

PART II.

N the former part of this communication I discussed the correlations of the principal chlorous elements, proceeding from the most chlorous downwards. I propose now to commence at the other extremity of the scale, and in considering the purely metallic groups, to proceed in the order of their basylous characters.

GROUP V. Lithium, Sodium, Potassium.

The mutual resemblance of these three metals is admitted on all hands. Their chief points of association are the following. Their atomic weights occur in sequence, that of sodium being the mean of the three:

[merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small]

The three metals are soft, fusible, volatile, and of low specific gravity; they decompose water at all temperatures, and are readily combustible. Each metal forms only one class of salts. Nearly all the salts are soluble in water, and undecomposed by heat. Corresponding potassium, sodium, and probably lithium salts are isodimorphous; but the isomorphism of the group is not striking, inasmuch as the corresponding salts frequently crystallize with different proportions of water of crystallization. Very many of the salts present gradational properties thus the carbonate of lithium is very sparingly soluble in water, that of sodium moderately soluble, that of potassium extremely soluble. The specific gravities of the three metals, Li=0.593, Na=0·972, and K=0.865, are not sequential. The atomic volume of sodium approximates to the mean of the three. Thus

[blocks in formation]

The atomic heat of sodium and potassium is double that of most of the metals.

GROUP VI. Calcium, Strontium, Barium.

All chemists are agreed in their association of these three metals into one group, which, however, is usually made to include magnesium also. The principal points of analogy are, the relation of the atomic weights, that of strontium being intermediate between the other two.

[blocks in formation]

The saline atomic volumes of the three metals, as estimated by Kopp, present us with a similar sequence:

[blocks in formation]

The three metals are soft, readily fusible, decompose water at all temperatures, are easily combustible, and oxidize rapidly even in dry air. Each metal forms only one class of salts. The hydrates, sulphides, sulphydrates, chlorides, and nitrates are soluble in water; the carbonates, sulphates, borates, and phosphates are insoluble. The carbonates are all decomposed at a red heat. Of the three sulphates, that of calcium is the most soluble, then that of strontium, then that of barium. The nitrate and chloride of calcium are very deliquescent, those of strontium less so, those of barium not at all so. The corresponding salts are for the most part isomorphous. Thus BaCl. H2O is isomorphous with CaCl. H2O, and SrC1.3H2O with CaCl.3H2O. Witherite is isomorphous with calc-spar, strontianite with arragonite. Heavy spar, celestine and anhydrite, all crystallize in the right prismatic system, &c.

The metals of the fifth and sixth groups are the only ones that form decidedly soluble hydrates and sulphides. The soluble sulphates of lithium and sodium are associated with the insoluble sulphates of strontium and barium by means of the sparingly soluble sulphates of potassium and calcium. Of the three alkaline metals, however, lithium seems to approximate most to those of the alkaline earths. A curious isomorphism of calcium with potassium and sodium is exhibited between the nitrates and carbonates. We have

and

Na NO3 isomorphous with Ca2 CO3 in calc-spar,

KNO3 isomorphous with Ca2 CO3 in arragonite.

GROUP VII. Magnesium, Zinc, Cadmium.

These three metals are generally distributed in three separate groups, notwithstanding that many of their analogies are extremely well known. In this group, as in the preceding, we have the remarkable sequence of the atomic weights :

[blocks in formation]

The three atomic volumes, as determined by Kopp, present a similar sequence :—

Mg = 40
Zn = 58

179

[blocks in formation]

59.6 mean.

3

[blocks in formation]

The number for magnesium is the saline volume. Those for zinc and cadmium represent both the primitive and the saline volumes. All three metals are soft, white, readily fusible, malleable, and crystallize in octahedra. Magnesium is the least volatile; zinc, from its volatility, is prepared by distillation; while cadmium is one of the most volatile of all the metals. Unlike the metals of the two preceding groups, the metals magnesium, zinc, and cadmium are all permanent in dry air, and acquire only a film of oxide in moist air. All three are combustible at a high temperature. All the metals of this group are remarkable for the facility with which they precipitate other metals, iron, copper, and tin for example. Magnesium decomposes boiling water, though very slowly; zinc requires a slight acidulation, cadmium a stronger acidulation. Each metal forms only one class of salts. The salts are for the most part colourless, like those of the two former groups. The three oxides are insoluble in water. The three hydrates, MgHO, ZnHO, and CdHO, are insoluble in water but soluble in ammonia. The three sulphides are insoluble in water. Sulphide of magnesium is soluble in acetic acid,-of zinc in very dilute chlorhydric acid, -of cadmium in stronger chlorhydric acid. Sulphide and sulphydrate of zinc correspond with magnesian sulphide and sulphydrate in having a white colour. The sulphides of zinc and magnesium are the only two insoluble sulphides that are white. The

corresponding salts are for the most part isomorphous, and present great analogy of properties. The magnesian salts are most neutral, then those of zinc, then those of cadmium. The three chlorides are deliquescent. The hydrated chlorides are decomposed by heat, hydrochloric acid being liberated,-most readily in the case of magnesium, less so in that of zinc, least of all in that of cadmium. The three sulphates are soluble in water, and are decomposed by heat,-that of cadmium most readily, then that of zinc, and that of magnesium only at a white heat. The sulphates of magnesium and zinc are isomorphous, Mg2 SO4.7H2O with Zn2 SO4.7H2O. The sulphate of cadmium crystallizes with four and eight atoms of water. The double sulphates of magnesium, zinc, and cadmium, with potassium, or sodium, or ammonium, are isomorphous,-MgKSO4. 6H2O, Zn KS2 O1. 6H2O, and CdKSO4. 6H2O. The crystalline ammonio-phosphate of zinc corresponds in composition with the ammonio-phosphate of magne.. sium dried at 100°C.; Zn2NH4 PO4. H2O with Mg2 NH4PO4. H2O. All three carbonates, borates, and phosphates are insoluble in water but soluble in chlorhydric acid. The respective nitrates and chlorates of magnesium and zinc correspond,

and

MgClO3. 3H2O with ZnCIO3. 3H20,

MgNO3.3H2O with ZnNO3. 3H2O.

Nitrate of cadmium crystallizes with two atoms of water. Zinc hydrate differs from the magnesian and cadmic hydrates in being soluble in potash. Moreover, cadmic oxide and sulphide differ from the magnesian and zincic oxides and sulphides in colour.

The members of this seventh group, more particularly magnesium, present some general relations to those of the sixth, but not in any way sufficient to warrant the usual disposal of magnesium in the calcic rather than in the zincic family. Magnesium differs from calcium and agrees with zinc in the character of its metal, of its hydrate, of its sulphide, of its sulphate, and of its hydrated chloride. Magnesite is isomorphous with bitterspar, calc-spar, and calamine. Moreover, the constitution of bitter-spar, Ca M CO3, tends rather against, than in favour of the association of calcium and magnesium, as in mineral chemistry we rarely have legitimate double salts of two metals belonging to the same natural family. In the solubility of their sulphates, the members of this group correspond rather with those of the fifth than of the sixth group.

These three groups present a very curious relation in their atomic weights :

« PreviousContinue »