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It is lately stated that osmium may be obtained in crystals by the same means as those used for boron and silicon, but I have as yet seen no account of the form which it assumes.

Deville has furnished another interesting fact with respect to osmium, by determining the density of the vapor of osmic acid, which he has found = 8*88. This, if we take the generally received atomic weight for osmium, gives the atomic volume 131*6

=14*82, indicating a condensation to 2 vols. If we now

o*88

calculate back to the theoretical atomic weight we get (14*57 X 8*88)—32=97*38, a number closely approaching 97, which, as we have seen, brings the equivalent of osmium into simple and harmonious relation with those of the other elements of the arsenic group.

The specific gravity of fused metallic osmium having been lately determined by Deville =21*4, there can be little doubt that all the metals of the platinum family possess the same atomic volume when in the free state, about 4*6 or 4*7; the specific gravity of ruthenium is not yet known with accuracy, but such experiments as have been made render it improbable that it will prove an exception. This number is about one-fourth the mean of the at. vols, of the long recognized members of the arsenic group, but these latter differ so widely among themselves* that the comparison is of little or no value. It would be desirable to get a good determination of the density of osmic acid in the solid state, so that its at. vol. might be calculated and compared with that of antimonious acid.

The specific heat of osmium, so far as its value as a physical character goes, opposes the introduction of this element into the arsenic group. It has been determined by Eegnault ='03063; multiplying now by the equivalent 97, we have the product, 2*9711, thus placing osmium in the list of the elements (including the majority) for which the product of sp. ht. by at. wt. is nearly 3, while for phosphorus, arsenic, antimony and bismuth the product thus obtained is twice as great, or about *6. In this respect, however, osmium probably resembles nitrogen—the latter examined, as it necessarily is, in the gaseous form.

It is to be hoped that the conducting power for heat and electricity of compact osmium will soon be examined; nothing is as yet known of these characters.

31

* Phosphorus, - —— , , =16 94

r 1-83 {Schrotter)

16

Arsenic, .... ^{fferap(Uh) = 13-28
Antimony, .... TM =17 76

Bismuth, ^(j^r«12

Lastly, as regards the magnetic relations of the element—it is placed, with some doubt, by Faraday in the paramagnetic class; the metal and its protoxyd were found to act feebly in this sense, while pure osmic acid is said to have shown itself clearly diamagnetic. The strongly diamagnetic character of phosphorus, antimony and bismuth would render a re-examination of this point interesting. Arsenic, however, is said to be very feebly diamagnetic, and is placed by Faraday close to osmium in the list of metals examined, though on the opposite side of the line of magnetic neutrality or indifference.

Reviewing, now, the united physical and chemical characters of osmium, and comparing them with those of the generally recognized members of the "arsenic group," we are, I think, justified in concluding that here this curious metal should be placed in a natural arrangement of the elements—while important distinctions seem to separate it from some, at least, of the platinum metals, with which it is usually associated and described.

Tuscaloosa, Ala., Nov. 1, 1889.

Art. TIL—The Comas and Tails of Comets; by Prof. W. H. C. Bartlett, U. S. Military Academy at West Point.

Comets have, at all times, been objects of curiosity and wonder; and the question in regard to the nature of their luminous appendages, has exercised the speculative ingenuity of philosophers from the earliest records of astronomy. Everything written about them is read with interest, and the most extravagant theories in respect to their constitution and the laws of their being find a ready favor with the public. They are still among the enigmas of the heavens. Among the recent and remarkable efforts at solution, is one by the ablest mathematician of the country, perhaps of the age: and granting the premises, there is no avoiding the conclusions of the comprehensive and searching analysis for which this eminent man is so remarkable. But the assumption, that the attractive energy which summons a comet from the depths of space to the presence of the sun, retains its nature unchanged and strengthens with the diminution of distance for a part of the approaching mass, and yet reverses its character and becomes repulsive for another part, in order to obtain material to build up the tail, appears so unsupported by the analogies of nature as to give to his results the taint of improbability. Indeed, a theory which demands such an exercise of faith in matters of science, and from such friends, can only inspire doubt, and should yield the place it has too long occupied to some other, founded in better ascertained laws of matter. The question is not one of pure mathematics, but of physics.

The material elements of all bodies of which we have any knowledge, are united by some conditions of aggregation, determined by the reciprocal action of molecular forces; and the circumstances of their relative motion will come from the equation

in which m is the mass of an element, xyz its coordinates of place, XYZ the sums of the components of impressed accelerations in the direction of the axes xyz, respectively.

The conditions of aggregation may be expressed in some functions of the coordinates of molecular places. As three coordinates determine the place of a single molecule, there will be three times as many coordinates as molecules; and if i" be the number of molecules and 1 the number of equations that give the conditions of aggregation, then will 3,« — i=n be the number of coordinates which, if given, would reduce the number of unknown coordinates to the number of equations. These unknown coordinates could then be found, and the places of the molecules at the corresponding instant determined.

Denote the 1 coordinates by xyz, x'y'z', &c., and the n coordinates by afty, a'P'f, &c. The former of these coordinates, as also the forces, may be expressed in functions of the latter, and both eliminated from the general equation of motion. And if f V t, £' v' t't &C-) be the increments of « 0 y, <*' 0' f, &c, at any instant and due to any transmitted initial disturbance, it is easily shown that

? =:^:R.N?.sin(/.Ve-»-).

?i = J£R.N„.wn (<.y?-r),

£ =^:R.Nrsin (t.*/j-r),

£'= <fec. <kc. <tc. In which there are n terms comprehended by the sign 2, and in which 9 will, in general, have different values from one term to another. When these values of p are real and positive, the different terms in the values of £ i £, &c, will disappear periodically, the precise times of disappearance being given by t.\fit— r=an; t'ts/if> r'=a!n; <fec, &c.

or t= ~-; t' = J—; &c, &c.

in which o is any whole number. The intervals of disappearance will be

z±r. z±r!. &c. &<..

and if these intervals be commensurable, all the terms will disappear simultaneously, and &c., reduce to zero, at equal intervals of which the duration is 7i + r n + r'

V? V?'

The vast atmosphere of ether which pervades all space is ever busy transmitting luminiferous waves from the sun and other heavenly bodies. Its molecules are ever on the move with velocities and in orbits determined by the relative places and intensities of existing wave sources. Any cause which will perturbate the etherial molecules of any limited volume of ether from these orbits, regarded as initial, and by the quantities f v £, {' i?' &c, will make such volume self-luminous; and if the perturbation be great enough, it will be visible from all directions.

Comets are known to exist in a state of great tenuity, their densities being almost insignificant in comparison with that of the fleecy clouds that float in the upper atmosphere. The luminiferous waves from the sun, entering such bodies with great' ease, their intromitted greatly preponderate over their reflected components. The former of these components modify and determine the internal motions of comets, and make them self-luminous. The internal cometary elements become so many centres of disturbance. They throw their waves in all directions, and are simultaneously sources of molecular perturbations to the surrounding ether, each giving rise to a term R.N-Sm (t-*/§ y), in the general value of the perturbating functions £ i? £, &c, and thus making the ether also self-luminous. The degree of illumination will vary with the maximum values of the perturbating functions. These will result, in any case, from the extent of the initial disturbance and the distance, at right angles thereto, over which the disturbance may have been propagated; decreasing, according to the principle of wave divergence, as the square of this distance increases. The components of the initial disturbance perpendicular to the line drawn from the comet to the sun, is, from the principle of wave propagation, much greater than in any other direction; and hence the much greater extent of the illumination on the side of the comet opposite the sun. The comet's head can have no phases, from its self-luminosity; neither can the coma and tail have sharply defined outlines, from the gradual degradation of molecular perturbations towards their borders.

This view denies the presence of cometary material in the coma and tail altogether, and regards these appendages but as phenomena due to the reciprocal action of the etherial and cometary molecular forces. According to it, the coma and tail become, as it were, a luminous shadow, a part of which is literally "cast before," and the dark cap which envelopes the head and stretches away through the tail, a region of wave interference. No wonder, then, that comets turn their tails from the sun, and, at perihelion, whisk them, though of enormous lengths, through celestial arcs well nigh equal to a semi-circumference, in a few hours. This is no more surprising than that opaque bodies throw their shadows from the luminous sources whose light they intercept. The curvature, which is so remarkable a feature in the tail, is but the simple effect of the comet's orbital velocity, and the progressive motion of light.

If the principles here cited be well founded, then will the zodiacal light find an easy solution; and the great oblateness of its spheroidal figure must be taken as evidence that the component molecular motions in the sun are greater in the direction of the solar axis than in any other.

West Point, Oct 25th, 1859.

Art. VIII.—On Sodalite and Elaeolite from Salem, Massachusetts; by J. P. Kimball, Ph.D.

For a knowledge of this locality of the occurrence of the two rare silicates, sodalite and elaeolite, we are greatly indebted to Gilbert L. Streeter, Esq., of Salem, as well as to several other gentlemen of the same city. Fortunately, Mr. Streeter very carefully observed their mode of occurrence, and, together with G. F. Cheever, Esq., and Kev. S. Johnson, Jr., collected choice specimens of them. The best of these are in the possession of the Essex Institute, Salem, to the curator of which, Dr. Henry Wheatland, I owe in a great measure the privilege of examining them.

The locality in which the minerals were found is "a pit or quarry, a short distance below the Almshouse upon the road passing along the northern side of the Neck, towards Hospital Point."* They were first noticed in a "block of compact syenite resting upon the bank, the end of which presented a beautiful coloring of blue and greenish white, with specks of black. Upon examination these conspicuous minerals were seen to be in a vein, a portion of which was connected with the block of syenite."f

Mr. Streeter subsequently discovered what undoubtedly was the continuation of the same vein. This traversed an erratic block of the same rock, imbedded in the drift, of which the small block, just mentioned, was a fragment. The vein is described to have been about six feet in width, and to have dimin

* G. L. Streeter: Essex Institute Proceedings, ii, 153. f lb.

SECOND SERIES. Vol. XXIX, No. 83.—JAN., I860.

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