SCIENTIFIC

INTELLIGENCE.

I. CHEMISTRY AND PHYSICS.

1. Artificial production of polychroism in crystallized substances.— SENARMONT has communicated to the Academy of Sciences the results of experiments upon this subject which are very unexpected and important. The capital fact which the author has discovered is, in his own words, expressed as follows: "A coloring matter disseminated continuously in the interior of a crystal, between its layers of increment but absolutely foreign to its substance, chemically inert and capable of being spontaneously eliminated by successive solutions and crystallizations in pure water, may nevertheless communicate to it in the highest degree the properties of polychroism, and an energy of absorb ing action comparable if not superior to that of substances naturally colored in which it shows itself in the most decided manner." As a proof of the correctness of this assertion, the author exhibited large crystals of nitrate of strontia formed in a concentrated tincture of campeachy wood rendered purple by a few drops of ammonia. In these crystals white light developed by transmission under certain incidences, a red color, and under others a blue or violet. Observed with a doubly refracting prism the crystals gave two images, the one red and the other dark violet, according to the thickness, and these images exchanged their colors, passing through identity, as the crystallized plate was made to turn in its own plane. Two similar and perfectly transparent laminæ superposed with a parallel orientation allowed a portion of the incident white light to pass with a purple color; superposed with a right angled orientation, they arrest the light like tourmalines, or at least reduce it to a violet shade so obscure that we may consider the light as extinct. Finally we may detach from these crystals perfectly pure and homogeneous plates slightly inclined to the optic axes. By placing such a plate very near the eye and using white natural light, we see alternately in the direction of each of the axes, a brilliant orange spot traversed by a hyperbolic branch. These open to the right and left of the principal section under the form of curved brushes composed of two equal parts of violet and sombre blue and dividing the field of the plate into two regions in which the purple tints regularly degenerate on both sides of their common limit. The dark tufts interrupted by the luminous spot are moreover fringed towards the point with a little yel low and blue, colors which are altogether local, and which arise mani. festly from the dispersion of the optic axes corresponding to the different colors. These phenomena are characteristic of polychroism in crystals with two optic axes, and perfectly similar to those which Brew. ster observed in Cordierite, and Haidinger in Brazilian Andalusite. The author obtained similar results with other coloring matters and other crystals, and promises more ample details hereafter.- Comples Rendus, xxxviii, 101, Janvier, 1854.

[Note. The results obtained by Senarmont clearly demonstrate that the existence of polychroism in crystals by no means necessarily implies their chemical homogeneity, since this polychroism may in any SECOND SERIES, Vol. XVII, No. 51.-May, 1854.

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case be produced by the mechanical admixture of foreign coloring maller. Is it not possible that the difficulty of expressing the composi tion of certain minerals by chemical formulas may arise from such an admixture of coloring matter, the optical characters of these minerals having hitherto entirely misled us?—w. G.]

2. Rate of transmission of impressions made upon the nerves.— HELMHOLTZ has communicated to the Physico-Agricultural Society of Königsberg a paper on the methods of measuring very small portions of time, and on their application to physiological purposes. The author alludes in the first place to the remarkable difference observed by astronomers between the observations of different individuals and termed by them the personal equation. The measurements of each observer agree well among themselves, but differ more or less by a constant quantity from those obtained by other persons. The apparatus of Siemens for the measure of the velocity of a musket or cannon ball is next described in its general features. This apparatus only dif fers from the common revolving cylinder and point now so well known in this country, by the arrangement of this point, which is here fixed, and presses against the surface of the cylinder: when an electric spark passes from the point to the cylinder it leaves a fine dark spot. The passage of the ball establishes metallic connection so that the discharge takes place at the instant of this passage. In this manner the time during which the ball traverses a space of half a line may be measured. The author next alludes to the principle of the revolving mirror due to Wheatstone, and used with so much success by Fizeau and Foucault. Finally he gives an account of the method of Pouillet as modified and used by himself. This consists in causing the galvanic current to act upon an oscillating magnet, observing this magnet by Gauss and Weber's method, and determining the constant factor necessary to convert differences of oscillation into differences of time. In this manner accurate determinations could be made up to the Tooth of a second of

The physiological questions which the author sought to solve were these. In the transmission of intelligence, is a measurable time necessary for the ends of the nerves to communicate to the brain the impression made upon them; and on the other hand, is time required for the conveyance of the commands of the will from the brain to a distant muscle? By operating with the muscle of a frog severed from the body of the animal but connected with the nerves proceeding from it, the author found that the activity of the muscle is by no means instantaneous, but appears sometime after the excitation of the muscle, increases gradually to a maximum, and then sinks, first quickly, afterwards slowly, so that while the greater part disappears in about onethird of a second, the remaining portion requires several seconds afterwards. The object was to show that the different stages of activity of the muscle take place later when the excitation has to pass through a greater length of nerve, and this is actually the case. The most probable value of the velocity of propagation in the motor nerves of the frog was found to be 26-4 metres or about 80 feet per second. The results of the author's experiments upon the human subject were as follows: The intelligence of an impression made upon the ends of the nerves in communication with the skin is transmitted to the brain with

a velocity which does not vary in different individuals, nor at different times, of about 60 metres, or 195 feet per second. Arrived at the brain an interval of about 5th of a second passes before the will, even when the attention is strung to the uttermost, is able to give the command to the nerves that certain muscles shall execute a certain motion. This interval varies in different persons, and depends chiefly upon the degree of attention. It varies also at different times in the case of the same person. When the attention is lax, it is very irregular, but when fixed very regular. The command travels probably with the above ve locity toward the muscle. Finally about the 5th of a second passes after the receipt of the command before the muscle is in full activity. In all therefore from the excitation of the sensitive nerves till the mov ing of the muscle 14 to two-tenths of a second are consumed. [For more ample details and for much matter of a highly suggestive character, we must refer to the original lecture.]-L. & E. Phil. Mag., Nor., 1853.

3. Preparation of large crystals of sulphate of iodo-quinine for optical purposes.-HERAPATH has succeeded in obtaining crystals of this very interesting and valuable substance of sufficient size to be substituted for tourmaline in polarizing light. For the details of the process however, we must refer to the original paper.-L. & E. Phil. Mag., Nov., 1853.

4. On the law of induction in magnetic and paramagnetic substances.-PLUCKER has communicated an elaborate memoir on this subject from which we shall content ourselves with abstracting the summary of results. These are as follows:

(1.) In all magnetic and diamagnetic substances the same general law gives the intensity of the induced magnetism as a function of the exciting force. For each substance this law is particularized by the values of two constants. Of these two constants the one gives, as the inducing force vanishes, the ratio of this force to the induced magnetisin (constant of induction), and the second determines the resistance which prevents the induced magnetism from increasing proportional to the inductive force (constant of resistance).

(2.) For every substance there is a point of saturation to which it constantly approximates as the inducing force increases.

(3.) Diamagnetic substances (Bismuth, Phosphorus,) so far as the law of the intensity of their excitation is concerned, behave precisely like magnetic substances, though they exert a directly opposite action upon the inducing magnetic pole. This similar behavior compels us, in my view, to assume that the condition of a diamagnetically excited body is in itself in no wise different from the condition of a magneti cally excited body, that furthermore polarity is also present in the exci tation of diamagnetic substances, but that this is called forth by an induction which is the opposite to that which occurs in magnetic bodies.

(4.) The curves which represent the law of induction for diamagnetic substances are surrounded on both sides by the curves for magnetic substances. They show that the resistance which is opposed to the excitation of diamagnetic bodies is less than in most magnetic substances, but by no means vanishes; on the contrary it is greater than in oxygen and hydrate of oxyd of cobalt.

(5.) There can be no assumption of a specific magnetism in a substance in a general signification as we speak of specific gravity. Cobalt is precisely as magnetic as iron, when we use a definite magnetic force, which in intensity lies between that produced by one and that produced by two Grove's cups. Iron is more strongly magnetic with a greater, cobalt more strongly magnetic with a less magnetic force. If we compare the specific magnetisin of hydrate of oxyd of cobalt with that of hydrate of oxyd of nickel, we obtain numbers which differ from each other by the multiple 24, according as we use a single element or a battery of 16 elements. The specific magnetism (iron as the point of comparison) of the hydrate is with the last exciting force twice as great as with the first. For oxygen and bismuth this ratio descends to about 1.9 and 18. This partly explains the fact that while Faraday's estimate of the magnetism of oxygen, with a nearly equal force and similar mode of observation, corresponds well with my own, other phys icists find much smaller numbers for oxygen and bismuth.

(6.) The alternation between magnetic attraction and diamagnetic repulsion in mixtures of magnetic and diamagnetic substances, is completely explained by the change of the specific magnetism with the magnitude of the inducing force. When we mix 10 million parts of bismuth with 310 parts of iron, and fill a small flask with the mixture, we observe neither attraction nor repulsion if we apply a current which is twice as powerful as that which corrresponds to a single Grove's ele ment; with a less powerful current the mixture is attracted, with a more powerful one it is repelled. We can calculate the repulsion like the attraction for a given force. When we use a single Grove's cell, the attraction is nearly one-third as great as the repulsion which, with an equal force, would occur if the whole vessel were filled with bismuth. By employing a battery of 6 elements, we obtain in place of the attraction just determined a nearly equal repulsion. With a batte. ry of 16 elements we obtain finally a repulsion about ths of that which pure bismuth experiences with a single element. These results would appear yet more striking if bismuth were mixed with nickel instead of iron. They would make their appearance in the opposite order if the metal were mixed with hydrate oxyd of cobalt.

(7.) Those substances which oppose a lesser resistance to magnetization appear also more easily to retain the magnetism once received. At least we find it asserted of pure nickel that it retains no magnetism while one hydrate of oxyd of cobalt does this; the same is the case with oxygen, which as the author first remarked is attracted or repelled at pleasure by changing the poles of the electro-magnet when contained in an indifferent glass sphere.

In conclusion the author promises us a memoir on the nature of the coercive force.—Pogg. Ann., xci, 1, Jan., 1854.

5. On the laws of the attraction of electro-magnets.-DUB has published a continuation of his interesting and valuable researches on magnetic forces. Before stating however the results of this investigation, we must premise that the author distinguishes between magnetism, attraction, and sustaining power in electro-magnets. By magnetism he understands the magnetic excitation of a piece of soft iron by the gal vanic current; Lenz and Jacobi measured this by means of the induced

current excited by the vanishing of the magnetism, to which it is proportional. When a second bar of soft iron is caused to approach the first, this also becomes magnetic and by n-fold magnetism, n2 times the attraction is produced. As however the author's experiments have shown that in immediate contact the attraction is not proportional to the squares of the currents it is clearly necessary to distinguish between these two cases of attraction in contact and attraction at a distance. The author gives the following summary of his results:

(1.) The attraction of U-shaped electro-magnets with an equal num. ber of windings of the electro-magnetic spirals is proportional to the squares of the magnetizing current force.

(2.) The attraction of U magnets is, with equal currents, proportional to the square of the number of windings of the magnetizing spirals. (3a.) The attraction of U magnets is proportional to the square of the current-force multiplied by the square of the number of windings. (36.) The attraction and the sustaining force both of straight magnets and U magnets is proportional to the square of the current-force multiplied by the square of the number of windings.

(4.) The magnetism of massive cylinders of iron of equal length, which are magnetized by galvanic currents of equal force, and by spirals of an equal number of windings closely surrounding the core, is accurately proportional to the square roots of the diameters of these cylinders.

(5.) For the particular case in which the surface of contact does not disturb the result, the attraction and the sustaining force are, with equal magnetizing forces, proportional to the diameters of the bar or U mag.

nets.

(6.) The attraction of bar and U electro-magnets, with equal magnetizing forces, increases the nearer the whole of the windings are to the poles.

(7.) The attraction, like the sustaining force of U electro-magnets, ceteris paribus, remains the same whatever be the distance of the branches of the magnet.

(8.) The length of the branches of a U electro-magnet has no influence on its attractive or sustaining force if the windings of the spiral surround its whole length.

In addition to these laws the author has found that the attraction which a helix or spiral exerts upon a soft iron bar placed in its axis follows the same law as an electro-magnet, so that we have

(9.) The attraction of a spiral is proportional to the square of the magnetizing current multiplied by the square of the number of windings.-Pogg. Ann., xc, 248, 436, Oct. and Nov., 1853.

6. Identity of Niobium and Pelopium.-H. Rosé has communicated to the Royal Academy of Sciences at Berlin a memoir in which he admits the identity of these metals. The author alludes in the first place to the extraordinary difficulties which have accompanied his investigations from their outset several years since. In earlier memoirs the analogy between pelopic and tantalic acids had been pointed out, as well as the marked difference between pelopic and niobic acids. Continued researches have, however, completely established the fact that pelopic and tantalic acids are essentially different, while between