APPENDIX TO NOTICE OF SCHOENBEIN.

[The phenomenon called passivity of metals mentioned in the foregoing eulogy as one of the discoveries of Schoenbein, consists in the fact that iron, for example, which, under ordinary conditions, is readily disolved in nitric acid, may while in a peculiar state remain for weeks in the same liquid without being acted on. This phenomenon is, without doubt, due to a galvanic action, which, when the iron is first plunged into the liquid deposits a coating of oxide which protects the metal from the further action of the acid. To illustrate this let a piece of clean iron wire be immersed in strong nitric acid together with a slip of platinum, the former being introduced first and the two connected with the ends of the wire of a galvanometer, a powerful current will be inducted at the first completion of the circuit, the iron acting as the positive metal, but the strength of this current will quickly decline to a small amount and then remain constant for several days. The iron thus treated is no longer attacked when plunged alone into nitric acid, and is said to be passive. Instead of using a galvanometer, which was merely introduced to prove the existence of a galvanic current, the same effect will be produced by touching the iron wire while the acid is acting on it with a piece of gold or platinum, also immersed in the liquid, the action will immediately cease and the iron become passive. When an iron wire in the passive state is plunged into nitric acid and the upper end touched with another iron wire, as soon as dipped into the acid the latter also becomes passive. In these experiment, the iron, which is rendered passive, acts as the zinc element of a galvanic pair, and is rapidly covered with an oxide which protects it from further action except of a very feeble character. In this state it may serve as the copper or negative metal of a galvanic pair, and really performs this part in the second experiment in which a galvanic couple is formed by the contact, while in the acid, of the passive and non-passive iron. The formation of an oxide sufficiently thick to protect the iron is not produced in nitric acid, of ordinary strength, unless a galvanic arrangement such as we have described is adopted, but if it be plunged into very strong acid the action, though violent for an instant, will soon cease, and the metal assume the passive condition. If this wire be withdrawn from the acid and exposed to the air for a short time, or rubbed with sandpaper, it will resume its ordinary state. An iron wire may also be rendered passive by holding it for a few minutes in the flame of a spirit lamp.

Other metals—namely, silver, copper, tin, aluminum, and especially bismuth, may be brought into the passive state by the methods we have mentioned, but the effect is not as marked as with iron. Dr. Hare constructed a galvanic battery in which the platinum was represented by iron in the passive state, but the action was capricious; though at one moment powerful at another it became almost nothing.

Another discovery of Schoenbein, mentioned in the foregoing sketch, is that of gun-cotton, a very explosive substance, produced by steeping cotton-wool in fuming nitric acid, or in a mixture of nitric and sulphuric acids, afterwards washing and drying the product. This discovery was announced by Schoenbein in 1845, but the mode of preparation was kept secret. It was, however, soon rediscovered independently by Bettger and Otto, while Kopt improved the process of production by the addition of sulphuric acid to the active liquid. From the first it was proposed as a substitute for gunpowder, over which it possesses the advantages of burning without smoke, and leaving no residuum to foul the chamber of the cannon. Large establishments were erected for its preparation, but the

occurrence of several severe accidents during its production, attended with great loss of life, caused it to be regarded as too dangerous for military purposes, and, accordingly, its manufacture was for a time almost abandoned. Within the last few years, however, the attempt to make use of it as a substitute for gunpowder has been renewed and brought to a successful issue by an Austrian officer of artillery.

Gun-cotton is used in military operations in the form of a spun yarn, in which it conducts combustion slowly in the open air at a rate of not more than one foot per second. This yarn is used to form cartridges for large guns, by being wound round a bobbin, so as to form a hollow spindle and thus give an interior surface for the action of the flame and the production of the most effective explosion. The effect of the explosion of gun-cotton under water is remarkable; the action is so instantaneous that the water has no time to yield, and consequently transmits the impulse as a solid material; hence it is unnecessary to place the charge in immediate contact with the body to be destroyed. In one experiment two parallel rows of piers, 10 inches thick, in water 13 feet deep, with stones between them, were blown to pieces by a barrel of 100 pounds of gun-cotton, placed at a distance of three feet from one side and eight feet under water. It made a breach of 15 feet, and threw the water to a height of 200 feet. In another experiment with 400 pounds of gun-cotton a vessel was blown up, the pieces projected into the air to a height of 400 feet, and the fishes for nearly half a mile around were so stunned as to float on the water. The rapidity of expansion and great elastic force of gun-cotton renders it a valuable agent in blasting. Its power when exerted against a great resistance, as in the case of splitting a rock, when compared with that of gunpowder, is in the ratio of 64 to 1.

The discovery which has rendered the name of Schoenbein most extensively known is that of ozone. Before the end of the last century Van Marum, of Holland, had observed that when an electric discharge was passed through oxygen the latter acquired a peculiar smell and the power of attacking mercury, but it was not until 1840 that any notice was taken of these facts, when Schoenbein published his first paper on ozone. In this he announced the fact that in the decomposition of water, by means of a galvanic battery, an odorous gas was given off at the positive pole, and that this might be preserved for a long time in a well-closed vessel. He also pointed out the fact of the similarity of this odor to that which accompanies a discharge of electricity, especially from points, and also the slow oxidation of phosphorus. Opinions as to the cause of the odor were long divided, but through the experiments of Schoenbein and the investigations of Andrews, and Tait, and others, it is now generally referred to oxygen in a changed or allotropic condition.

One of the simplest methods of exhibiting the production of ozone consists in transmitting a current of oxygen through a glass tube, into the sides of which a pair of platinum wires have been sealed, with their points a small distance apart. On connecting one of these wires with the prime conductor of an electric machine, in active operation, while the other is connected with the ground, the odor of ozone is immediately perceptible in the stream of gas. But in order to produce a maximum effect it is necessary to transmit the discharge silently in the form of a brush or a star, since sparks appear to produce an opposite effect, and are, therefore, to be avoided. Ozonized air may also be obtained by placing a stick of clean, moist phosphorus in a bottle of air or oxygen, when, after an hour or so, the smell of ozone will be obvious. The stick of phosphorus is then to be taken out and the gas washed with water to remove the phosphorous acid. Or ozone may at once be produced by plunging a heated glass rod into a mixture of air and a vapor of ether. The galvanic decomposition of water acidulated with sulphuric acid, or better, perhaps, with the addition of chromic acid, affords at the positive pole a large supply of ozone. The general characteristics of ozone are those of an oxydizing agent; it corrodes organic matter, as shown in

its energetic action on the caoutchouc tubes through which it is conducted. It bleaches most vegetable colors; it changes the black sulphide of lead into white sulphate, the yellow ferrocyanide of potassium into the red ferrocyanide. It oxydizes moist filings of iron, copper, mercury, and silver. In some cases, however, ozone acts as a deoxydizing agent. It decomposes peroxide of iron and barium. It exists in variable quantities in the atmosphere, and its presence is indicated by what is called ozone test-paper, namely, paper steeped in iodine of potassium, which is rendered brown by the liberation of the iodine. If starch be added to the solution in which the paper is steeped the ozone produces a blue color; but according to some authorities this test is not as reliable as that of the solution of the iodide of potassium alone. As ozone is an energetic oxydizing agent, it combines with animal matter and other impurities in the air, and hence its absence, as evinced by the want of coloration in the test-paper, is considered as an indication of the presence of malaria in the atmosphere of the locality in which such indications are observed. It is evident from what has been stated that ozone must be produced in the atmosphere by electrical discharges, but whether it exists from other sources in the air is at present unknown. Neither are the test-papers we have mentioned decisive proofs of its relative quantity, since there are other substances generally present in the air which are competent to produce similar effects.

One of the most plausible hypotheses as to the nature of ozone is that of Clausius, who considers all gases, whether simple or compound, as made up of a num ber of atoms combined together to form molecules. That, for instance, a molecule of oxygen consists of at least two atoms, and that it may happen that a portion of each of the great number of molecules which exists in a given quantity of oxygen can be decomposed into two atoms which distribute themselves in their separate state among the remaining undecomposed molecules, and that these isolated atoms, which in their relations to foreign bodies must differ from the molecules of ordinary oxygen, constitute ozone.

In accordance with this hypothesis, the production of ozone by passing electricity through oxygen or atmospheric air may be attributed simply to the repulsive power of the electricity by virtue of which the two atoms of oxygen, being charged with the same kind of electricity, are driven apart, as in the case of the well known experiment of two pith-balls. When oxygen is evolved in the decomposition of water, a similar repulsive separation takes place at each pole or electrode, but most of the atoms immediately combine again upon the electrodes to form ordinary oxygen. A small portion only of the atoms remain in a separate condition, and these constitute the ozone with which the oxygen is mixed. Finally, in the case in which ozone is developed during the oxidation of phosphorus in moist air or oxygen, we may suppose that the atoms which make up the oxygen molecules are in different states or degrees of electricity; that one of these tends more energetically to combine with the phosphorus than the other; and that the latter, removed from the sphere of its attraction by the heat generated in the combination of the former, remains in an isolated condition. The fact that these atoms do not immediately recombine into molecules to form ordinary oxygen may be due to their similar electrical state. When ozonized air is heated, the ozone disappears, because the high temperature determines the union of the atoms as it does of hydrogen and oxygen in the application of a flame to a mixture of these two gases. It has been found that the ozonification of oxygen by the electrical spark or brush can only be carried on to a certain extent if the ozone remain mixed with the oxygen; but it the ozone be removed as rapidly as it is formed by the oxidation of silver, all the oxygen may be gradually converted into ozone. In this case, when the number of separate atoms become too great in a given space, they are brought within the sphere of mutual attraction; combination ensues, and the ozone dis

appears as fast as it is produced in the reproduction of ordinary oxygen. The power of combination with metals and other bodies exhibited by ozone becomes a consequence of this hypothesis, inasmuch as separate atoms must from analogy have more combining power with foreign bodies than atoms which are already in combination with each other.

The hypothesis of Clausius is very suggestive, and with a few supplementary assumptions can be made not only to render a plausible explanation of known phenomena, but also to indicate new experiments. It must be stated, however, that it is at variance, as presented in the foregoing sketch, with the experiments of Soret on the density of ozone. This chemist finds, from an elaborate investigation, that when ordinary oxygen is converted into ozone its density is increased instead of being diminished, as it should be, according to the hypothesis of Clausius. This result was arrived at by two different methods, that of absorption and that of diffusion. Both gave approximately the same result, from which it appears that the density of ozone is one and a half times that of oxygen. According to the hypothesis of Clausius a molecule of oxygen consists of two atoms, and may be represented by 00, while an atom of ozone would be indicated by 0. From this it is evident that the density of ozone should be only onehalf of that of oxygen. In order to make the hypothesis of Clausius agree with the result obtained by Soret, we must suppose that while an element of oxygen consists of two atoms, and is represented by 00, an element of ozone consists of three atoms represented by 00,0; that when by electrical repulsion or other action, the two atoms of oxygen are separated, one of them immediately unites with a molecule of ordinary oxygen and the other to a second molecule, forming two molecules of ozone out of three molecules of oxygen. Or, in other words, by the decomposition of one of three molecules 00 00 00 of oxygen, and recomposition with the remaining two, we shall have two molecules 00,0 00,0 of ozone. It is not necessary that we should limit a molecule of oxygen to two atoms; on the contrary, we may suppose that it consists of an indefinite number provided we admit that under the action of electricity or other forces it is divided into two portions, each containing an equal number of atoms. In the present state of science, if we adopt the atomic constitution of matter, we must consider what was formerly assumed as the ultimate atoms of bodies, as groups of atoms held in relative position by attracting and repelling forces. It is only by an assumption of this kind that we are enabled to obtain a mechanical conception of matter in any degree applicable to various chemical and physical phenomena.] J. H.

MEMOIR OF ENCKÉ.

BY G. HAGEN.

Translated for the Smithsonian Institution by C. A. Alexander.

Last year died the director of our observatory, Professor Encke. Besides his other scientific and serviceable labors, he acted for eight and thirty years as secretary of the physico-mathematical class of our Academy, and during that long interval administered the affairs pertaining to the office with the utmost disinterestedness, skill and discretion.

His

Johann Franz Encke was born in Hamburg, September 23, 1791. father, archdeacon in the Jacobi-church at that place, died four years afterwards. Although his mother brought to the rearing of her eight children remarkable energy of character, yet the moderate pension which the family still drew from the church by no means sufficed for the expense of extensive studies.

As a preparation for the business of life Encke first resorted to a private school kept by Hipp, the author of several mathematical works, and later, from 1805 to 1810, frequented the Johanneum, where Hipp was still his teacher. Under these circumstances he very early developed a singular predilection for mathematical studies. At this time he voluntarily imposed on himself the task of repeatedly going over the collection of problems propounded by Meyer-Hirsch, and is stated in the parting certificate awarded him, October 11, 1810, to have been a model. to his school-fellows for diligence, correctness of deportment, and modesty.

It was now that he expressed to his mother the wish to study astronomy, and his two elder brothers, who had entered into trade and who recognized his talent, devoted themselves to the furtherance of his purpose, which they were enabled to gratify through the intervention of the pastor, Schäfer. During a year he attended a gymnasium in Hamburg, and proceeded in 1811, shortly after the death of his mother, to Göttingen. Here an older fellow-countryman named Gerling introduced him to Gauss, whom he was accustomed afterwards to regard as pre-eminently his instructor, and to whom he referred almost exclusively his mathematical and astronomical culture. Especially instructive did he consider an entirely private course (privatissimum) which, together with Gerling, he attended in the summer of 1812, at the residence of Gauss, who, on his own part, in a letter to Schumacher of this date, already calls Encke "his highly accomplished and well-informed pupil."

Political events led Encke, in the beginning of 1813, to enter into the Hanseatic artillery service. He was engaged in the bloody fight for the fortress of Göhrde, September 16, where Wallmoden attacked and defeated the corps which Davoust had despatched thither under Pecheux. He also took part, the following month, in Tettenborn's bold advance upon Bremen. In the honorable discharge granted him, June 24, 1814, he is styled sergeant-major of cavalry.

He resumed his studies in Göttingen, but as the war broke out anew the following spring, he at once decided, in company with his younger brother who

Abhandlungen der königl. Academie der Wissenschaften zu Berlin, 1866. Read 5th July, 1866, before the Royal Academy of Sciences at Berlin