Sulphur instead of phosphorus gave no action until it was fused by means of a hot metal ring; the galvanometer was then instantly deflected. In another experiment both tubes of the battery were charged with nitrogen, but one was provided with phosphorus, the other with iodine. After closing, a decided current appeared which lasted for months. The nitrogen did not change in volume, but the liquid became gradually colored. Here the vapor of iodine acted like oxygen; the vapor of phosphorus like hydrogen. § 46. The pole changer.-It is well known that if two homogeneous plates, say of platinum, be immersed in dilute acid, the poles being connected even with only a single voltaic element, the galvanic polarization which they undergo if connected after the interruption of the primary current, is sufficiently strong to cause a current in the oppoFig. 33. site direction. For example, let a, in Fig. 33, be a vol a b tameter, b a galvanic element, sending its current through the latter; the current being interrupted, connect the terminal wires of a multiplier c with the two plates, and this will indicate a current of polarization which, however, will soon cease. In this manner a whole series of plates can be polarized, and thus we obtain Ritter's secondary pile, for charging which a primitive battery of many pairs of plates is always used. The electro-motive force, which sets in motion the current of the secondary battery, is evidently less than that of the primary charging battery. Poggendorff has invented a contrivance for charging, with a simple voltaic battery a secondary battery of any number of plates, and thus obtains a current of far greater electro-motive force than that of the charging battery itself. (Pog. Ann. LX, 568.) The process is as follows: Suppose we have a series of pairs of platinum plates, in cells, filled with dilute sulphuric acid, as shown in Fig. 34. Fig. 34. Plates 1 and 2 are in the first cell, 3 and 4 in the second, &c. Now, if plates 1, 3. 5, and 7 be connected with the positive pole of the simple battery, and the plates 2, 4, 6, and 8 with the negative pole, the plates denoted by the odd numbers will be negatively polarized, (since oxygen escapes at their surfaces,) and the plates denoted by even numbers will be positively polarized, (by hydrogen.) After this connexion has existed only a very short time, it must be suddenly broken, the charged plates connected according to the principle of the battery, and the circuit closed by a voltameter; this will now be traversed by a current of much greater tension than the primitive one, because in this combination the electro-motive force of all the polarized pairs of plates is added together. For this purpose, the plates 1 and 8 must be placed in conducting connexion with the voltameter, while 2 and 3, 4 and 5, 6 and 7, must be joined by metallic wires. Poggendorff has invented an apparatus, called the pole-changer, for effecting these changes and discharges in rapid succession. But his instrument requires the use of mercury, and I propose for the purpose the apparatus represented in Fig. 35. On a vertical board to the left of the figure is a series of brass pillars, which serve for fastening metal wires. The screw which is used for this purpose is represented only in the one at H; all the other posts are also provided with screws. These pillars all stand on metallic springs, rubbing against a movable cylinder; the first and last pillars stand a little below the level of the others. At each end of the cylinder a copper ring is placed. The spring of the first post (the wire from which passes towards P) rubs on the first copper ring, and the spring of the last post (whose wire goes to Z) rubs on the further ring. These wires pass to the platinum and the zine plates of the charging element. The wires O and H, leading to the platinum plates of the voltameter, are screwed to the first and last of the more elevated pillars. The wires 1, 2, 3, 4, 5, 6, 7, 8, which are screwed in the other posts, pass to the platinum plates of the secondary battery. On the movable wooden cylinder are placed four semicircular wooden bars, 90° apart, which are partly covered with bands of copper; the springs of the high posts rub upon these alternately during the revolution. On the bar which is represented as uppermost in the cut, and on which the springs are resting, the copper bands are so arranged that 1 is brought into conducting connexion with 0, 2 with 3, 4 with 5, 6 with 7, and 8 with H; in like manner the platinum plates from 1 to 8 are combined according to the principle of the battery, and closed by the voltameter. The lower wooden bar has exactly the same construction as the upper one. The other two bars opposite each other, to the right and left of the cylinder, are also alike, and so constructed that when they come into contact with the springs, the plates 2, 4, 6, and 8 are in conducting connexion with the carbon cylinder, and 1, 3, 5, and 7 with the zinc cylinder of the charging element. For ready expression, we shall call the rollers which are above and below in the cut the discharging rollers; the others the charging. rollers. The construction of the charging rollers is as follows: Eight copper bands are placed on the wooden roller in such a way that they may come in contact with the eight springs corresponding to the eight platinum plates. Half of these bands (the 2d, 4th, 6th, and 8th from the first in one figure) are connected with a copper strip, which passes to the front ring of the cylinder, and thus to P. In the same manner the other half of the copper bands (1, 3, 5, and 7) are connected with a similar strip of copper, which, lying on the other side of the wooden bar, is not visible in the figure, and which passes to the farther copper ring of the cylinder, thence to Z; thus the bands 1, 3, 5, and 7 are in connexion with the zinc cylinder, and 2, 4, 6, and 8 with the carbon cylinder of the charging element, when the charging roller is uppermost. The cylinder is turned by the crank; at each revolution there is a double charge and discharge of the secondary pile. The most suitable dimensions for the cylinder are 12 centimetres long, (for a pile of 4 pair of plates,) and (without the bars) 2 to 3 centimetres in diam eter. It is well known that with one Grove's element very little water can be decomposed; the voltameter plates become coated with gas bubbles, but very few ascend. But on using the simple battery through the medium of the pole-changer for charging the secondary battery, in whose circuit the voltameter is inserted, a lively decomposition of water is obtained as soon as the pole-changer is set in motion, which is a striking proof that the electro-motive force of the secondary current is considerably stronger than that of the primary. With a voltameter whose plates presented a surface on each side of about 3 square inches, sulphuric acid being added to the water, Poggendorff obtained from 5 to 6 cubic centimetres of explosive gas per minute, when in this time the circuit was closed and opened about 80 times. 1 n The secondary current thus obtained has an electro-motive force which exceeds that of the primary current in proportion as the pairs of plates of the secondary battery are more numerous. On the other hand, the entire chemical effect which the secondary current produces in the voltameter is only (if the secondary battery consists of n pair of plates) of that which the primary current had previously produced in each separate cell for charging the plates. For, while 6 cubic centimetres of explosive gas were collected in the voltameter in the abovementioned experiment, 6 cubic centimetres of this gas had to unite to form water in each of the four cells of the charging battery, and this quantity of gas was first released from the water by the action of the primary battery. Therefore by the action of this battery in the 4 cells together, the water of 6 x 4 = 24 cubic centimetres of gas must be electrolyzed per minute, in order that 6 cubic centimetres may be released in the voltameter. Without the pole-changer and by the direct action of the simple battery in the four cells, (which in this combination represent a large pair of plates,) not over 0.1 cubic centimetre of gas would be evolved, because the gas, which appears at the first moment of the passage of the current, produces at once a polarization of the plates, in consequence of which only an exceedingly feeble current can circulate; but by the pole-changer this polarization is immediately removed, and thus an undiminished action of the charging cells is rendered possible. The platinum plates, of which Poggendorff constructed his secondary battery, were platinized. If the secondary current is to be tolerably strong, this is very necessary; at least the negative plates of the secondary battery must be platinized, i. e. those at which the primary current has evolved oxygen, and to which the secondary current carries hydrogen. The influence of platinizing appears from the following experiments made by Poggendorff: In five minutes a battery of two pairs of platinum plates connected with a small Grove's element and the pole-changer yielded the following quantities of gas: 1. All the plates uncoated........ 4. All the plates platinized.. 1 c. c. (a little over.) 13 to 14 c. c. The positive plates are those at which the original current evolves hydrogen. This is not due to the platinized plates being more strongly polarized, for in fact they are less so than the naked ones; but, in Poggendorff's opinion, the action of the platinum coating consists in favoring the combination of the oxygen, separated at its surface by the primary current, with the nascent hydrogen evolved in consequence of the secondary. Much might be said in opposition to the modus operandi as explained by Poggendorff; but this is not exactly the place for the discussion. Poggendorff has successfully used plates of Bunsen's carbon in constructing secondary batteries. A battery of two pairs of such plates 1 inch wide and 1.5 deep, immersed in dilute acid, yielded 8 cubic centimetres in five minutes. The current of polarization which such a secondary battery yields by means of the pole-changer is considerably stronger than that of a Grove's gas-battery. The intermitting current of a secondary battery of two pairs of plates gave in one minute with the pole-changer = 2.8 c. c. of gas, while the continuous current of a gas-battery of ten cells yield only 2.1 cubic inches in thirty-six hours; thus only about 0.016 cubic centimetre of gas per minute. § 47. Old observations on the relation of iron to nitric acid.-On immersing an iron wire in nitric acid of the specific gravity 1.4, it instantly turns brown, while red vapor escapes with more or less effervescence. This, however, soon ceases; the iron recovers its metallic lustre, and retains it as long as it remains in the acid without being further attacked. Once placed in this state of chemical inactivity, such a wire will remain so even in dilute acid, which of itself could not have produced this condition. This remarkable relation of iron to nitric acid was observed as early as the last century by James Keir, and published in the Phil. Trans. for 1790; but the phenomenon was too much isolated to allow a true determination of its nature, and thus Keir's observation was forgotten. After the lapse of thirty-seven years, Wetzlar made similar observations, which he published in Schweigger's Jahrbuch der Chemie und Physik; Bd. 49, S. 470; Bd. 50, S. 88 and 129; Bd. 56, S. 206. In England, Herschell took up this subject, (Pogg. Ann., XXXII, 211; Ann. de Chemie et de Phys., 1833, vol. LIV, 87,) and Fechner observed similar phenomena in the action of nitrate of silver on iron. Schönbein has prosecuted this subject most zealously, and to him belongs the merit of having extended, more than any one else, the circle of the phenomena relating to it. Since Schönbein has investigated the phenomena of the passivity of iron (a term which was introduced by himself) the most thoroughly, it may be advisable to take our facts chiefly from his memoirs. This distinguished natural philosopher, however, will, I hope, not take offence if I should venture the remark, that the peculiar diffuseness which characterizes these papers renders them difficult to understand. § 48. Schönbein's observations on the passivity of iron.-His first paper on this subject may be found in Poggendorff's Annalen, XXXVII, 390. "It has long been known," Schönbein begins, "that very concentrated nitric acid does not attack many metals, which are oxidized with violence by the same acid containing more water. Of these metals tin is one, but iron more especially has this characteristic. "An iron shaving perfectly free from rust was not attacked by nitric acid of the specific gravity of 1.5. Even after adding to the acid as much water as will dilute it to the degree at which it would attack fresh iron shavings violently, the shaving thus treated will remain perfectly passive. "It is not only the treatment with concentrated nitric acid which produces this passivity. Iron filings, heated for only a few seconds |