also, the intensity of the atmospherical electricity is generally dimi nished, probably because the strata of air containing different quantities of the fluid are brought successively to the ground, and thus there is produced a nearly uniform distribution of the fluid between the earth and the atmosphere. It may be easily conceived that, in stormy weather, the variations of the atmospherical electricity will be very irregular; for currents of air in the upper regions, driving the strata of clouds in different directions, the electrical actions between the clouds and the atmosphere below must be extremely complex.

M. de Saussure has observed that, during summer and winter, by night as well as by day, when the atmosphere is free from clouds, the electricity of the air is positive; and Mr. Read (Phil. Trans.,' 1794) has shown that out of 404 observations made in one year, the air was positively electrical in 241, negatively in 156, and that the electricity was insensible in 7 observations only. It seems probable, in fact, that the negative electricity which may be observed in a pure atmosphere is caused by the discharge of its positive electricity into the earth or a cloud, when one or the other of these is in a contrary state. Mr. Ronayne, however, states (Phil. Trans.,' 1772) that in Ireland the electricity of the atmosphere is positive in winter when the air is clear: he observes that it diminishes in frosty or foggy weather, and that he could detect no electricity in the air during the summer except when a fog came on; the electricity was then positive, but it was less intense than during the winter fogs. It was an observation of Saussure that electricity is strongest in the open air, and that it is weak in streets, in houses, and under trees. In close rooms and hospitals the electricity of the air has been always found to be negative; such also is the electricity of the atmosphere when it is vitiated by exhalations from lime, paint, and decaying vegetables. [OZONE.]

All observations concur in showing diurnal variations in the intensity of atmospherical electricity, but there is some uncertainty concerning the precise times at which the intensities are the greatest and the least. Cavallo observed that in dry weather the electricity was weakest at sunrise; that it attained the maximum of strength in the day-time, and continued in that state till sunset, when the intensity diminished. He conceived that this diminution was much more rapid as the air became more humid; and he observed that in winter, when a dry wind prevailed, if the sky were free from clouds, the electricity became very strong after sunset. M. de Saussure observed at Geneva (1785) that, during winter, the intensity of atmospherical electricity attained its first maximum at 9 A.M.; that it diminished till 6 P.M., when it was in a minimum state; it afterwards increased, and attained its second maximum at 8 P.M.; after which it continued to diminish till it was again a minimum at 6 on the following morning. The same philosopher found that in summer the diurnal variations were less perceptible: on a dry warm day he found the electricity increase from sunrise, when it was almost insensible, till 3 or 4 P.M., when it became a maximum; it appeared then to diminish till the dew fell, when it became stronger, but it was scarely sensible during the night. Lastly, the experiments of Mr. Read exhibit two maxima and two minima in twenty-four hours. The atmospherical electricity seemed strongest at two or three hours after sunrise, and again about sunset; it was weakest at noon and at 4 P.M. The experiments of Mr. Crosse show also that, in the ordinary state of the atmosphere, the electricity is positive, and that it increases in proportion to the elevation above the earth's surface; the same philosopher observes that it is most intense at sunrise and sunset, and weakest at noon and during the night. He finds that the approach of a thundercloud produces a change in the electricity of the atmosphere, rendering it positive if it were before negative; and the contrary: whatever be the nature of the change which takes place, the intensity of the electricity increases to a certain degree; it then diminishes and disappears, and is succeeded by an opposite electricity: this gradually increases till it becomes of higher intensity than the former kind, and then it decreases till it vanishes: it is again succeeded by the first kind. These changes are often found to take place several times successively. Fogs, rain, snow, &c., also change the electricity from positive to negative, again from negative to positive, and so on; the change taking place every three or four minutes. A cold rain, in large drops, is frequently accompanied by intense electricity; and during a driving fog or rain the electricity is occasionally as strong as during a thunder-storm. A warm small rain is weakly electrified; and a weak positive electricity generally prevails during cloudy weather. Mr. Crosse finds also that the electricity of the air is very weak during the north-east winds which in winter and spring times produce extreme cold and dryness.

The intensity of atmospherical electricity has been observed to undergo annual changes; it increases from July to November inclusive; so that the greatest intensity occurs in winter, and the least in

summer.

Any of the different kinds of electrometer may be employed to determine the nature and intensity of atmospherical electricity. [ELECTROMETER.] Professor Loomis states (Poggendorff's Annalen, Band 100), that the telegraphic wires are very sensitive to an approaching thunder-storm, and often become highly charged even when a storm is so distant that neither thunder nor lightning can be appreciated; but if the thunder-cloud be near, the wires may become so highly charged as to injure the magnetic apparatus and expose the

clerks to considerable danger. Such effects, however, are not peculiar to America. Later observations have confirmed the conclusion that the usual electrical condition of the air is positive. Out of 15,170 observations made at the Kew Observatory, during a period of 5 years, 14,515 are of positive electricity, and 655 of negative. During the years 1845, 1846, and 1847, out of 10,500 observations, 10,176 showed positive, and only 324 negative electricity; the latter being usually accompanied by heavy rain. The tension of atmospheric electricity was found to be at its minimum at 2 a.m., from which hour there was a gradual increase until 6 a.m.; after which the tension increased more rapidly, its value at 8 a.m. being nearly double that at 6 a.m.: the increase then became more gradual until 10 a.m., the period of the first or morning maximum: from this hour it gradually declined until 4 p.m., when it was only a little higher than at 8 a.m.; it then increased rapidly until 8 p.m., and after a slight rise at 10 p.m., the time of the principal or evening maximum, the tension was found no longer to increase. The evening maximum was much higher than that of the morning. Between 10 p.m. and midnight the tension decreased nearly to that of the diurnal minimum. With regard to the annual period the lowest tensions were found to be in June and August, the tension during July being slightly superior to that of those two months. There is a slight elevation in September, which increases in October, and more rapidly from November to January. The maximum is in February. In March a rapid diminution sets in, and proceeds to the minimum in June.

During four years' observations made at Brussels by M. Quetelet, the maximum was found to be in January, and the minimum in June. Expressed numerically the maximum had the value of 605°, and the minimum 47°; so that the electricity in January is thirteen times more energetic than in June. The difference was found to be much more sensible during serene than cloudy weather, and, setting out from June, the electricity of the air in serene weather was found to exceed the electricity observed during a clouded sky, in proportion as January was approached, in which month the ratio was more than 4 to 1. The powerful electric intensity of the air during a serene sky in winter must be considered as a remarkable fact. A high electric intensity was always observed during fog and snow.

Atmospheric electricity seems to depend for its supply chiefly on evaporation. It can be proved by experiment that the evaporation of water from heated bodies disturbs the electrical equilibrium, the + electricity being carried off by the vapour, while the electricity is left behind in the vessel, or is carried off by its support. In order to produce this result, however, some chemical change seems to be necessary, since the evaporation of pure water fails to produce electrical disturbance. When water is evaporated from alkaline solutions the vapour carries off and leaves + electricity behind; but the reverse of this takes place when water is evaporated from an acid or from neutral saline solutions, including sea-salt. During the processes of vegetation, in which water is largely separated from the other constituents of plants, there is also electrical disturbance. Volcanic eruptions and conflagrations are also accompanied by strong electrical excitement. The friction of wind, dust, &c., is also a source of atmos pheric electricity. Dr. Livingstone (Missionary Travels, &c., in South Africa,' 1857) states that in the Kalahari Desert, during the dry season, a hot wind occasionally blows, and it is "in such an electric state that a bunch of ostrich feathers held a few seconds against it, becomes as highly charged as if attached to a powerful electrical machine, and clasps the advancing hand with a sharp crackling sound." Even the motion of a native in his kaross is sufficient to produce therein a stream of small sparks.

There are many difficulties in accounting for the presence of free electricity in the air. Perhaps the best theory is that started by Eales (Phil. Trans.' 1757-8), and extended by Sir John Herschel (Encyc. Brit.,' 8th ed., Art. Meteorology). We will state sufficient to indicate the nature of the theory. When water evaporates it is supposed to pass off in the form of little vesicles of moisture, each vesicle charged with electricity of too low a tension to discharge itself into neighbouring vesicles, or to escape by conduction through a moist atmosphere. If, however, a tall pointed conductor, a flame, or a column of smoke, be introduced, the air may give off a portion of electricity by contact discharge. But if we suppose a number of the particles of vapour thus electrified to be condensed by cold into a drop of water, the whole of the electricity would be collected on the surface of such drop with a corresponding increase in tension. Suppose one such drop to be 1000 times the size of one of its constituent particles, the diameter will be increased 10 times and the surface 100 times, while the electricity being the sum of the 1000 globules will be increased 1000 times on the surface of the drop with a tenfold density or tension. The comparatively high electric state of fog, and of snow, is thus explained. Every globule of moisture of which it consists has a coating of electricity, which it will part with to the surface of any conductor, and the denser the fog, and the larger its globules, the greater is the quantity of electricity set free. This theory also explains the electricity of the air as occasioned by the deposit of dew, and also the daily variation in tension; for the air losing a great deal of vapour by a deposit of dew, by night the electricity is at its minimum. increases as fresh vapour arises under the influence of the ascending

It

so. NO6

This hypothesis of the constitution of oxygen salts is supported by the law of equivalent electrolysis, which we shall discuss directly. It requires, however, the assumption of hypothetical substances, SO, NO, &c., which are not separated as such, like chlorine, but must be decomposed, as soon as formed, into oxygen, which appears, and the anhydrous acid, SO,, NO,, &c., which finds water with which to combine. It is these hypothetical molecules, SO,, NO, &c., which must be supposed to be successively liberated by the liberated atoms of hydrogen or metal from the molecules of the salt, in the same manner as is the case with the halogens in binary compounds. However, the necessity for this hypothesis cannot be considered as very hostile to the haloidal constitution of oxygen salts, for many instances are known where the actual ultimate products of electrolysis (or ions) appear, by the strongest possible analogies, to be products due to the chemical action between the initial ions and the unaltered electrolyte in contact with them. Thus, if instead of chloride of copper, chloride of sodium be submitted to electrolysis, chlorine as before is evolved at the positive pole, while at the negative not sodium but hydrogen is evolved. Now Cu Cl and Na Cl are certainly chemically similar. Further, if the anhydrous salt be employed in the fused state, sodium is actually liberated: finally, if sodium were set free in an aqueous solution at the negative pole, the products would be the same as they are actually found to be, for water would be decomposed, soda formed, and hydrogen evolved. Again, by altering the nature of the positive electrode, we may affect the nature of the ion appearing there. Thus, if the electrolyte be dilute sulphuric acid, and the positive electrode be of zinc amalgam, the oxygen which, had the positive electrode been platinum, would have been liberated there, combines with the zinc and dissolves as sulphate of zinc in the sulphuric acid. Or, further, even by altering the strength of the electrolyte (sulphuric acid), the products may be affected; for unless the acid be dilute, a part of the oxygen, instead of escaping as such, combines with the water to form HO,, which dissolves in the electrolyte. It is, indeed, on the power we have of altering the nature of the ions, that the construction of constant batteries depends. In Daniell's for instance, the hydrogen, which would collect at the negative pole (which is of copper) and check the current by interposing a partial cushion of a non-conducting gas, is made, by placing a solution of sulphate of copper round this pole, to precipitate an equivalent (chemical and electrical) of metallic itself being converted into water and combining as such with the sul phuric acid of the sulphate of copper; whereby the electrode continues to be coated with fresh portions of metallic copper. In Grove's and Bunsen's batteries, again, the hydrogen which would be liberated at the negative pole is there instantly oxidised to water by the nitric

acid which surrounds it.

copper,

In organic compounds, where the electrolyte is often of a very complex nature, it is frequently difficult or impossible to recognise the primitive ion, and to distinguish it from the products of its action upon the electrolyte in contact with it.

It is in considering the products of electrolysis, or ions of an electrolyte, that the atomic theory receives prominent illustration; or, conversely, it is from the atomic hypothesis that the clearest light is

thrown upon the process of electrolysis.

Suppose the current from a battery to pass by one electrode into a vessel of water: let the other pole be immersed in a vessel of chloride of copper, and let an arc of platinum be placed with one limb in each vessel. The current from the battery will enter the water by the positive electrode P,, will traverse the water and escape by the negative electrode N,; passing along the arc, it will enter the solution of copper by the positive electrode P,, traverse it, and escape by the negative electrode N, thus regaining the battery after having traversed both vessels. Now from what has been said before, it is self evident that for every atom or equivalent of oxygen liberated at P, there must be an equivalent of hydrogen liberated at N,, that is, if the whole of each ion set free escapes. Further, that for every atom or equivalent of chlorine liberated at P2, there must be an atom or equivalent of copper deposited at N, (under the same restriction). In other words, the hydrogen and oxygen will be in the proportion of 1 to 8, and the copper and oxygen in that of 1 to 0:25. So much is in accordance with the purely chemical law. But it is further found that the quantity of hydrogen in the first vessel is to that of copper in the second as 1 to 32, and therefore to that of chlorine as 1 to 355, so that the quantities of all four ions are in the proportion of their equivalents. The same is true, however many electrolytes a current has to decompose in its course, and this constitutes the great law of equivalent electrolysis. It is easy to understand that the same law is essentially true when the primitive products of decomposition undergo further change in presence of the electrolytes. Thus, if a current traverses two neutral

solutions, the one of sulphate of potash the other of phosphate of soda, each of which is divided into two portions by a porous diaphragm in such manner that the sulphuric and phosphoric acids, freed at the positive electrodes, shall not mix with the potash and soda at the respective negative poles; then, however long the current has passed through, not only will the liberated phosphoric acid exactly neutralise the liberated soda of the same cell, it will also neutralise the caustic potash of the other one; and the same is true mutatis mutandis of the sulphuric acid. For the metals potassium and sodium, initially separated, being equivalent, will by their oxidation give rise to equivalent quantities of potash and soda. The meaning then of the law of equivalent electrolysis is, that the power which a current possesses of effecting chemical change is exerted throughout its course in equal degrees wherever such chemical change is effected.

the nascent state.

It follows that by ascertaining the whole amount of one ion liberated at any electrode of a current which passes through any number of electrolytes, we ascertain the quantity of every other ion liberated by the same current (or, of its equivalent derived by secondary change). A practical application of this is exemplified in the construction and use of the voltaic electrometer or voltameter. This instrument is, in fact, nothing more than a cell containing acidulated water, which is introduced into the current and decomposed by its means between two platinum electrodes. By a simple arrangement, the amount of hydrogen liberated at the negative electrode is collected and measured. Or the oxygen and hydrogen from the positive and negative poles are collected together. In the latter case, of course, for every 9 parts by weight of the mixed gases, every other ion freed in the circuit has been liberated in quantity proportional to its equivalent. The hydrogen liberated at the negative, and the oxygen liberated at the positive pole exhibit all the powerfully developed affinities of these elements in Thus many oxygenated or chlorinated organic bodies which refuse to part with their oxygen or chlorine under ordinary conditions, and which may refuse to conduct the current when pure, will give up their electro-negative elements to the hydrogen at the negative pole of a galvanic current when, by mixture with an acid and aqueous solution, they are made to conduct, and hydrogen is furnished. Under the same conditions many substances may be oxidised by the oxygen liberated at the positive pole. Thus by mixing chloroform with alcohol and dilute sulphuric acid, and submitting the mixture to electrolysis, the hydrogen at the negative pole replaces the chlorine of the chloroform, and hydride of methyl, or marsh gas is formed. Again, if a solution of caustic potash be submitted to the action of a current, the positive electrode of which is metallic iron,-the iron becomes peroxidised to ferric acid, and ferrate of iron is formed at this pole. So, a solution of cyanide of potassium gives when similarly treated, cyanate of potash at the positive pole. The same powerful affinity is exhibited by other electronegative ions; thus, for instance, when chloride of ammonium in solution is electrolysed, chloride of nitrogen is formed at the positive pole. It will, however, be borne in mind, that a part of the oxidising action of electrolytic oxygen may be due to the formation of binoxide of hydrogen, as before mentioned, or perhaps, to that of a still higher oxide (ozone). That the electrode itself may undergo change by the action of the ion liberated upon it, has already been exemplified. An instructive instance is furnished by the electrolysis of fused chloride of silver, when the electrodes are of silver. In this case silver is deposited in crystals upon the negative electrode, while the chlorine formed. Thus the amount of chlorine in the electrolyte remains at the positive one combines with the silver of which that electrode is unchanged, but fresh portions of silver are continually being transferred through it, from the positive to the negative electrode.

metal precipitated at the negative pole is not an alloy of all the metals

If a current be led through a solution of various metallic salts, the present. Some metals are precipitated in preference to others, and the whole quantity of one metal present is precipitated before the second commences to be thrown down. Thus, from a solution of copper and iron salts, the copper is precipitated before the iron, from zinc; finally, from a mixture of copper, iron, and zinc, the copper a mixture of iron and zinc salts the iron is precipitated before the is precipitated first, then the iron, and lastly the zinc. Zinc is thus said to be more electro-positive than iron, iron more electro-positive than copper, &c. Compared with regard to their electro-positive characters the metals are related to one another as follows:—

sodium at the negative one. The chlorides of aluminium, magnesium, barium, calcium, etc., yield up their metal at the negative pole. In such electrolysis, special precautions must be taken to prevent the metal from becoming re-oxidised, for the details of reduction by such means, see ALUMINIUM, CALCIUM, MAGNESIUM, &c. It may be here mentioned, that for this purpose it is usual to employ mixed chlorides, the melting points of such mixtures being lower than those of the simple chlorides, and the purity of the reduced metal not being impaired by the presence of other metals in the electrolyte, because the more electro-positive is separated in preference to the less electropositive. An example of the importance of the light which may be thrown upon the constitution of a compound substance by the electrolytic separation of its parts into the most electro-positive and electro-negative of its constituents, is furnished by the behaviour of the salts of the fatty acids towards the electric current. If valerianate of potash, whose empirical formula is KOCH,O,, be electrolysed by means of platinum electrodes separated by a porous diaphragm, potassium (that is, potash and hydrogen), appear at the negative pole, while carbonic acid and butyl appear at the positive.

10

KOCH,O3+ nHO = H + KO + C, H, + 2CO2+ (n-1) HO Valerianate of potash.

Butyl.

fine platinum points, minute bubbles of hydrogen and oxygen collect at the poles. Mixtures of oxygen and hydrogen, or hydrogen and chlorine, are exploded on passing a spark through them.

The following are examples of the formation of high tension elec tricity by chemical change:

The insulated grate of a stove in which coal is burning is found to be negatively electrical. If hydrogen passes through a metal tube, and is then burned, the tube is found to be negatively electrical. If dry air and chlorine be passed through a thin copper tube, the latter becomes negatively electrical; if the product be then passed through a platinum tube, the latter becomes positively electrical.

With regard to the relative powers of low and high tension elec tricity to effect chemical change, it has been found that "A wire of platinum and a wire of zinc, each th of an inch in diameter, placed ths of an inch apart, and immersed to a depth of ths of an inch in an acid consisting of one drop of oil of vitriol în four ounces of distilled water, at a temperature of 60° Fahr., and connected at their other extremities by a copper wire 18 feet long and th of an inch thick, yield as much electricity in about three seconds of time as a Leyden battery exposing a surface of 3500 square inches, charged by 30 turns of a plate-glass machine 50 inches in diameter in full action.' ELECTRODE. [ELECTRO-CHEMISTRY.]

ELECTRO-DYNAMICS. In ordinary electricity, that fluid when

HI, CH,, 2CO3.

If a piece of zinc be immersed in a solution of dilute sulphuric acid, so that a part of it is above the liquid, an inactive metal, such as platinum (that is, one which is not attacked by the acid), which connects the part of the zinc attacked with the part above the liquid, is found to be traversed by an electric current. This is the type of the formation of all electrical currents by chemical change. And if we assume arbitrarily that the current passes only in one direction,namely, from the part of the zinc attacked to the part unattacked,— then the direction of the current in this example gives us the direction of the current in all other cases where metals are attacked by acids. Hence when, in the same acid, two metals are placed, of which the one is attacked, the other unattacked, by the acid, a current passes through the liquid from the attacked to the unattacked metal, if metallic contact be established between them above the liquid. But if such metallic connector be broken, and the ends plunged into a liquid containing an electrolyte (fig. 1), then such an arrangement of active and passive metal constitutes a battery. The most simple method of interpreting such an arrangement with regard to the "signs" of its parts electrically considered, is by remembering that the part of the zinc immersed is positive; the part out of the acid, together with the connecting wire and attacked electrode, is negative; the opposite electrode, where the chlorous element is eliminated, together with the whole of its platinum connection, as far as the surface of the liquid in the battery, is positive. The part immersed in the liquid of the battery is negative. This comes to the same thing as saying that the positive ion is liberated at the electrode in connection with the positive element of the battery, the negative ion at that connected with the negative element. It must be borne in mind, that the ions appearing at the two electrodes are not absolutely positive and negative towards one another. All that is known is that they are in the same relation with regard to sign, as are the elements of the battery by whose action they are separated; and that thus, with regard to sign, all we know on decomposing chloride of silver by means of a Grove's battery is that

Ag: Cl: Zn : Pt.

That electric currents are established between the active and passive points of a metal, and that the positive current passes from the active to the passive part, is seen in the example of so-called passive iron. If clean iron be dipped into strong nitric acid, no evolution of gas occurs, and the iron is not acted on. If another piece of iron be first plunged into dilute nitric acid, and then into the strong nitric acid, the decomposition set up in the dilute acid is carried on in the strong acid. It is possible thus in one and the same acid to have two pieces of the same metal, the one of which is active and the other passive. On connecting the two, a current passes through the connector, just as if platinum had been employed instead of the passive iron. Nickel may by the same means be brought into the passive state, and a battery formed of elements of active and passive nickel. It is beyond the scope of this article to describe the conditions under which the passivity of iron is destroyed or preserved.

The power possessed by high tension or frictional electricity of effecting chemical change is very small in comparison with that of low tension or galvanic electricity; and in many cases where chemical change accompanies frictional electricity, it may be traced to the accompanying forces of light and heat.

If an electric spark be made to pass through an atmosphere of mixed oxygen, nitrogen, and water, the nitrogen combines with the oxygen in part and in part with the hydrogen, the two products so formed combining together.

If a spark be made to pass through water by means of exceedingly

power of the medium on which it is disposed, on the non-conducting power of the medium by which it is enveloped, and on the law of force, whether of attraction or repulsion, between the elementary portions of electricity. The motions of electrised bodies are only results of the statical equilibrium of this fluid, and do not therefore belong to electrodynamics. The mode of calculating such effects may be found under the head ELECTRICITY, COMMON. These effects are, moreover, of the same nature, whether the source of electricity be by means of friction, or by chemical action, as in the voltaic pile, the nature of the elec tricities in these cases differing from each other only in the mode of their production [ELECTRICITY]; but when the contrary electricities are no sooner produced than re combined, again re-produced, and again re-combined, a new class of phenomena arises belonging to electricity as it were in motion. Suppose, for example, that the plate A is a constant ů

source of positive electricity, the plate в in like manner a constant source of negative electricity of equal intensity; that A C, B c, are two conducting rods communicating with each; the electricities immediately combine when the conductors are made to touch at c, and for an instant the whole may be conceived to be in the neutral state, but a being the next instant replenished with positive and B with negative electricity, the same combination takes place over again, the same neutrality succeeds, and so on indefinitely. The rod A CB is in a different condition from one in its natural state, since electrical charges are continually pouring through it from A and B; and again it is in a different condition from an electrised rod, since we cannot at any moment say that it is charged positively rather than negatively. Hence we cannot infer that it should attract rather than repel an electrised ball D, since there is as much reason for one event as the other, and in point of fact we find that it neither will attract nor repel D. We have here a posi tive current of electricity issuing from a and a negative from B, and no effect of attraction or repulsion is produced on an electrised point as in statical electricity. How then is its state recognised? First by touch; for if we touch the rod A CB, a series of shocks is felt, the interval between two succeeding ones being inappreciable; and secondly, powerful chemical decomposition may be effected. [GALVANISM; by presenting to A B another rod A'B' under exactly similar circumELECTRO-CHEMISTRY.] But, thirdly, we may recognise it mechanically

A

A

B

B'

stances, when the effects of the currents in a B, A'B', will be recognised by the visible motions of the rods, provided they be free to move while their communication with the proper sources of electricity remains unbroken: for example, if their extremities be immersed in cups of mercury communicating with the constant sources of the positive and negative electricities. The laws of the mutual action of electrical currents constitute the science of electro-dynamics; and previous to its study it would be desirable that the reader should be acquainted with the construction and applications of the galvanic apparatus, the opposite poles of which afford the two constant sources (A B) of electricity which we have supposed. These will be found under the head GALVANISM.

To discover the laws of the mutual actions of electrical currents we must have recourse to experiment. The apparatus required for the purpose is figured in books on physics, together with a description of the mode of performing the various experiments by which these laws a current is convenient when speaking of more than one; for instance, have become known. [ELECTRO-MAGNETISM.] The term direction of