positive. This is exhibited in the following table, drawn from the former experiments. The range was

Although, as Faraday himself remarks, these numbers require considerable correction, the general result is striking and the differences in several cases very great.

It appears clearly from these experiments that different gases have not equal capacities for insulation. Considering the mean values of u, (for positive charges of s and S,) we perceive that a stratum 0.62 of an inch of

that is, an electrical discharge passes as easily through a stratum of air 0.370 of an inch thick, as through one of hydrogen of 0.62 of an inch; an electrical spark penetrates a stratum of air 1.105 inch thick as easily as one of 0.62 of an inch of muriatic acid gas; an electrical spark passes with decidedly more ease through oxygen, hydrogen, and coal gas than through an equal stratum of air; but muriatic acid gas and olefiant gas present a decidedly greater resistance to the transmission of the spark than an equal thickness of air does.

Similar results were obtained from later but less reliable experiments. (Pog. Ann., XLVIII, 281.)

The mean values of u are not equal with positive and negative charges of S and s; for many gases u has a greater mean value with a positive charge of S and s than with a negative; for other gases it is the reverse; but to draw the conclusion from these experiments that many gases more readily allow the negative and others the positive discharge through them, seems to me unwarranted, because the differences of the above table in this respect are within the limit of errors of observation.

Thus, according to the table given above, the mean value of u with a positive charge of s and 8 is, for

while Faraday obtained from a subsequent series of experiments, similarly arranged, the following values:

Hydrogen
Carbonic acid...

Olefiant gas.

0.40

0.78

0.96

Evidently the corresponding values of u, obtained by positive charges of s and S, and which should be exactly equal, differ as much from each other as the corresponding values for the positive and negative charges of s and S; from which it appears that we are justified in assigning no great value to these differences. But there is a further reason for ascribing these differences to errors of observation, arising from the fact that when air is in the receiver, and the spark accordingly takes place through air, the positive and negative mean values for u are found unequal, namely: with s and S positive u = 0.695; with s and 8 negative u 0.635. These differences can be ascribed only to accidental disturbances, which produce the errors of observation; for why should the spark, with a positive charge of s and S, pass more easily through the air at v, and with a negative charge, more easily at u, also through the air? Air being in the receiver, and + and — charges imparted to s and S, the values for u would be nearly identi cal, unless the errors of observation were too considerable.

Faraday himself does not consider these experiments decisive in this respect, but brings forward some facts which seem to indicate some such difference between the positive and the negative discharge; making u = 0.8 of an inch, and filling the receiver with muriatic acid gas, the discharge always took place, with a positive discharge of s and S, at u, through air, but with a negative charge of s and Sat v, through the muriatic acid gas.

It also appeared that when the conductor was connected only with the muriatic acid gas apparatus the discharge occurred more readily with a negative discharge of the small ball s than with a positive; for in the latter case much of the electricity passed off as brush discharge through the air from the connecting wire; but in the former case it all seemed to go through the muriatic acid.-(Pog. Ann., XLVII, 287.)

$80. Unequal striking distances of positive and negative discharge.— Many known phenomena coincide in showing that positive and negative discharges do not take place with equal facility. When a small ball, connected with the conductor and thus made inductive, is placed opposite a larger one, which is uninsulated, a spark is obtained twice as long, the conductor being charged positively, as when negatively charged.

Faraday has closely investigated this phenomenon, and obtained the following facts:

He passed the discharges between two balls of the respective diameters of 2 inches and 0.25 of an inch. The larger ball being connected

Sparks alone up to an interval of .....................

val was greater than....

with the conductor, and thus made inductive, there appeared with a

positive conductor

Negative brush, from the small ball alone, when the inter

0.49 in.

0.52"

With a negative conductor

Sparks alone up to an interval of...........

1.15 "

Positive brush, from the small ball alone, when the interval was greater than........

1.65 "

Between these limits he obtained sparks and brushes mixed. The balls were then exchanged, the small ball being connected with the conductor, and the large one uninsulated. The result with a positive conductor was

0.4 in.

Sparks alone to an interval of........
Negative brush alone, when the interval was greater than.. 0.44 "
From these experiments it follows that—

1. Longer sparks are obtained when the small ball is positively electrified.

2. Longer sparks are obtained when the large ball is the inducing, and the small one the inducteous ball.

When the small ball discharges electricity in the form of brushes, they are much more numerous, and each one seems to carry off much less electrical force when the discharged electricity is negative than when positive.

This appears to indicate that a small ball requires a greater tension for discharging when positive than when negative.

Fig. 73.

A

To illustrate this important point, Faraday arranged an apparatus, represented in fig. 73. A fork, A, carrying a large and a small ball, was connected with the conductor of a machine; a perfectly similar fork, B, was connected with a discharging train; the small ball on each fork was placed opposite the larger one on the other. The intervals at n and o were equal. The conductor being negative, the discharge always happened at n, which is not surprising, because the negative charge of the small inducing ball at n is always stronger than the positive charge of the small inductious ball at o. But had the discharge taken place at o with a positive charge of the conductor, it would have appeared that the weak negative charge of the small inducteous ball discharges with greater facility than the far stronger positive charge of the small inducing ball at n, which would have been a decisive proof of the more facile discharge of negative electricity.. But such a decisive result the experiments did not give; when the intervals at n and o were 0.9 of an inch, or 0.6, the discharge always took place at n, whether the conductor was positive or negative.

B

The interval n being made 0.79 and o 0.58 of an inch, if the conductor was positive, the discharge at both n and o was about equal, but if negative, the discharges mostly happened at n, which signified,

evidently, that the small ball discharged in the negative state somewhat more easily than in the positive, yet their result is not perfectly decisive.

A contrivance, similar to that of fig. 73, was placed inside a glass vessel, which could be filled with different gases. With equal intervals at n and o, Faraday obtained quite decided results for carbonic acid. When the conductor was positive the discharge took place mostly at o, when negative always at n; here, then, the negative discharge was decidedly the more easy, and in coal gas the preponderance of the negative discharge was just as decided. In air and in oxygen the greater facility of the negative discharge appeared somewhat doubtful; in nitrogen and in hydrogen there appeared some probability of an opposite relation.

Belli has made experiments, from which it follows that negative electricity escapes more easily into air than positive.-(Pog. Ann. XXXV, 73.)

After fastening a quadrant electrometer on a horizontal insulated conductor and electrifying it positively, he found, as a mean of three experiments, that the electrometer required a period of ten minutes to sink from 20° to 10°; but with negative electricity only 4.5 minutes were required.

§ 81. Sparks in different gases.-The phenomena attendant on sparks in different gases have been often observed and described. Faraday has made experiments on this subject also, and describes them in the twelfth series of his Experimental Researches.-(Pog. Ann. XLVII, 536.)

The gases were under the pressure of the atmosphere; the sparks passed between brass balls.

"In air," says Faraday, "the sparks have that intense light and bluish color which are so well known, and often have faint or dark parts in their course, when the quantity of electricity passing is not great.

"In nitrogen they are very beautiful, having the same general appearance as in air, but have decidedly more color, of a bluish or purple character, and, as I thought, were remarkably sonorous.

"In oxygen the sparks were whiter than in air or nitrogen, and I think not so brilliant.

"In hydrogen they had a very fine crimson color"-" very little sound was produced in this gas.'

"In carbonic acid gas the color was similar to that of the spark in air, but with a little green in it. The sparks were remarkably irregular in form, more so than in common air.

"In muriatic acid gas the spark was nearly white. It was always bright throughout, never presenting those dark spots which happen in air, nitrogen, and other gases.

"In coal gas the spark was sometimes green, sometimes red, and occasionally one part was green and another red; black parts also occurred very suddenly in the line of the spark."

Sparks may be obtained in media, which are far denser than airas in oil of turpentine, olive oil, resin, glass, spermaceti, water, &c.

H

§ 82. The electrical brush.-The most important facts which Faraday has obtained in reference to the brush are the following,) Pog. Ann., XLVII:)

"The brush and spark gradually pass into each other." (Faraday calls the electrical brush "a spark to air.") "Making a small ball positive by a good electrical machine with a large prime conductor, and approaching a large uninsulated discharging ball towards it, very beautiful variations from the spark to the brush may be obtained. The drawings of long and powerful sparks, given by Van Marum, (description of the large machine in Taylor's museum, German translation of 1786, Tab. III, fig. 1 ;) Harris, (Phila. Trans., 1834, p. 243,) and others, also indicate the same phenomena," namely, a ramification of the spark by which its transition to the brush is made.-(Faraday's Researches, § 1448.)

"If an insulated conductor, connected with the positive conductor of an electrical machine, have a metal rod 0.3 of an inch in diameter projecting from it outwards from the machine and terminating by a rounded end or a small ball, it will generally give good brushes; or if the machine be not in good action, then many ways of assisting the formation of the brush can be resorted to; thus, the hand or any large conducting surface may be approached towards the termination;" or the termination may be smaller and of badly conducting matter, as wood; or sparks may be taken between the prime conductor and the secondary conductor, to which the termination giving brushes belongs;" "or the air around the termination may be rarefied.”— (1425.)

66

That the brush is not a continuous discharge is evinced in the gradual transition of the spark to the brush. By proper proportion, in the size of the small knob to the power of the machine, brushes are obtained which show immediately that they consist of ramified sparks rapidly following each other; the machine being worked more rapidly, or with the same working of the machine substituting a still smaller discharging knob, the brush assumes a more uniform appearance, which Faraday very well describes in the following words: "A short conical bright part or root appeared at the middle part of the ball, projecting directly from it, which, at a little distance from the ball, broke out suddenly into a wide brush of pale ramifications, having a quivering motion, and being accompanied at the same time with a low, dull, chattering sound."—(1426.)

At first such a brush seems continuous, but Wheatstone has shown that it consists of successive intermitting discharges, (Philos. Trans., 1834, p. 586,) which was to be expected from the gradual transition of the spark to the brush. Faraday gives a very simple method for decomposing the apparently continuous brush into its elementary parts without the help of Wheatstone's rotating mirror; he says: "If the eye be passed rapidly, not by a motion of the head, but of the eyeball itself, across the direction of the brush, by first looking steadfastly about 10° or 15° above, and then instantly as much below it, the general brush will be resolved into a number of individual brushes."-(1427.) This method of analyzing has not succeeded perfectly in my trials.