micrometer) has no influence upon the brightness of the electrical spark if the extent of the coating remain the same. The size of the plate, however, has considerable influence.

With unchanged distance of the constant light and of the striking distance, and with the same thickness of glass, the surface of the coating was varied, and each time the corresponding distance of the spark from the rotating disk, at which the sectors just ceased to be perceived, was observed. The results of such a series of observations are given in the following table:

The first vertical column contains the size of the surface of the coating, the second is the ratio of the first and second surfaces, and then of the first and third. The third column gives the corresponding Y, the fourth the square of this distance, and the last the ratio of the numbers of the first Y2 to the second, and of the first to the third. Comparing the second and fifth columns we have very plainly

that is, the superficial content F of the coating of the condenser is in proportion to the square of the distance Y, the illumination of the disk ab by the electrical spark remaining constant.

But the constant illumination of the disk is

J = c Y2

J denoting the intensity of the light of the spark, and e a constant factor, or

or, the brightness of the electrical spark is proportional to the surface of the coating.

§ 92. Relation between the intensity of the electrical spark and the thickness of the condenser.-The surface of the coating remaining the same, the thickness of the glass plate was changed; and for each plate the distance Y was observed at which the sectors just ceased to be visible, the illumination by the lamp remaining constant. The following table contains a few of the results obtained.

Now, 114

1.18 and 101.15, that is, the two quotients are

therefore the values of Y are nearly in the in

verse ratio of the square roots of the corresponding thicknesses of the glass; that is

that is, the intensity of the spark is inversely proportional to the thickness of the condenser.

The remaining experiments which Masson made on this point did not coincide generally so well with the above deductions. This he ascribes to the circumstance that he could not measure the thickness of the glass with sufficient accuracy, and that the different condensers may have had unequal "capacities for condensation."

§ 93. Influence of the nature of the pole on the electrical spark.-Masson found that the spark is somewhat more intense, if, under circumstances otherwise the same, it be passed between lead, zinc, and tin balls, than when the balls (equal in size) are of copper, brass, or iron. Masson thinks that this depends upon the unequal tenacity of the metals. In all his experiments there were traces of a transportation of the metal from one pole to the other; now since lead, for example, is less tenacious than copper, more lead will be carried off than copper with the same tension of the electricity; the conducting circuit then will have its capacity for conduction suddenly increased, and the light must become more brilliant in consequence.

This opinion is sustained by the fact that the intensity of the spark is very considerably increased if polished brass balls be exchanged for such as have their surface amalgamated, where evidently the transference is greatly facilitated.

The spark, with the carbon used for Bunsen's battery, is very white in the middle, reddish at the edges, and looks a little like a flame.

§ 94. Nature of electrical light.-There are two hypotheses as to the nature of the electrical spark; the first regards it as a motion which is communicated to the ether by the electrical spark; according to the second hypothesis, electrical light is produced by incandescent ponderable matter transported by the electricity.

Masson inclines to the first hypothesis, with which also his experiments coincide, since the intensity of the spark depends in no respect upon the fusibility or oxidibility of the balls, but upon their tenacity. If, in consequence of the lower tenacity of the metals, more particles are carried off, the facility of the circuit for conduction is increased; hence the same quantity of electricity is discharged in a shorter time, whereby a more brilliant light is produced. All of the laws of the brightness of electrical light just mentioned are comprised by the following formula:

in which

X28

J=H

Y2e

J denotes the intensity of the spark;

X the striking distance;

s the surface of the condenser;

(1)

Y the distance of the spark from the rotating disk of the photometer; e the thickness of the condenser, and H a constant factor depending upon elements which are not yet determined.

Substituting in equation (1), X=p, which is allowable, since Riess has shown that the striking distance is proportional to the electrical density (§ 31), we have

m

J =
q2
Y 2 2

Hp2

e

(2)

by making =m, that is, equal to a constant factor which is admissible so long as the thickness e of the condenser does not vary.

q2
82

the

Hence the intensity of the electrical light is proportional to square of the electrical density, or which is the same, to the tension of the electricity and the surface s of the condenser. Equation (1) may also be written

H

8

J=yze X 8 X

Y2e

Substituting for the last X its value p2, we get

8

(3)

or in words, the intensity of the spark is proportional to the striking distance and quantity of electricity.

According to equation (2),

is the quantity of heat which is set free in the wire by discharging through it a quantity of electricity q, collected on the surface s.

Hence if the discharge stroke of an electrical battery produces a spark at any interruption of the circuit, the intensity of the light is proportional to the heat which the same discharge produces in a piece of wire forming part of the circuit.

At the conclusion of his memoir, Masson proposes the spark generated under determinate conditions as the photometric unit, by which it will be possible to compare the intensity of the most diverse constant sources of light with a common standard.

SECTION FIFTH.

ELECTRICAL ODOR.

§ 95. Ozone and its reactions.-When we are in the neighborhood of a powerful electrical machine we perceive, when the electricity issues from points, or when a series of sparks are passed from the conductor, a very peculiar odor, which, for sake of brevity, we will term electrical odor, or ozone odor. This electrical odor is very probably that which is observed after a stroke of lightning; and which, by those who do not know how to characterize it properly, is termed a sulphurous smell. Schönbein observed in the vicinity of a place where lightning had struck a decided odor of ozone, even some time after the stroke.

Until recently we were quite in the dark as to the nature of this odor. Some physicists supposed that it was owing to a peculiar affection of the organs of smell, produced by electricity; an explanation which, in addition to its error, did great injury, by preventing further investigation and discussion.

Others advanced the hypothesis that electrical odor was owing to fine metallic particles carried off by the escaping electricity. But this view also is entirely inadmissible, because the nature of the emitting points does not in the least change the nature of the odor.

Schönbein has the great credit of having restored this question to the current of scientific activity. He has shown that the electrical odor comes from a peculiar gas, produced during the electrical emission, which he calls ozone. He has investigated the properties of this substance for years with the greatest zeal, and although, as yet, it has not been obtained in an isolated state, many of its important chemical and physical relations have been ascertained, and further researches on the subject promise most interesting discoveries in the field of chemistry.

The first memoir of Schönbein on ozone is in the "Denkschriften der Müncheuer Akademie." It is also printed in Poggendorf's Annalen. Bd. L. p. 616.

A small pamphlet with the title, "On the production of ozone in the chemical way," evidently by Schönbein, was published in 1844 by Schönbein & Schweighauser, in Basel.

The most important treatises on this subject which then followed are to be found in Poggendorfs Annalen, by reference to the index of names, appended to the LXXV volume."

In these papers the historical course of Schönbein's discoveries may be followed out. I will omit this historical investigation on account of its great extent, and I will not refer to the contents of the separate papers, but describe the most essential experiments which show the nature and most important relations of ozone, in the order in which Professor Schönbein had the goodness to show them to me in the year 1849, and, passing over their earlier phases, present his views upon its nature as now held, after many years investigation. The prime conductor of an electrical machine being provided at the end with a round-pointed wire, a, b, about 1 line in diameter, (fig. 82.) When the machine is turned the peculiar electrical odor will be perceived in the vicinity of the end a of the wire.

Fig. 82.

That this odor is not to be ascribed to a mere subjective affection of the organ of smell, but is owing to a peculiar gas, is certain from the fact that this odorous principle produces a series of chemical and physical effects, having the greatest similarity to the chemical reactions and physical relations of other gases. Indeed, Schönbein has succeeded in preparing this odorous principle, ozone, in a purely chemical way, and in producing the same reactions with it which are observed when electricity is issuing from points.

If we hold before the point, at the distance of about or 1 inch, a piece of paper covered with a paste of starch and iodide of potassium, the paste will at once turn blue.

To make this preparation two teaspoons full of starch with a small crystal of iodide of potassium are to be boiled to a paste, with ten times their volume of water.

The ozone acts upon this paste as chlorine does; it decomposes the iodide of potassium, and the iodine set free, colors the starch blue.