through a long circuit, including the fine igniting wires, with the negative pole, so that B may be considered the negative pole and A the positive. The current circulates and produces ignition in the iron wire as soon as A and B come into contact.

In order to pull the disk B towards A from a distance, two pieces of twine fastened to B pass through holes in the disk A, and at E are connected with the long string that reaches to the place whence the person who is to close the circuit stands. An accidental discharge is prevented by a peg F between A and B, which must be removed before the two disks can come in contact. But besides lessening the danger this method of blasting offers other considerable advantages; it enables us without much difficulty to explode powder under water. For this purpose the entire charge is to be enclosed in a water-tight tin box and this put in the place where its action is desired.

The application of galvanic ignition is also very advantageous when great masses of rock are to be blasted. Formerly, in such cases, it was necessary to use a heavy charge in one great mine, but several smaller properly distributed charges would produce a much greater effect if they could be ignited simultaneously. This can now be done by the aid of the galvanic current; the connecting wires have only to be so arranged that all the holes are at the same time in the circuit. In this way immense effects have been obtained in England.

What power the battery must have in each case can easily be ascertained from preceding sections. From section 59 can be ascertained what force of current is required to make the thin iron wire incandescent, (the diameter of which must of course be known,) and after computing the resistance in the conducting wires, it is easy to determine how many cups or pairs of plates of any give point must be used and how they must be arranged to produce this force of current.

§ 64. The voltaic arc.-By the construction of the constant battery, the production of the arc of light which Davy was the first to observe is greatly facilitated, and hence this interesting phenomenon has been several times investigated, though much is still left for further researches.

De la Rive paid great attention to the galvanic arc; we take the following from his elaborate treatise on this subject, published in Phil. Trans., f. 1847, (Pogg. Ann., LXXVI, 170.)

The voltaic arc can be produced not only between carbon points but also between points of different metals. It is greater with the more fusible or oxydisable metals, as zinc or iron, than with platinum or silver. The size of the arc of light is proportioned to the greater or less facility with which the substance of the electrode disintegrates; for since this phenomenon is produced by minute particles of matter carried over from one electrode to the other, its formation must necessarily be favored by a less cohesion of the electrodes; this is also the reason why, under otherwise like conditions, the greatest arc of light is always obtained between carbon points. The transference of the matter is always from the positive to the negative pole. In the air and with metallic electrodes, the deposit upon the negative pole

always consists of oxydized particles of the metal used as the positive electrode.

If the negative pole has the form of a plate, while the positive pole is a point, the deposit of the transferred matter upon the plate forms a very regular ring, the centre of which is the projection of the point upon the plate.

When the arc of light is taken between a metallic point and an opposite surface of mercury, the latter, when positive rises in a cone, but forms a cavity when negative. In this case it is very difficult to observe accurately the minutia of the phenomenon, on account of the great quantity of mercurial vapor evolved.

De la Rive made experiments with plates and points of platinum, iron, silver, and copper, but I cannot enter upon the details of the experiments, because there is much that is not clear to my mind; in many cases, for instance, I cannot see in the individual experiments the proof and confirmation of the generalizations announced. repetition of these experiments and an accurate description, illustrated when practicable with figures, seems therefore very desirable.

A

§ 65. Intensity of light of the voltaic arc.-Casselmann has made experiments upon the intensity of light of the voltaic arc, which have been described in the memoir already mentioned. They were afterwards also copied into Poggendorf's Annals.—(Pog. Ann., LXIII, 576.) The photometer used in his experiments was constructed upon the same principles as that described in the third edition of my Lehrbuch der Physik, vol. II, 674. The carbon pieces, between which the arc was taken, were of the same composition as that used in the cylinders of Bunsen's battery, but prepared also in other ways, as some of them were saturated in solutions of nitrate of strontium, boracic acid, &c., and then intensely ignited. Thus prepared they gave a very steady light, differently colored, according to the solution employed; and the carbon points could (with a Bunsen battery of 44 cups) be removed to a distance of 7 to 8 millimetres before it disappeared, while the unsteady light of unprepared carbon went out at a distance of 5 millimetres.

A tangent compass was at the same time inserted into the circuit, so that for each measurement of the intensity of light the corresponding force of current could be determined.

The brightest parts of the whole light, it is well known, are at the points of the two pieces of carbon, upon which the arc rests. In the following table the intensity of the whole light is compared with that of a stearine candle, and for each kind of carbon, with the points once at a very small, and then at the greatest possible distance. The values of the force of current are reduced to the chemical unit.

This table shows that on increasing the distance of the points the intensity of light and the force of current decrease. By most of the substances with which the carbon had been prepared, the arc of light was made more steady and allowed of a greater distance of the points, but the intensity was not greater, except with the carbon prepared with borax and sulphuric acid.

But the results in the above table are only approximately accurate, since the changeable position of the most brilliant points at the origin of the arc may have prevented the light from acting with its full intensity upon the photometer. In another series of experiments, an abstract of which is given in the following table, this error was avoided, the arc of light having been directed towards the photometer by means of a magnet. In these experiments only 34 Bunsen's cups were used, the distance of the carbon points was not changed, and the intensity of the light was measured for different degrees of force of

The carbon saturated with sulphate of soda was not heated to redness before use.

It follows from these experiments that the intensity of light increases in a somewhat greater ratio than the force of the current.

It is to be regretted that we have no measures of the intensities of the galvanic light, when different metals are used instead of the carbon points.

Fizeau and Foucault have also made comparative experiments on the intensity of the galvanic arc light, but from another point of view.— (Ann. de Chim. et de Phys. ser. III, T. XI, pp. 370; Pog. Ann., LXIII, 463.) They did not compare the intensity of the light from different sources, but its chemical effect. In this way they compared the galvanic light with that of the sun, and of lime incandescent in detonating gas. The experiment was conducted in the following manner: An iodized silver plate was inserted in a camera obscura, in the place where the image of the sun or of the light, emanating from the carbon or lime, was formed. After a short action of the light the camera obscura was closed, and the position changed, so that another image of the object was shown upon the prepared silver plate beside the first one; the exposure was somewhat longer than before; for a third place still longer, &c. The plate was then put into the mercury bath and examined, in order to find which one of the images became visible by the action of the mercurial vapor. In this way it was ascertained how long the light had to act in order to produce that change in the iodide of silver, which is necessary for the condensation of the mercurial vapor.

If all the other circumstances were entirely identical, the time required for the production of the Daguerrean image would be nearly inversely proportional to the chemical intensity of the corresponding sources of light.

But Fizeau and Foucault used for their experiments with the artificial light lens of shorter focus than for obtaining images of the sun; the aperture of the lens also was varied by means of diaphragms. These circumstances have, therefore, to be taken into account.

If the image is n times further from the lens, it will, cæteris paribus, be n times greater in its linear dimensions, and will, therefore, cover a surface n times as large, and consequently the intensity of light at each point of the image will be n2 times less. The chemical power of the source of light may, therefore, be considered proportional to the square of the distance of the image formed from the lens.

But it is also, as easily perceived, inversely proportional to the surface of the opening of the lens, i. e., to the square of its radius, and therefore

d
J=
t. r2

when J denotes the chemical power of the source of light, d the distance of the image from the lens, r the radius of its opening, and t the time required to produce a Daguerrean image.

If we denote by a the angle which the radius of the aperture of the lens subtends at the place of the image, then

By this, or rather by a similar equivalent formula, Fizeau and Foucault computed the results of their observations, and thus obtained the following relative values for the intensity of the sources of light. Sun-light, in August and September, at noon, with a clear sky Carbon-light, produced by 46 Bunsen's zinc-carbon cups........ Lime-light.......

1000

235

6.8

The lime-light appears to be surprisingly little; but Fizeau and Foucault found with the common photometrical method a similar relation between the lights from lime and carbon. No other comparative measurements are known to which these can be referred; careful experimental re-examination of the matter is, therefore, desirable.

a

In reference to the change of the intensity of the carbon-light with the number and magnitude of the galvanic elements we find the following data in this memoir: While a battery of 46 Bunsen's elements gave an intensity of light of 235, this was increased to 238 only when the number of cups was augmented to 80; but a battery of 46 triple cups gave an intensity of 385, after having been already in action for one hour.

In consequence of the rapid alteration of the fluid-the diluted sulphuric acid becoming gradually a solution of sulphate of zinc-the force of the battery, and with it the intensity of the arc of light produced by it, decreases rapidly. While 80 cups afforded at first an intensity of 238, this after three hours was diminished to 159.

It is to be regretted that these physicists have not measured the force of current corresponding to the intensity of light, whereby the value of the above given numerical relations would have been very much enhanced.

§ 66. Production of heat by the voltaic arc.-The heat developed at the poles, between which the arc is taken, is entirely too great to be attributed to the mere passage of the electric current through these conductors. According to the experiments mentioned in § 57, a current, to make a platinum wire of 0.75 mm. in diameter incandescent by its passage, must have at least a force of 160. Therefore, to make a platinum wire of 3 mm. in diameter only white-hot requires, at the very least, the enormous force of current of 640; and yet with the current of a Bunsen's battery of 44 cups and a force of 80 to 100, we can produce an arc in which the point of a platinum wire of more than 3 mm. in diameter may easily be melted into a globule, if used as one pole of the battery while the other is formed by a carbon point. The combustion of carbon is so trifling that it cannot essentially contribute to the great heat produced; besides, the fusion of the platinum wire by the galvanic arc takes place in a vacuum as readily as in the open

air.

The electric current, therefore, besides producing heat by its mere passage through the conductors, in forming the arc must act at the pole itself to produce heat in some other way, of which as yet we know nothing.

The development of heat is not equal at the two poles of the arc; it is greater at the positive than in the negative. De la Rive, in his