hole in the disk, and concentric with it, there is a thin sheet-iron pipe 4 inches in diameter, whose upper end almost touches the disk while its lower end descends to within 4 or 5 inches of the bottom of the cylinder. When the fan is made to revolve rapidly, the air passes up through the central pipe with a velocity of 460 feet per minute. At its upper end the spindle carries a small grooved pulley, d, which is made to revolve by means of an endless cord passing round it and over another large grooved driving wheel D. The lower end of the cylinder rests on an iron ring screwed down to the top of the wooden gallery BC, in which there is an opening corresponding to the size of the cylinder. The cylinder is attached to one side of the iron ring by a hinge which allows it to be folded back into the position shown in fig. 4. At the other side it can be fastened by a screw in an upright position as shown in the other figures.

The gallery BC, consists of a range of wooden pipes 14 inches square inside, and, altogether, 79 feet long. The centre of the cylinder A is 5 feet from the end B, and there is a valve just below the point b, by means of which the part of the gallery towards B can be isolated from the remainder. The separate pipes have broad wooden flanges which are put close together when they are placed so as to form a gallery, but they are not fastened to each other in any way; they rest on wooden blocks a a, and any one of them can be drawn out from between the two on each side without disturbing the others. This is shown by the dotted lines in fig. 6. Near the end B there is a sheetiron cylinder 3 feet long by 10 inches in diameter, closed at each end, and having an inlet pipe for steam at q, and an outlet pipe at t for condensed water and steam. At the same end also there is a branch e, f, g, leading to K, which is the horizontal part d c, of the apparatus represented in fig. 1. This branch is also connected with a blowing fan F, driven by a steam turbine of which I and h are the steam and exhaust pipes. The pipe bringing fire-damp from below ground, referred to in my former paper, can be made to deliver its fire-damp into the air inlets of the fan in the same way as in the former experiIt will now be evident that currents of air of various qualities can be made to traverse the gallery B C from B towards C. Thus, if the valve s is open, while the valve s' is shut, the return air of the upcast shaft passes through the apparatus and escapes at C; but if s' is open and s is shut, the return air is cut off, and by setting the fan F in motion, we obtain either pure air, or air and fire-damp mixed, as we may desire. In every case, also, the air passes over, and is heated by, the steam cylinder p, so that, even when return air is used, the interior of the gallery CD can be kept dry.

ments.

The interior of the cylinder A is lined with wood about 2 inch thick, and its capacity is about 4.648 cubic feet. For the purpose of obtaining the explosive mixture required, the cylinder E, 8 inches in diameter,

and 2 feet long, is filled with fire-damp, of which a certain measured proportion is afterwards transferred to the interior of the cylinder A through the india-rubber tube r n. This is done by admitting water through an india-rubber pipe attached at the point O. At the same time as the gas is flowing in at the top of the cylinder A, air is allowed to escape at the point b near its bottom by taking out a plug for that purpose. The plug is put in immediately after the operation is completed. The amount of fire-damp employed is as near as may be 456 cubic foot. The cylinder E is refilled with fire-damp by shutting a stop-cock at n, opening another at m, which is connected with the fire-damp pipe by means of an india-rubber tube not shown in the drawing, and lowering the water bucket below the level of the bottom of the cylinder E. Before the fire-damp is admitted to the interior of the cylinder A, a paper diaphragm is inserted between its lower end and the ring to which it is hinged and screwed, so as to isolate it from the gallery BC. The explosive mixture in the cylinder A is ignited by the spark of a powerful magneto-electric machine which Messrs. Cross Brothers, of Cardiff, most kindly lent to me for the purposes of these experiments. The wires pass through the plug at b, and are brought together just inside the cylinder.

The method of forming an explosive mixture in the cylinder A, will now be sufficiently plain, but I will repeat the description of the operations in regular order. When the cylinder is in the position shown in fig. 4, several sheets of paper are laid over the opening in the top of the gallery; the cylinder is then raised to an upright position and fastened by means of the screw. The plug b is opened and fire-damp is made to flow through the pipe rn, displacing a corresponding volume of air which escapes at b. As soon as the requisite volume of gas has been obtained the cock r is shut and the plug b is replaced. The driving wheel D is next made to revolve at the rate of about eighty turns per minute; twenty-five or thirty turns being found quite sufficient to make a perfect mixture; and, thereafter, a spark from the magneto-electric machine causes the explosion.

When there is no coal-dust in the gallery BC, the flame of the fire-damp explosion does not extend further than from 7 to 9 feet. from the bottom of the cylinder A. It should be understood that the valve at b is always closed just before the spark is passed.

When the gallery contains coal-dust, on the other hand, scattered along its floor, and lying on a few shelves, whose position will be given immediately, and when it is filled with the return air of the upcast shaft, the flame of the explosion traverses its whole length, and shoots out into the air at the end of C, to distances varying from from 4 to 15 feet beyond it. At first, it appeared to me that the wooden gallery might be prolonged indefinitely with the same result; but on adding another pipe at the end of C, I was surprised to find.

that I could not, by any possibility, get the flame to travel more than one-half or two-thirds of the former distance, and I came to the conclusion that the initial impulse, which raises the coal-dust, is insufficient to overcome the resistance under the altered conditions. Again, I had 60 feet of nearly air-tight pipe prepared, thinking thereby to prevent the energy of the wave created by the fire-damp explosion from being dissipated; but here, once more, I found that it was impossible to get the flame to travel to a distance of more than 30 or 40 feet from the origin, and in this case I concluded that the expanded part of the wave extinguished the flame of the coal-dust. The best results were obtained when the wooden pipes had open seams along the junction of the boards of which they are formed.

At the beginning of the present month (March, 1879), Professor G. G. Stokes, F.R.S., communicated to me the suggestion that if a weak solution of chloride of calcium were used for watering the roadways of mines, instead of ordinary water, the deliquescent salt, would tend to retard evaporation, and a smaller quantity of water would serve the purpose of keeping the workings damp. Accordingly, I have begun an experiment with such a solution in a dry mine, but it is not yet sufficiently advanced to enable me to state any results in the present paper.

The temperature of the air current passing along the gallery varied from 74° F. near the explosion-cylinder to 60° at the end C. The wooden shelves spoken of above were in sets of three (one above the other at equal distances) the shelves themselves being about 6 inches broad. One set was placed at each of the points ×, fig. 6; and a brick was placed so as to obstruct the passage below the lowest shelf of the first, third, and fourth sets for the purpose of causing the force of the explosion to exert itself more powerfully in sweeping the dust off the shelves and mixing it with the air.

The arrangements whereby pure air, or pure air and fire-damp, can be employed, were only completed before the weather became unsuitable for continuing the experiments, which, I need hardly say, are made in the open air. I obtained sufficient results, however, to show that the absence of even the small proportion of fire-damp contained in the return air of Llwynypia Colliery, makes a great difference in the force of the explosion and the distance to which the flame will travel along the gallery. I found, also, that when two per cent. of fire-damp was added to the current of pure air entering the fan F, even better results were obtained than with the return air of the mine.

Although this apparatus appears to be on too small a scale to solve the coal-dust question unequivocally, I think the results obtained with it are sufficiently conclusive to enable us to affirm, that a firedamp explosion, occurring in a dry coal mine, is liable to be in

definitely extended by the mixture of air and coal-dust produced by the disturbance which it initiates.

The dangers due to the presence of coal-dust in dry mines can be very easily avoided by sprinkling water plentifully on the principal roadways along which the air currents pass, in going to, and coming from, the working places. For example, Llwynypia Colliery, which was formerly one of the driest and most dusty of the mines in the South Wales basin, is now kept constantly damp or wet in this way with a daily expenditure of about 1,800 gallons of water. The amount of air passing through it at present is over 80,000 cubic feet per minute, and its out-put of coal is, on the average, about 800 tons per day.

III. "The Contact Theory of Voltaic Action." No. III. By Professors W. E. AYRTON and JOHN PERRY. Communicated by Dr. C. W. SIEMENS, F.R.S. Received February 19, 1879.

(Abstract.)

The authors commence by referring to the experiments that had been made prior to 1876, on the difference of potentials of a solid in contact with a liquid, and of two liquids in contact with one another, and they point out that :

1. The earlier experiments were not carried out with apparatus susceptible of giving accurate results.

2. Owing to the incompleteness of the apparatus assumptions had to be made not justified by the experiments.

3. No direct experiments had been performed to determine the difference of potential of two liquids in contact, with the exception of a few by Kohlrausch, using a method which appeared to the authors quite inadmissible as regards accuracy of result.

In consequence of this great vagueness existed as to whether the contact difference of potentials between two substances, when one or both were liquids, was a constant depending only on the substances and the temperature, or whether it was a variable dependent upon what other substance was in contact with either. Some authorities regarded it as a variable. Gerland considered he had proved it to be a constant, but first, the agreement of the value of the electromotive force of each of his cells with the algebraical sum of the separate differences of potential at the various surfaces of separation, and which was the test of the accuracy of his theory, was so striking, and so much greater than polarisation, &c., usually allows one to obtain in experiments of such delicacy, that one could not help feeling doubtful regarding his

conclusions; secondly, his apparatus did not allow of his experimenting with two liquids in contact, consequently he could not legitimately draw any conclusion in this latter case. And although Kohlrausch had made some few experiments on the difference of potentials of liquids in contact, still since he employed moist blotting paper surfaces instead of the surfaces of the liquids themselves, the authors considered for that reason alone, if for no other, that his results did not carry the conviction the distinguished position of the experimenter might have led them to anticipate.

They therefore designed a method and an apparatus for carrying it out, by means of which they could measure the difference of potentials in volts at each separate contact of dissimilar substances in the ordinary galvanic cells, from which they could ascertain whether the algebraical sum of all the contact differences of potential was, or was not, equal to the electromotive force of the particular cell in question. From the results they obtained, and which are given in Papers Nos. I and II, "Proc. Roy. Soc.," No. 186, 1878, they concluded within the limits of their experiments that if AB, BC, CD, &c., were the contact differences of potential measured separately of the substances A in contact with B, B in contact with C, &c., then, any one or more of the substances being solid or liquid, if any number A, B, C - - - - - K were joined together, and the electromotive force of the combination AK, measured, the following equation was found true:—

which proved that each surface of separation produced its effect independently of any other.

Their method by which any single contact difference of potentials was measured was as follows:-Let 3 and 4 be two insulated gilt brass plates connected with the electrodes of a delicate quadrant electrometer. Let 1 under 3, and 2 under 4 be the surfaces whose contact difference of potential is to be measured; 3 and 4 are first connected together and then insulated, but remain connected with their respective electrometer quadrants. Now 1 and 2 are made to change places with one another, 1 being now under 4 and 2 under 3, then the deflection of the electrometer needle will give a measure of the difference of potentials between 1 and 2; and in the present paper it is proved that in order that the observed difference of potentials in the electrometer quadrants shall be proportional to the contact difference of potentials desired to be measured, either there must be perfect symmetry in the induction apparatus before and after reversal of 1 and 2, a condition very difficult to be obtained, or else the plates 3 and 4 must, in addition to being connected together, be also put to earth, or reduced to zero potential, before each reversal, and also the mean potential of the substance under test