To charge the battery will take about twenty turns of this large Holtz machine. Observe how the pith ball of the electroscope rises as the machine is worked, showing that the charge is going in. And now it remains stationary; which is a sign that the battery is fully charged, and can receive no more. You will notice that the outside coating of the battery has been already connected with one of the brass rods dipping into the tumbler of water. By means of this discharger I will now bring the inside coating into connection with the other rod. And see, before contact is actually made, the spark has leaped across, and our tumbler is violently burst asunder from top to bottom. [Illustration: GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY.] This will probably appear to you a very small affair, when compared with the tearing asunder of solid masonry, and the hurling about of stones by the ton weight. No doubt it is; and that is just one of the lessons we have to learn from the experiment we have made. For, not only does it show us that effects of this kind may be caused by electricity artificially produced, but it brings home forcibly to the mind how incomparably more powerful is the lightning of the clouds than the electricity of our machines. The property which electricity has of setting fire to combustible substances may be easily illustrated. This india-rubber tube is connected with the gas pipe under the floor, and to the end of the tube is fitted a brass stop-cock which I hold in my hand. I open the cock, and allow the jet of gas to flow toward the conductor of Carré’s machine, while my assistant turns the handle; a spark passes, and the gas is lit. Again, my assistant stands on this insulating stool, placing his hand on the large conductor of the machine, while I turn the handle. His body becomes electrified, and when he presents his knuckle to this vessel of spirits of wine, which is electrically connected with the earth, a spark leaps across, and the spirits of wine are at once in a blaze. Once more; I tie a little gun-cotton around one knob of the discharging rod, and then use it to discharge a small Leyden jar; at the moment of the discharge the gun-cotton is set on fire. It would be easy to explode gunpowder with the electric spark, but the smoke of the explosion would make the lecture-hall very unpleasant for the remainder of the lecture. I propose, therefore, to substitute for gunpowder an explosive mixture of oxygen and hydrogen, with which I have filled this little metal flask, commonly known as Volta’s pistol. By a very simple contrivance, the electric spark is discharged through the mixture, when I hold the flask toward the conductor of the machine. A cork is fitted tightly into the neck of the flask, and at the moment the spark passes you hear a loud explosion, and you see the cork driven violently up to the ceiling. [Illustration: GUN-COTTON SET ON FIRE BY ELECTRIC SPARK.]