The reason the filament in an incandescent lamp is not burned up is because there is _no oxygen_ inside the globe. After the carbon is put in its place all the oxygen is drawn out through a tube, and the glass is sealed up so that no more oxygen can get in. This is called obtaining a "vacuum," and vacuum means a space without air. There being no oxygen in the globe, it is impossible for the carbon to burn up; so the incandescent lamp will continue to give its light for a very long time, some of them lasting for thousands of hours. Some day, however, from a great variety of obscure causes, the filament becomes weak in some particular spot and breaks, and the light ceases. When this happens, we unscrew the lamp and put another one in, and the light goes on as usual. Now you have learned how the incandescent lamp is made to give light. We will add that it is a beautiful, soft, white light, almost without heat, it will not explode, throws off no poisonous fumes like gas or oil lamps, and has many other points of comfort and convenience which make it very desirable. ELECTRIC-LIGHT WIRES Before closing the subject of electric light you would perhaps like to know something about the way in which we place the wires leading to the lamps. [Illustration: Fig. 25] If you remember what we told you about measurements in the beginning of this book, it will be easy to understand what follows: You know that if you have a very great pressure you can force a quantity through a small conductor. This is the principle upon which the arc-lamps are run. Every arc-lamp takes about 40 to 50 volts and from 5 to 10 ampères to produce the light, and they are connected with the wires as shown in Fig. 25. This is called running lamps in "series," and, as you will see from the sketch, the wire starts out from the dynamo and connects with one carbon of the first arc-lamp, and to the other carbon is connected another wire which goes on to the next lamp, and so on until the last lamp is reached, and then the wire goes back to the dynamo. This forms, practically, one continuous loop from one brush to the other of the dynamo. The current starts out, makes its way through the first lamp, goes on to the next, makes its way through that, and so on till it has jumped the last one; then it goes back to the dynamo. Now, as each of these jumps requires a pressure of 40 or 50 volts, you will easily see that the total pressure, in volts, of the electricity must be as many times 40 or 50 volts as there are lamps to be lighted; so, if there were 60 lamps in circuit, there would be 2,400 to 3,000 volts pressure, which, while it gives very fine lights, might cause instant death to any one touching the wires.