In the first place, let the radiator-box and receiver-box be placed a few feet apart with their open mouths facing each other, like two guns arranged to fire down each other’s throats. Then, if all is in order when we make an electric spark between the two balls of the radiator, the electric bell in connection with the receiver will begin to ring, showing that the coherer in the receiver-box has been affected and made conductive by the electric wave sent out from the radiator-box. If a smart rap is then given to the receiver-box the clinging metallic filings in the ebonite box will be separated again and, the circuit being interrupted, the bell will stop ringing. This being done, the radiator-box is then turned a little on one side by rotating it round its hollow trunnions like a gun until the open mouths of the two boxes no longer face each other. It will then be found, on repeating the former experiment, that the bell will not ring when a spark is made between the balls. A little experimenting will show that the action which affects the coherer is propagated out from the radiator-box in straight lines like the light from a lamp, and that we are here dealing with something which has all the character of radiation. In the next place, let the receiver- and radiator-boxes be again arranged with their open mouths facing each other. We make a spark and again secure the responsive action of the bell. We shall now proceed to prove that this effect, which is called _electric radiation_, passes quite freely through certain substances, but is more or less completely stopped by others. For instance, if we hold a sheet of iron, tinfoil, or even paper covered with silver leaf between the open mouths of the radiator and receiver, we find that the bell of the receiver will not ring even when a rapid series of oscillatory sparks are made in the radiator. These sheets of metal, thick or thin, are quite _opaque_ to the electric radiation proceeding from the spark-balls. On the other hand, we find a sheet of paper or card, a wooden board, a sheet of glass, a slab of wax or bitumen, sulphur, marble, or slate, are all quite pervious or transparent, and when held between the radiator and receiver do not hinder at all perceptibly the action of the former on the latter. We conclude, therefore, that some bodies are opaque and some transparent to the electric radiation. But the classification does not agree with the classification as regards opacity or transparency for light. Wood, marble, and pitch are optically opaque, but electrically transparent. The general law, however, which decides the question of opacity or transparency for electric radiation, is as follows: _All good electrical conductors are opaque to electric radiation, and all good insulators or non-conductors are transparent._ Hence we see at once why metal sheets are opaque, and wood, wax, or glass transparent, to the electric radiation from the spark-balls.