Hold the vibrating fork over the mouth of an empty fruit-jar and there will probably be little or no reinforcement; but gently pour in water, thereby shortening the air column within the jar, and the sound of the fork will be gradually intensified until at a certain point it becomes quite loud. If you pour in still more water the sound will gradually become feebler. This shows that _for every tone an air column of a certain size most powerfully reinforces that tone_. Experiment V As a sequence to the last experiment, take two fruit-jars of the same size, and, having learned to what point to fill them for the greatest resonance, fill one jar (after warming it) to the required point with hot water, the other with cold water, and you will find that the resonance of the heated, therefore expanded, air is much less than the denser air of the cold jar. This shows that _the degree of density of the air affects its resonance_. Experiment VI To demonstrate the resonance of the oral cavity, apart from the voice, hold a vibrating tuning-fork before the open mouth. Vary the shape and size of the cavity until the sound of the fork suddenly increases in volume, showing that the right adjustment for resonance has been made. _This intensification of the sound is due to the vibration of the air in the mouth cavity, together with the sympathetic vibration of the surrounding walls._ Experiment VII As an illustration of sympathetic resonance without contact, sing forcibly a tone that is within easy range, and at the same time silently hold down the corresponding key of the piano. On ceasing to sing you will hear the tone sounding in the piano. This may be further illustrated by playing on the open string of one violin while another, tuned to the same pitch, rests untouched near by. Through _sympathetic resonance_ the corresponding string of the second violin will vibrate and sound its note. The louder the first violin is played the louder will be the sympathetic tone of the second. The deep pedal-tones of a church organ often induce sympathetic resonance that may be felt beneath the feet of the listener. One writer, a singer, speaks of living in the same house with two deaf-mutes. He lodged on the first floor, they on the third. One day, meeting at luncheon, one of the deaf-mutes told the singer that he had begun practice earlier that morning than usual. Surprised, the writer asked how he knew. The deaf-mute replied that they always knew when he was singing because they felt the floor of their room vibrate. If tone vibrations can be transmitted so readily throughout a house, it is not difficult to understand how easily the vibrations of bone and tissue can be transmitted until the whole framework of the body responds in perceptible vibration.