Notice how certain sounds, such as _b_ and _p_ and also _t_ are represented by very high notches or teeth in this line of light. These sounds are called _explosive consonants_, and if you examine the manner in which they are made by your mouth, you will notice that it consists in closing the mouth by the lips or tongue placed between the teeth, and then suddenly withdrawing the obstruction so as to allow the air from the lungs to rush forcibly out. Hence the air outside, and in this case the diaphragm, receives a sudden blow, which is represented by this tall tooth or notch in the luminous band. The experiment teaches us that whereas _musical tones_ are caused by certain very regular and uniform vibrations of the sounding body, _vocal sounds_ and _noises_ are caused by very irregular movements. Also that loud sounds are created by large motions, and feeble ones by small motions. Again, that the difference between tones in music is a difference in the rate of vibration of the sounding body. We may infer also that the difference between the quality of sounds is connected with the _form_ of the wave-motion made by them. Having established these facts, we must, in the next place, proceed to notice a little more closely the nature of an _air wave_. It will be necessary to remind you of certain qualities possessed not only by the air we breathe, but by all gases as well. Here is a cylinder with a closely fitting piston, and a tap at the bottom of the tube. If I close the tap and try to force down the piston, I feel some resistance, which increases as the piston is pushed forward. If the pressure is removed, the piston flies back to its old position, as if there were a spring underneath it. The air in the tube is an elastic substance, and it resists compression. At constant temperature the volume into which the air is squeezed is inversely as the pressure applied. The air, therefore, possesses _elasticity of bulk_, as it is called, and it resists being made to occupy a smaller volume. Again, the air possesses _inertia_, and when it is set in motion it continues to move like any other heavy body, after the moving force is withdrawn. We have, therefore, present in it the two essential qualities for the production of a wave-motion, as explained in the first lecture. The air _resists_ compression in virtue of elasticity, and when it is allowed to expand again back, it _persists_ in motion in virtue of inertia.