37. =Influence of Heat on the Bulk of Liquids.=--Although the interstices between the particles of liquids can not be varied by mechanical pressure, they can be by variations of temperature. Liquids are dilated or expanded by heat; that is, their particles are put farther apart. They are contracted or compressed by cold; that is, their particles are brought nearer together by the abstraction of heat. The most familiar example that we have is in the thermometer. The mercury rises in the tube when the heat increases the interstices between its particles; and it falls when the loss of heat allows the particles to come near together. The same effects are seen when alcohol is used in the thermometer, as is done in the arctic regions, because mercury may freeze there. A thermometer with water in it would answer if we wished only to measure temperatures between the freezing point and the boiling point of water. The expansive influence of heat will be particularly treated of hereafter. 38. =Compressibility of Aeriform Substances.=--Aeriform bodies are more compressible than any other substances, showing that in their ordinary condition there is a great deal of space among their particles. While they are thus unlike liquids in compressibility, they are affected by heat in the same way that liquids are. [Illustration: Fig. 4. Fig. 5.] 39. =Elasticity.=--Closely allied with the compressibility of matter is its elasticity. We see this property strikingly exemplified in India-rubber. It occasions the rebounding of a ball of this substance when thrown down. Observe flow exactly what occurs in this case. The ball as it meets the resistance of the floor is flattened, as represented in Fig. 4. Then, as it assumes the round shape, as seen in Fig. 5, it pushes downward upon the floor. It is this sudden pushing downward that makes it rebound. It is as if there were a compressed spring between the ball and floor. It may be likened also to jumping. When one jumps he bends his limbs at the thigh and knee joints, and then, in straightening himself up, gives a sudden push, like that given by the ball as it assumes its round shape, and so is thrown forward or upward, according to the direction in which the pushing force is made. The same flattening occurs in an ivory ball, though not to the same degree. You can prove that it does occur by experiment. Let a marble slab be wet and drop the ball upon it. Quite a spot will be made dry by the blow of the ball, showing that it touched more of the marble than it does when it is merely placed upon it.