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(26.) It is a physical law, of high generality, that an increase in the temperature, or degree of heat by which a body is affected, is accompanied by an increase of volume; and that a diminution of temperature is accompanied by a diminution of volume. The exceptions to this law will be noticed and explained in our treatise on Heat. Hence it appears that the reduction of temperature is an effect which, considered mechanically, is equivalent to compression or condensation, since it diminishes the volume without altering the mass; and since this is an effect of which all bodies whatever are susceptible, it follows that all bodies whatever have pores. (ssss1.)

The fact, that the elevation of temperature produces an increase of volume, is manifested by numerous experiments.

(27.) If a flaccid bladder be tied at the mouth, so as to stop the escape of air, and be then held before a fire, it will gradually swell, and assume the appearance of being fully inflated. The small quantity of air contained in the bladder is, in this case, so much dilated by the heat, that it occupies a considerably increased space, and fills the bladder, of which it before only occupied a small part. When the bladder is removed from the fire, and allowed to resume its former temperature, the air returns to its former dimensions, and the bladder becomes again flaccid.