(25.) _Dilatability._--This quality is the opposite of compressibility. It is the capability observed in bodies to have their volume enlarged without increasing their mass. This effect may be produced in several ways. In ordinary circumstances, a body may exist under the constant action of a pressure by which its volume and density are determined. It may happen, that on the occasional removal of that pressure, the body will _dilate_ by a quality inherent in its constitution. This is the case with common air. Dilatation may also be the effect of heat, as will presently appear. The several qualities of bodies which we have noticed in this chapter, when viewed in relation to each other, present many circumstances worthy of attention. (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_. (23.) 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. (28.) Let A B, _fig. 2._ be a glass tube, with a bulb at the end A; and let the bulb A, and a part of the tube, be filled with any liquid, coloured so as to be visible. Let C be the level of the liquid in the tube. If the bulb be now exposed to heat, by being plunged in hot water, the level of the liquid C will rapidly rise towards B. This effect is produced by the dilatation of the liquid in the bulb, which filling a greater space, a part of it is forced into the tube. This experiment may easily be made with a common glass tube and a little port wine. Thermometers are constructed on this principle, the rise of the liquid in the tube being used as an indication of the degree of heat which causes it. A particular account of these useful instruments will be found in our treatise on HEAT.