MATTER AND ITS PHYSICAL PROPERTIES. 29 



with air, are pervaded by the water, between which and the stone there sub- 

 sists a physical relation by which the one renders the other perfectly trans- 

 parent. 



Larger mineral masses exhibit degrees of porosity not less striking. Water 

 percolates through the sides and roofs of caverns and grottoes ; and being im- 

 pregnated with calcareous and other earths, forms stalactites or pendent protu- 

 berances, which present a curious appearance. 



COMPRESSIBILITY. 



That quality in virtue of which a body allows its volume to be diminished 

 without diminishing its mass, is called compressibility. This effect is produced 

 by bringing the constituent particles more closely together, and thereby in- 

 creasing the density and diminishing the pores. This effect may be produced 

 in several ways, but the name compressibility is applied to it when it is caused 

 by the agency of mechanical force, as by pressure or percussion. All known 

 bodies, whatever be their nature, are capable of having their dimensions re- 

 duced without diminishing their mass, and this is one of the most conclusive 

 proofs that all bodies are porous, or that the constituent atoms are not in con- 

 tact ; for the space by which the volume may be diminished, must, before the 

 diminution, consist of pores. Some bodies, when compressed by the agency 

 of mechanical force, will resume their former dimensions with a certain force 

 when relieved from the operation of the force which has compressed them. 

 This property is called elasticity, and it follows from this definition that all elas- 

 tic bodies must be compressible, although the converse is not true compressi- 

 bility not necessarily implying elasticity. 



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 circum- 

 stances worthy of attention. It is a physical law, to which there is no real 

 exception, 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 diminu- 

 tion of temperature is accompanied by a diminution of volume. The apparent 

 exceptions to this law will be noticed and explained in our discourses on hat. 

 Hence it appears that the reduction of temperature is an effect which, consid- 

 ered mechanically, is equivalent to compression or condensation, since it di- 

 minishes 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. 



The fact that the elevation of temperature produces an increase of volume, 

 is manifested by numerous experiments. 



If a flaccid bladder be tied at the mouth so as to stop the passage 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, 



