POROSITY. 31 



4. POROSITY. 



Nature of porosity. We have seen that the molecules of any 

 substance are not in absolute contact, but that there are spaces 

 between them which we call intermolecular spaces, and the 

 property of matter to have spaces between the particles com- 

 posing it is known as porosity. 



In the case of solids, these spaces or pores are sometimes of 

 considerable size, visible even to the naked eye, as, for instance, 

 in charcoal, whilst in most cases these spaces cannot be dis- 

 covered, even by the microscope. That even apparently very 

 dense substances are porous, can be demonstrated by the fact 

 that liquids may be pressed through metallic disks of consid- 

 erable thickness, or that gases may be caused to pass through 

 plates of metal or stone, and finally also by the fact that sub- 

 stances suifer expansion or contraction in consequence of in- 

 creased or diminished heat, or in consequence of mechanical 

 pressure. 



Surface. In every-day life the expression surface refers to that 

 part of a substance which is open to our senses, visible and 

 measurable ; but from a more scientific point of view, we have 

 also to take into consideration those surfaces which, in conse- 

 quence of their porosity, extend to the interior of matter and are 

 invisible to our eyes and absolutely immeasurable by instru- 

 ments. 



Surface-action. Attraction acts differently under different 

 conditions, and accordingly we assign different names to it. 

 We call it cohesion when it acts between molecules, gravitation 

 when acting between masses, and surface-action or surface- 

 attraction when the attraction is exerted either by the visible 

 surface or by that surface which pervades the whole interior of 

 matter. The phenomena caused by this surface-action are ex- 

 tremely manifold, and some are of sufficient interest to be taken 

 into consideration. 



Adhesion. Most solid substances, when immersed in water, 

 alcohol, or many other liquids, become moist; immersed in mer- 

 cury, they remain dry. We explain this fact by saying that the 



