32 INTRODUCTION. 



4. POROSITY. 



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

 stance are not in absolute contact, but that there are spaces between 

 them which we call intermolecular spaces ; the property of matter to 

 have spaces between the particles composing it is known as porosity. 



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

 siderable size, visible even to the naked eye, as, for instance, in 

 charcoal, whilst in most cases they cannot be discovered, 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 considerable thickness, that gases may be caused to 

 pass through plates of metal or stone^that solids dissolve in liquids 

 without showing a corresponding increase in volume of the solution 

 thus obtained, and, finally, also by the fact that substances suffer ex- 

 pansion or contraction in consequence of increased 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 measur- 

 able ; but from a more scientific point of view, we have also to take 

 into consideration those surfaces which, inconsequence of porosity, 

 extend to the interior of matter and are invisible to our eyes and 

 absolutely immeasurable by instruments. 



Surface-action. Attraction acts differently under different condi- 

 tions, 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 extremely manifold, and some are of suffi- 

 cient interest to be taken into consideration. 



Adhesion. Most solid substances, when immersed in water, 

 alcohol, or many other liquids, become moist ; immersed in mercury, 

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

 of most solid substances exert an attraction for the particles of such 

 liquids as water and alcohol to such an extent that these particles 

 adhere to the surface of the solids. Such an attraction, however, 

 does not manifest itself for the particles of mercury. This form of 



