36 CHEMICAL PHYSICS. 



Changes in the atmospheric pressure. The height of the mer- 

 cury column in a barometer is not the same at all times, but varies 

 within certain limits. These variations are due to a number of causes 

 disturbing the density of the atmosphere, and are chiefly atmospheric 

 currents, temperature, and the amount of moisture contained in the 

 atmosphere. 



As the height and with it the density of the atmosphere diminishes 

 gradually from the level of the sea upward, the height of the mercury 

 column will be lower in localities situated at an elevation. This 

 diminution of pressure is so constant that the barometer is used for 

 estimating elevations. 



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 ; 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 scmetimes of con- 

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

 charcoal, while 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, in consequence 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 



