POROSITY. 33 



temperature, show a decrease in weight whilst yet warm. Upon 

 cooling, the original weight will be restored. This fact cannot 

 be explained otherwise than that some substance or substances 

 must have been expelled by heat, and that this substance or 

 these substances are reabsorbed on cooling. 



This is actually the case, and the substances expelled and re- 

 absorbed are the gaseous constituents of the atmospheric air, 

 chiefly the aqueous vapor. 



Every solid substance upon our earth condenses upon its sur- 

 face more or less of the gaseous constituents of the atmosphere. 

 This condensation takes place upon the outer as well as upon 

 the inner surface. The amount of gas absorbed depends upon 

 the nature of the gas as well as upon the nature of the absorbing 

 solid. Some of the so-called porous substances, such as char- 

 coal, generally condense or absorb larger quantities than solids 

 of a more dense and compact structure. Heat, as stated above, 

 counteracts this absorbing power. 



Surface-attraction of solids for liquids or for solids held in solution. 

 When a mixture of different liquids, or a mixture of different 

 solids dissolved in a liquid, is brought in contact with or filtered 

 through a porous solid substance, such as charcoal or bone- 

 black, it will be found that the surface of the solid substance 

 retains a certain amount of the liquids or of the solids held in 

 solution, and that it retains more of one kind than of another. 



It is this peculiarity of surface-attraction which is made use 

 of in purifying drinking water by allowing it to pass through 

 charcoal. Bone-black is similarly used for decolorizing sugar- 

 syrup and other liquids. 



Absorbing power of liquids. In a similar manner as in the case 

 of solids, liquids also exert an attraction for gases. When a 

 gas is condensed within the pores or upon the surface of a solid, 

 or when it is taken up and condensed by a liquid, we call this 

 process absorption. This absorbing power of different liquids 

 for different gases varies greatly ; it is facilitated by low tem- 

 perature and high pressure, and counteracted by high tempera- 

 ture and removal of pressure. Thus: One volume of water ab- 

 sorbs at ordinary temperature and pressure about 0.03 volume 



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