FUNDAMENTAL PROPERTIES OF MATTER. 39 



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 charcoal, generally condense 

 or absorb larger quantities than solids of a more dense and compact 

 structure. Heat, as stated above, counteracts this absorbing power. 



The absorptive power of charcoal for gases varies with the nature of the 

 charcoal and the gas absorbed, as will be seen from the following table: 



Unit volume of charcoal absorbs 



Boxwood. Cocoanut. 



Ammonia gas 90 vols. 172 vols. 



Hydrochloric acid gas 85 " 



Sulphur dioxide 65 ' 



Hydrogen sulphide 55 ' 



Nitrogen monoxide 40 ' 86 vols. 



Carbon dioxide 35 ' 68 " 



Ethylene gas 35 ' 75 " 



Carbon monoxide . 9.42 ' 21 " 



Oxygen 9.25 ' 18 " 



Nitrogen 7.5 ' 15 " 



Hydrogen 1.75 ' 4 " 



Pine charcoal has about one-half the absorptive power of boxwood char- 

 coal. Platinum sponge absorbs about 250 times its volume of oxygen. Other 

 porous substances, as meerschaum, gypsum, silk, etc., are also very absorbent. 



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 the process 

 absorption. This absorbing power of different liquids for different 

 gases varies greatly ; it is facilitated by low temperature and high 



