34 INTRODUCTION. 



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 sub- 

 stances are reabsorbed on cooling. 



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

 sorbed are the gaseous constituents of the atmospheric air, chiefly the 

 aqueous vapor. 



Every solid substance upon our earth condenses upon its surface 

 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. 



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 varie^ greatly ; it is facilitated by low temperature and high 

 pressure, and counteracted by high temperature and removal of 

 pressure. Thus : One volume of water absorbs at ordinary tempera- 

 ture and pressure about 0.03 volume of oxygen, 1 volume of carbon 

 dioxide, 30 volumes of sulphur dioxide, and 800 volumes of ammonia. 



Diffusion. When a cylindrical glass vessel has been partially 

 filled with water, and alcohol, which is specifically lighter than 



