30 POROSITY. 



these bodies consist, the unassisted eye perceives many 

 interstices of variable size, termed pores. The property 

 of bodies to contain pores between their particles is 

 called porosity. In bodies like wood, cork, paper, sand- 

 stone, the pores are much smaller and can only be seen 

 with a magnifying glass. Many other bodies, even 

 when thus examined, present no visible pores. Never- 

 theless such interstices exist, although they are very 

 small. Thus hard stones are often coloured for com- 

 mercial purposes ; this would be impossible if the 

 colouring matter could not penetrate into their pores. 

 Again, by using great pressure, liquids may be forced 

 even through metals. 



Even in liquids, where we should least expect pores, 

 their existence may be demonstrated. A small bottle 

 with ground stopper, of about 100 or 200 CC capacity, 

 is filled half with water ; alcohol is then poured in 

 cautiously along the side of the bottle, so as not to mix 

 the two liquids, which may be seen to be separate by 

 bringing the middle of the bottle to the level of the 

 eyes. The bottle should be filled so that a few drops 

 of alcohol are pushed out when the stopper is inserted, 

 and no air-bubbles should remain in the bottle. In order 

 to be sure that no more alcohol leaves the bottle, that 

 which has run out should be carefully wiped off with 

 a cloth, without removing the bottle from the table on 

 which it stands. 1 Now raise the bottle between thumb 

 and fingers, press the forefinger firmly upon the stopper 

 and invert the bottle several times so as to mix the two 

 liquids thoroughly. Numerous tiny bubbles will be 



1 Alcohol injures the polish of the table ; it is therefore better to place 

 the bottle upon a china plate, if the table is polished. 



