"I can understand now, I think, sir," said Fred, as they walked home, "why porous bodies have the power of absorbing liquids. Our former lessons merely taught us that this absorption actually takes place. We saw the liquids rise in the porous bodies. Today"s lesson shows us the reason why they rise.""Well, Fred," said Mr. Wilson, "suppose you try and explain what really happens.""I think I should begin, sir, by tr ying to tell the difference between the force of cohesion and the force of adhesion. Our lesson on cohesion showed us that it is a very difficult matter to join the particles of a solid, because, smooth as their surfaces may appear to the naked eye, the magnifying glass proves them to be so rough and uneven that it is impossible to make them touch at all points, and without touching everywhere there can be no cohesion.
"On the other hand, it is quite easy to make a liquid and a solid join, because the liquid flows and fills up the inequalities in the surface of the solid, so that the two bodies touch everywhere, and the liquid holds to the solid.""Does the force of cohesion accomplish this too?" asked Mr. Wilson.
"No, sir," said Fred. "We call the force which holds the liquid to the solid by another name-adhesion-and the liquid is said to adhere to the solid. The experiment which you showed us with the fine hair-tube, sir, is all due to this adhesive force between the liquid and the solid. The colored water rose in the tube by capillary attraction. The word capillary means like a hair.
"Capillary attraction takes place because of the adhesive force between the liquid and the sides of the glass tube. This adhesive force draws the particles of water upwards and makes them adhere to the sides of the tube.""But you have not told us yet, Fred," said Mr. Wilson, "what all this has to do with the absorbing power of porous bodies.""I am coming to that now, sir," said Fred. "Every pore in a porous body is really a little tube, and the liquids areabsorbed into the pores by capillary attraction, just as the colored liquid rose in the capillary tube.""That"s capital," said Mr. Wilson. "You"ve mastered this matter well, Fred. Remember, it is the same capillary attraction that raises the oil in the lamp wick for the supply of the flame; it is the same capillary attraction that emptied the tumbler of water when you hung the piece of loose wick over the edge of the glass; it is the same capillary attraction that allows the soil in the flowerpot to suck up water from the saucer in which it is placed."Lesson 06
