"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.
