“Well done, my boy! Now, then, explain the reason of the leather’s adhesion to the surface, and of its being thus capable of retaining its hold, notwithstanding the gravity of the stone.” “In the first place,” answered Tom, “the edges of the wet leather, from being closely pressed, stuck with sufficient firmness to the smooth surface of the stone, to resist the force of the string as I pulled it upwards; the consequence was, that a hollow was formed in the middle part of the leather; and, as that hollow place cannot contain any air, it is called a _vacuum_.” “Very well,” replied his father, “so far you are right; but you have not informed me in what manner a _vacuum_ acts in preventing the stone from quitting the leather.” “It makes it adhere to it by some kind of _suction_, but I confess that I do not exactly understand the subject.” “Then let us proceed cautiously and deliberately in the explanation. In the first place, you have said, and said correctly, that the edges of the leather adhere to the stone; but what is the nature of the power to which this adhesion is to be referred? I perceive you are puzzled by the question: attend, then, to my explanation: you must know that there exists a tendency in all bodies to adhere together, provided the contact of their surfaces be sufficiently perfect; this property is termed _cohesion_, or cohesive attraction, from the Latin word _cohæreo_, which I need not inform you signifies to _stick together_. The dry leather will not adhere to a smooth surface, because, in that case, the contact cannot be rendered sufficiently perfect; but, when saturated with water, the interstices of the leather are filled with that fluid, and the inequalities of the surface, which must always prevent close contact, are removed. If two bodies, when placed together, be not sufficiently smooth, or polished, it will be vain to make any attempt to produce their cohesion; since the particles will, in such a state, touch each other only in a few points; it is for this reason that carpenters, when they intend to glue pieces of wood together, plane the surfaces perfectly smooth, before they apply the glue.” Tom here acknowledged that he had not before understood the reason of the leather’s adhesion to the stone. “Having, then, settled this point to your satisfaction,” continued Mr. Seymour, “let us proceed. Your idea of a _vacuum_ being formed in the hollow part of the leather is perfectly correct: for, as you draw up the central part by the string, the hollow thus produced must necessarily be a _vacuum_, since the air cannot pass through the leather to supply it; in this state, therefore, the atmosphere presses upon the exterior of the leather, and like any other weight prevents its rising from the stone.”