The particles of all bodies are subject to two opposite powers, _repulsion_ and _attraction_; between which they remain in equilibrium. While the _attractive_ force remains strongest, the body remains in a state of solidity; but if heat destroys this force, the particles lose their cohesion, and the body ceases to be solid. _Cohesion_, act of sticking together, union of the constituent parts of a body. Which is supposed to be the most natural state of all bodies? Solidity; for by the _combination_ of caloric with them we can reduce most substances to the fluid state; while the greatest number of _liquid_ substances take a _solid_ form by the loss of caloric. Thus, water congeals and forms ice; and even the gases show this disposition to become solid, when they lose their _elasticity_ by forming some _combination_. Explain the terms _Repulsion_ and _Attraction_. Repulsion is a peculiar property in the particles of matter, which gives them a constant tendency to recede from each other. Attraction is an unknown force, which causes bodies or their particles to approach each other. The particles of all bodies possess this property, which causes them to adhere, and preserves the various substances around us from falling in pieces. What different kinds of Attraction can you mention? Attraction may be distinguished into that which takes place between bodies at sensible distances, and that which manifests itself between the _particles_ of matter at insensible distances. Give an example of the first kind of attraction. One of the most familiar instances of attraction at sensible distances is seen in the descent of heavy bodies to the ground. When a stone is lifted up in the hand, the earth's attraction, which previously caused it to remain at its surface, is overcome; but, as soon as the hand is withdrawn, the stone falls to the earth. The force which causes this is called the _attraction of gravitation_, or simply _gravitation_. How is the second kind of attraction, or that between the particles of bodies, subdivided? Into the _attraction of aggregation_, or _cohesion_; and _chemical attraction_, or _affinity_. The former takes place between particles which are _similar_, and the latter between those which are _dissimilar_. All the operations of chemistry are founded upon the force of affinity which Nature has established between the particles of different kinds of matter, and which enables the chemist to produce _new_ compounds differing more or less from the substances by whose union they were formed. Is it, then, necessary for chemists to understand the relative nature of all substances? Yes; because the basis of this science consists in an _analytical_ examination of the works of Nature; an investigation of the properties and uses of all substances we are acquainted with; and the study of the effects of _heat_ and _mixture_, in order that we may find out their general and subordinate laws.