Country folks escape this severe penalty, because even when they half smother themselves by night, the thoroughly fresh air in which they spend the day goes a great way toward the removal of the mischief. Still they are by no means free from all penalty. You yourself have suffered from breathing bad air. Do you remember last autumn, when I came to see you sick in bed with the fever? Do you recollect how your limbs ached, and your skin burned then, and how you tossed restlessly from side to side, without being able to sleep, your mouth and tongue being brown and parched with dryness which water could not moisten? You could not raise your head from the pillow; and once when I asked you how you felt, you answered me by telling me something about the corn stacks and the last harvest, being quite unconscious of what you were saying. What do you think was the matter with you then? Your body and blood were full of poisonous vapor. And what do you think had made them so? Why, fresh air had not done its work of purification as it ought. You had been breathing a great deal of impure air, and were paying the penalty for having done so. If you had seen the prisoners in the Black Hole of Calcutta an hour or two before they died, you would have found them exactly in the same state. The term “fever” is taken from a Latin word which signifies “to burn.” The skin and the body feel burning hot in fever, because impure poisonous blood is flowing everywhere through their vessels, in the place of pure blood, and the blood is poisonous because it has not been freed from its poison-vapors as fast as they have been bred in, or thrown into its streams. In the worst forms of fever the blood gets so impure that it steams out, through the breath, vapors which are able to produce the same kind of disease in other people, and which are, under these circumstances, termed _infection_. The infectious poison-vapors of fever get so strong when they are received into close rooms, and are not allowed to be blown away, that they often kill persons who breathe them in that state, very quickly. But you want me to explain how all the mischief, which results from breathing foul air, may be prevented. Come down with me into the garden, and creatures that you believe to be of far inferior powers to yourself, shall give you a lesson.