Happiness is the rule of healthy existence; pain and misery are its exceptional conditions.  Nor is pain altogether an evil; it is rather a salutary warning.  It tells us that we have transgressed some rule, violated some law, disobeyed some physical obligation.  It is a monitor which warns us to amend our state of living.  It virtually says,—­Return to nature, observe her laws, and be restored to happiness.  Thus, paradoxical though it may seem, pain is one of the conditions of the physical well-being of man; as death, according to Dr. Thomas Brown, is one of the conditions of the enjoyment of life.

To enjoy physical happiness, therefore, the natural laws must be complied with.  To discover and observe these laws, man has been endowed with the gift of reason.  Does he fail to exercise this gift,—­does he neglect to comply with the law of his being,—­then pain and disease are the necessary consequence.

Man violates the laws of nature in his own person, and he suffers accordingly.  He is idle and overfeeds himself:  he is punished by gout, indigestion, or apoplexy.  He drinks too much:  he becomes bloated, trembling, and weak; his appetite falls off, his strength declines, his constitution decays; and he falls a victim to the numerous diseases which haunt the steps of the drunkard.

Society suffers in the same way.  It leaves districts undrained, and streets uncleaned.  Masses of the population are allowed to live crowded together in unwholesome dens, half poisoned by the mephitic air of the neighbourhood.  Then a fever breaks out,—­or a cholera, or a plague.  Disease spreads from the miserable abodes of the poor into the comfortable homes of the rich, carrying death and devastation before it.  The misery and suffering incurred in such cases, are nothing less than wilful, inasmuch as the knowledge necessary to avert them is within the reach of all.

Wherever any number of persons live together, the atmosphere becomes poisoned, unless means be provided for its constant change and renovation.  If there be not sufficient ventilation, the air becomes charged with carbonic acid, principally the product of respiration.  Whatever the body discharges, becomes poison to the body if introduced again through the lungs.  Hence the immense importance of pure air.  A deficiency of food may be considerably less injurious than a deficiency of pure air.  Every person above fourteen years of age requires about six hundred cubic feet of shut-up space to breathe in during the twenty-four hours.[1] If he sleeps in a room of smaller dimensions, he will suffer more or less, and gradually approach the condition of being smothered.

[Footnote 1:  Where six hundred cubic feet of space is allowed, the air requires to be changed, by ventilation, five times in the hour, in order to keep it pure.  The best amount of space to be allowed for a healthy adult is about eight hundred cubic feet.  The air which is breathed becomes so rapidly impure, that a constant supply of fresh air must be kept up to make the air of the shut-up space fit for breathing.  The following are some amounts of space per head which are met with in practice:—­