they keep their houses well aired; but the poor, and servants, are
not made to comprehend this matter properly; and therefore from
neglecting to open their windows, and breathing a foul, tainted air,
the greatest part of their time, many disorders are brought on, and
others rendered worse than they naturally would be. [6]

Having considered the purity of the air, let us next take a view of
the changes in temperature which it undergoes, and the effects which
these have upon the constitution.

We find the air sometimes considerably below the freezing point;
nay, even so much as 20 or 30 degrees; it is then intensely cold;
and on the other hand, the thermometer sometimes indicates a great
degree of heat. We then find ourselves much relaxed, and our
constitutions exhausted.

To understand how this happens, let us consider for a moment the
nature of heat, and cold.--Heat is one of those stimuli which act
upon the excitability, and support life: for if it was totally
withdrawn, we should not be able to exist even a few minutes; and
cold is only a diminution of heat. When heat is present, in a proper
degree, or the atmosphere is about that degree of heat which we call
temperate, it just gives such a stimulus, and keeps the excitability
exhausted to such a degree, as to preserve the body in health; but
if it continue for a considerable time to be much warmer than this
temperature, the consequence must be, from the laws already laid
down, an exhaustion of the excitability, and a consequent relaxation
and debility; for, when the excitability has been exhausted by the
violent application of heat, long continued, the common stimulant
powers which support life, cannot produce a sufficient effect upon