organs of the body. If therefore this air, which we inhale every instant,
be not pure, the whole mass of blood is very soon contaminated, and the
frame, in some part or other speedily experiences the bad effects. This
will explain to us the almost miraculous benefits which are obtained by
_change of air_, as well as the decided advantages of a free and copious
ventilation. The prejudices against a free circulation of air, especially
in the sick chamber, are productive of great evil. The rule as regards
this is plain and simple: admit as much fresh air as you can; provided it
does not _blow in_ upon you _in a stream_, and provided you are not in a
state of profuse perspiration at the time; for in accordance with the
Spanish proverb--


"If the wind blows on you through a hole
Make your will, and take care of your soul."


but if the _whole of the body be exposed at once_ to a cold atmosphere,
no bad consequences need be anticipated.

A great deal has been said about the necessary quantity of _sleep;_ that
is, how long one ought to indulge in sleeping. This question, like many
others, cannot be reduced to mathematical precision; for much must depend
upon habit, constitution, and the nature and duration of our occupations.
A person in good health, whose mental and physical occupations are not
particularly laborious, will find seven or eight hours' sleep quite
sufficient to refresh his frame. Those whose constitutions are
debilitated, or whose occupations are studious or laborious, require
rather more; but the best rule in all eases is to sleep till you are
refreshed, and then get up. If you feel inclined for a snug nap after