that startling hypothesis needless. A ray of
sunshine darting through a keyhole, reveals myriads
of motes floating about unsuspected.

Accessory matters in the air are odours,
pestilential miasms, and putrifiable vapours.
In some spots, noxious gases are generated in
abundance, and would accumulate to a
dangerous amount, were they not swept away by
storms and burnt by lightning. We do not yet
know what it is (although we may one day
discover) whose action, in concert with
sulphuretted hydrogen gas, renders the
neighbourhood of marshes and stagnant waters
unhealthy. In marshes which cannot be drained,
plantations of trees and shrubs are the best
means of diminishing the danger.

Here, again, we see the analogy between the
aerial and the watery oceans. The atmosphere
resembles the sea, in being the receptacle of all
sorts of gases and vapours, which escape into it
from the earth's surface, exactly as the sea is of
all sorts of waters and their solutions, which
drain into it from continent and island. The
air is the sewer for gaseous exhalations, as the
sea is for watery liquids.

Changes in the purity of the air which are
sufficient to produce disease and death, are still
so really slight and subtle that they are not
recognisable by chemical analysis in the
laboratory, though sure to be detected by their
effects on the nicer chemistry of the human
frame. Several years ago, the French Academy
sent out bottles, and caused specimens of air
from various parts of the world to be brought
home to be analysed. The nicest tests which
the most skilful chemists could apply, were
incapable of detecting any, the slightest, difference
as to ingredients in the specimens from either
side of the equator. To ascertain whether the
air is everywhere identical, it was requisite to
ascend to great heights, either by scaling the
loftiest mountains, or through the agency of
balloons. We have Saussure's observations
made on the top of Mont Blanc, and Humbolt's
on Chimborazo. With a clear sky, Gay-Lussac
mounted in a balloon to the height of twenty
thousand feet. He there found a temperature
of nine degrees centigrade below the freezing-
point; and he brought down air whose analysis
proved it to be of the same composition, in
respect to oxygen and azote, as our ordinary
circumambiant air here below.

Man, who is a veritable steam-engine, burns
some three-quarters of a pound of carbon per
day; and to burn it he requires more than a
pound and a half of oxygen, which must be
supplied to him by the air. A dearth of oxygen
wears and wearies him; want of it kills him.
To breathe freely in-doors, a man should have at
least seventy cubic yards of air. Patients in
hospitals, children in dormitories, have scarcely
half the necessary mass of air. The only way
of remedying the short allowance is frequently
to renew the vitiated air. In rooms which
have chimneys, lighting a fire is an obvious and
easy means of doing so. The air in the chimney,
heated by the fire, ascends and escapes, drawing
after it the air in the room which has already
passed through human lungs, and which is
replaced by fresh air entering at the door. No large
room in which people assemble in numbers
should be without a fireplace and a chimney, not
so much for warmth as for ventilation. The
close air of a crowded apartment which is heated
by a multitude of breaths and bodies, will be
cooled by lighting a small fire, as the surest and
the safest way of causing a stream of fresh air to
enter. Architects frequently pay too little
attention to ventilation. They reckon too much
on the fissures of doors and windows. Many a
snug apartment is consequently unhealthy. Dr.
Franklin had such faith in the virtues of fresh
air, that, besides breathing all he could, when
the weather was warm enough he used to take
air baths for hours at a time, reading, writing,
and pursuing his private occupations in the
costume adopted in Paradise.

For army stables, Vauban, the famous French
engineer, allowed a metre (a little more than a
yard) per horse, which system lasted until 1840.
Statistics show that the mortality among those
horses amounted to from ninety to ninety-five
per cent. Renaud (a distinguished veterinarian
who carefully studied ventilation, and who died
of marsh fever in Italy while watching the
typhus of horned cattle) proved that, to ventilate
stables properly, every horse ought to have
a space of a metre and a half. This reform,
put in practice, reduced the mortality to forty
per cent. It has been calculated that, since
Vauban's time, the narrowness of the boxes has
cost the French government no less than a
thousand million horses.

A curious calculator estimates that three
thousand men, located on an area of an acre of
ground, would, in thirty-four days, make by
their own transpiration an atmosphere eighty
feet in height, which, if not dissipated by winds,
would instantly become pestilential. Whence
towns and armies are warned of —what we only
know too well.

How minute must be the atoms composing
these miasms and infectious emanations, whose
presence science is unable to detect! Learned
and speculative men have discussed the question
whether matter be infinitely divisible or not.
That it is not so, is rendered probable by
Lavoisier's discovery respecting the proportions in
which simple bodies (or those believed to be so)
enter into chemical combination. But the
wonderful and extreme divisibility of matter is
illustrated in various ways.

One of the most subtle divisions of solid
matter is to be found in the black pulverulent
state of metals. It has been supposed that all
matter is black when extensively divided,
because the particles are too small to reflect
light; but the form of the black particles is
unknown to us, because, as Mr. Alfred Smee
informs us, the highest powers of the
microscope are insufficient to render them visible to
the eye. Professor Faraday showed a method
of dividing gold to an extreme amount. He
precipitated the metal from its solution by