1 1

legs, and body, and how in the lungs it changes
from dark purple to a bright red colour. In
Figure 11, A is the left side of the heart, from
which the pipes called arteries, B B, carry the pure
blood to the upper part of the body D, and to the
lower part of the body F; the blood passes into
the veins C C, and becomes dark-coloured, and, by
means of the veins, it gets to the right side of the
heart I; from the right side of the heart it is sent
by the vessels K K to the two lungs L L, through
the little bladders which are seen to be continued
with the windpipe E; it then passes into G and
H, the vessels which are shown by the arrows to
carry it into the left side of the heart A, from which
we started to describe the circulation.

14. Now it is quite certain that it is of very little use for the blood to be continually going to the small bladders or air-cells of the lungs in order to be made pure, if these little bladders do not contain fresh and pure air. It must also be understood that the air which has once been breathed becomes bad, from its having given off its good parts to the blood, and from its having become loaded with a large quantity of impure gas and moisture which come

from the body. This moisture is seen at once by breathing on a pane of glass, and it is often of a very unpleasant smell. In Figure 12, two of the little bladders of the lungs are seen; one containing pure air, the other dark one full of air which has been breathed.

FIGURE 12.

15. The air which has once been breathed is so bad that it is in fact no longer air, but

FIGURE 13.

has become an impure gas. To prove that the air is bad, take two bottles of clear lime - water, as in Figure 13, and with a straw blow for a few minutes into one, so that the air from the lungs may pass into it; at the end of this time the lime-water will become quite white and milky, as in the left bottle: but if fresh or common air be blown through the other bottle with a pair of bellows, the lime-water will remain quite clear, as in the bottle on the right.

16. The air which comes out of the lungs through the mouth or nose is always warm, and being on this account lighter than the air outside the

FIGURE 14.

body, it goes upward, and in sitting-rooms, schools, churches, and all other places which have a ceiling, the bad air rises to the upper parts, as in Figure 14. 17. As the bad air goes upward and cannot get away, in a very short time a room, or church, in which people are sitting, or a school or factory in which children are at work, gets quite full of bad air which has been breathed, and, as is seen in Figure 15, the bad air comes down again to be breathed, as it can only get away at the fireplace, which is below the heads of the people in the room. Ill health, shown by paleness of face and weakness, also colds, fever, and consumption, and other dis

FIGURE 15.

eases, are produced by breathing this bad air over and over again.

18. It is quite plain that, as the bad air which comes from the lungs goes upwards to the top of the room, there ought to be some way of letting out the air at the top of the room, to prevent its coming down again to be breathed. This is managed by Dr. Arnott's valve, Figure 16, which may be fixed by any bricklayer in a hole made into the chimney, close to the ceiling of the room; by means of it, the bad air, as is seen by the arrows, escapes from the room, and is drawn up the chimney.

FIGURE 16.

Room

Chimney

19. This valve, invented by Dr. Arnott, which is a very great boon to mankind, and, when brought into still more general use, will be the means of preserving the health of thousands, both

among rich and poor-is also seen in Figure