That a class which lives under the conditions already sketched and is so ill-provided with the most necessary means of subsistence, cannot be healthy and can reach no advanced age, is self-evident.  Let us review the circumstances once more with especial reference to the health of the workers.  The centralisation of population in great cities exercises of itself an unfavourable influence; the atmosphere of London can never be so pure, so rich in oxygen, as the air of the country; two and a half million pairs of lungs, two hundred and fifty thousand fires, crowded upon an area three to four miles square, consume an enormous amount of oxygen, which is replaced with difficulty, because the method of building cities in itself impedes ventilation.  The carbonic acid gas, engendered by respiration and fire, remains in the streets by reason of its specific gravity, and the chief air current passes over the roofs of the city.  The lungs of the inhabitants fail to receive the due supply of oxygen, and the consequence is mental and physical lassitude and low vitality.  For this reason, the dwellers in cities are far less exposed to acute, and especially to inflammatory, affections than rural populations, who live in a free, normal atmosphere; but they suffer the more from chronic affections.  And if life in large cities is, in itself, injurious to health, how great must be the harmful influence of an abnormal atmosphere in the working-people’s quarters, where, as we have seen, everything combines to poison the air.  In the country, it may, perhaps, be comparatively innoxious to keep a dung-heap adjoining one’s dwelling, because the air has free ingress from all sides; but in the midst of a large town, among closely built lanes and courts that shut out all movement of the atmosphere, the case is different.  All putrefying vegetable and animal substances give off gases decidedly injurious to health, and if these gases have no free way of escape, they inevitably poison the atmosphere.  The filth and stagnant pools of the working-people’s quarters in the great cities have, therefore, the worst effect upon the public health, because they produce precisely those gases which engender disease; so, too, the exhalations from contaminated streams.  But this is by no means all.  The manner in which the great multitude of the poor is treated by society to-day is revolting.  They are drawn into the large cities where they breathe a poorer atmosphere than in the country; they are relegated to districts which, by reason of the method of construction, are worse ventilated than any others; they are deprived of all means of cleanliness, of water itself, since pipes are laid only when paid for, and the rivers so polluted that they are useless for such purposes; they are obliged to throw all offal and garbage, all dirty water, often all disgusting drainage and excrement into the streets, being without other means of disposing of them; they are thus compelled to infect the region of their own dwellings.