many other continental towns. We We may readily imagine how such an arrangement, by preventing the access of light and the free escape of noxious vapours, must tend to foster and aggravate fevers and various epidemic disorders.

Change of the air without, is provided for in various ways. One great and important provision exists in the diffusion law, to which allusion has already been made, and by which, gases heavier than the atmosphere rise against their gravity, and become diffused through space. The different objects on the surface of the earth, according to their radiating power, throw out the heat derived from the sun, and in so doing, rarify the air in contact with them, cause it to ascend, and thus tend to promote its renewal. But the great promoters of external ventilation are the winds. These, in proportion to their violence, sweep before them the impurities with which the air may be charged, cause an effective change of this, and are especially serviceable in maintaining its purity and healthfulness in crowded towns.

"Winds from all quarters agitate the air,

And fit the limpid element for use

Else noxious; oceans, rivers, lakes and streams,
All feel the freshening impulse, and are cleansed
By restless undulation."

The air within our dwellings may be rendered impure by other causes than the respiration of, and emanations from human beings, and the combustion resulting from lights and fires. Waste fluids, useless animal and vegetable matters, ordinary refuse, will, if not effectively removed, undergo decomposition, give out noxious gases and emanations, and charge the atmosphere

that should give us health, with the seeds of our destruction. We all know how rapidly chloroform and ether, when inhaled, act upon the system and produce insensibility. We know too, how the poisonous emanations given out by individuals when suffering from certain forms of fever and disease, if inhaled by others, may infect and cause them to fall victims. Now, in precisely the same way, do the noxious products of decomposition with which the air may be vitiated, produce their effect; becoming absorbed by the blood spread over the respiratory air-cells, and by it circulated through the system. The rapidity, intensity, and peculiar character of their action depend upon the nature of the gaseous products and the degree to which the air is charged with them. The principal gas given out by stagnant refuse matter is sulphuretted hydrogen, a deadly poison, one part of which diluted with 800 parts of atmospheric air, has been shown sufficient to kill a dog, while a 1500th part diffused through the air was quickly fatal to small birds. The men employed in clearing out places where this and other gases existed in a concentrated form, have, by incautious exposure, immediately fallen victims; and where these gases have been more diluted, their inhalation has induced nausea, faintness, delirium, and insensibility. The frequent consequences of exposure to these poisonous agents are violent diarrhoea and irritability of stomach, followed by excessive prostration; symptoms, in short, resembling those of cholera; while in haunts thus rendered impure, fever is most frequent and virulent. It follows, therefore, that cleanliness and purification of the air within our

dwellings should be promoted in every possible way-by a free supply and use of water, by sinks communicating with drains and these again with the general sewers, by careful removal of refuse matter, by periodical emptying of dustbins and other receptacles. Cesspools, stables, cow sheds, pigstyes, and other sources of foul emanations should not be in immediate proximity to our dwellings, which should further be kept dry by effective roofing, gutters, pavement, &c. The Dutch make good use of the water with which they are on all sides so freely supplied, in maintaining the cleanliness of their houses both within and without; and Brock on the Zuyder Zee, enjoys the distinguished reputation of being the cleanest village in the world.

I

There are numerous causes existing without our dwellings by which the air may be vitiated, and for the removal of which we must look to the legislature rather than to individuals. can do little more than enumerate these causes. My object is to indicate principles. The suggestion of the various modes in which they best be carried out must be left to others.

may

Among the sources by which the external air may be rendered impure, I may notice the condition of the public roads, which should be cleansed periodically, solid matters being removed and those of more fluid character washed away and carried off by an effective sewerage system. Where such a system is provided, the very rain aids our efforts, offensive matters being swept away in the countless miniature torrents caused by the passing shower; but rain-water itself may, if allowed to stagnate, become a source of impurity.