But in countries like ours, where vast numbers of families are collected in cities, with narrow streets and lanes; where an open place like Stephen's Green or Merrion Square is anxiously sought after, and disproportionate rents paid for the houses which are around it, this immediate restoration of the injury done to the air by breathing, and the burning of lights and fuel, cannot occur. The air is vitiated permanently, and those resident in towns require for their health's sake to understand how the evil may be rendered as small as possible. Even in a town, the total quantity of air is so great, that if it all come into play, it can be but slightly injured. But such is often not the case. How often, when there is a fine healthful breeze outside the town, do we find, on entering a narrow street, the mass of air perfectly motionless, and all the mischievous vapours which are produced, collecting until they become almost irrespirable. This is a great source of disease in towns; and to prevent or remedy it, requires but frequent change of the air which a room or a street contains: it requires but ventilation. It is by means of a fireplace that a room is generally ventilated. The air which has served for the burning of the fuel is thereby made very hot, and hot air, being much lighter than cold air, rises up the chimney, generally mixed with soot, and is then called smoke. According as the hot air leaves the room, cold air enters to supply its place through the open doors or windows, or, if these be closed, through every little crevice which can give it passage. There is thus produced a rapid current of air, or draught, as it is termed. The air vitiated by the breathing of persons in the room is carried away along with that vitiated by the fire, and at any one moment the air in the room is found to be almost completely pure. It is therefore to proper ventilation that the inhabitants of towns must look for the maintenance of health. Disregard to this precaution has been the means of increasing to a frightful extent the mortality of large cities, and instances have been given, where an infectious disease, which had ravaged a number of low and confined streets in a large English town, stopped suddenly, and avoided a street otherwise no better than the rest, but which had been kept clean, and the rooms ventilated, by the exertions of some well-informed persons. For the preservation of the health of the poorer classes in large towns, medicine is of far less importance than cleanliness and ventilation.