As the demand becomes less, the difference diminishes. If ozone were taken into consideration, the difference would probably be much greater; but I do not know what allowance to make for that body, and leave it out of consideration. In the smoky towns there is none at all.
The conclusion has been drawn by some scientific men that in all spaces, and under every circumstance, the same amount of air must be supplied. Now, it is true that the same amount is to be actually breathed, and, if this breath is thoroughly mixed with all the air of the room, the same amount must be supplied for ventilation, whatever the size of the room. But let us suppose—the most common case—that the thorough mixing does not take place, and we have at once a different amount of air required.
As to the imperfect mixing, it is so various and characteristic that we cannot reduce it to rules; we may hold a smoking substance in the midst of an apartment, and find the smoke go directly to an opening without mixing with the air of the room. It goes, too, with a velocity greater than that of the air of the room, otherwise an opening must be supposed sufficient to change the whole air of the room in a few seconds, that being the time required for the smoke to reach the opening. If we could drive the impure air in a similar way in a narrow current toward its exit, we might manage ventilation with a very small amount of air. To do this is a matter of great importance, because the expense of building is becoming so great, that very few men can afford to pay for a large enough house, and, while rents are rising, the rooms of the middle classes have actually diminished in some places, and within these few years. Yet the evil of small rooms is great, because rapid currents are required for ventilation; cold currents are hurtful, and the warm difficult to obtain. If, however, we could obtain warm-air currents, it would not be important for us to have the rooms so large. It is a question of price. I believe the warmth must be obtained as the first demand of Nature, and without it civilization will go back. When men are cold, they give themselves to physical exercise, and, if that is impossible, to discomfort, in which the mind refuses to do more than to complain, if it cannot forget. Which is cheapest for us? Is it to build large rooms and to have less warmth with slow currents, or to build small rooms and to have more warmth with rapid currents? It is to be wished that the former should be the rule for private houses; more comfort and convenience are promised, and mechanism is not required; if it were, it could not be obtained. For hospitals, the use of mechanism is more within possibility. The expense of large rooms, when the architecture is of a kind intended to ornament a capital city, is very great. Should we not gain by a judicious system of warming? Our methods of warming are very cumbrous, and we seem to be behind ancient Rome and modern Russia. We warm the air which changes in a moment when a door
