It is true that the ventilation of a house is generally considered to be the business of the builder and architect, yet there are many unpleasant phenomena that come under the observation of the housewife which are due to this law of convection, and it will be useful to consider a few of them. Let us take first the universal annoyance to housewives caused by the sight of _dirt on the ceiling_. That all air is full of dust is seen when a stream of sunlight crosses a room; the particles of dust are then clearly perceived moving rapidly in all directions in the air. These dust particles, when air is at rest, constantly fall to the ground under the action of gravity, and are deposited on shelves and ledges, from which they have to be removed daily by the housemaid. When air is warmed and ascends it carries the dust particles with it, and these particles striking against any cold surface with which they come into contact stick to it. This is the cause of the necessity for the periodical sweeping of chimneys. The walls of the chimney are colder than the smoke that comes into contact with them, and the particles of soot in the smoke striking against them are deposited on them. In the house the effect of the bombardment of surfaces by dust-laden streams of air is seen most conspicuously over burning gas-lights. Burning gas does not itself produce all the dirt which is found on the ceiling above it, but it causes upward streams of hot air, which carry up the dust and deposit it on the ceiling. The practice of suspending a shade over the gas-light does not lessen the amount of dust and smoke in the air, but the shade serves to spread out the air over a larger surface, and thus to render the dirt on the ceiling less apparent. That the shade itself remains clean is due to the fact that it gets hot. A heated surface promotes the activity of the motion of the air-particles in its neighbourhood, and by this local activity the dust is repelled, so that a surface remains clean or becomes coated according as it is more or less hot than the invading current. The validity of this explanation may be tested by holding a cold spoon over a lighted candle when it will be seen that the spoon becomes blackened; if a hot spoon be substituted for the cold one it will remain clean. In order that the hot, vitiated air of a room may escape easily, it has been in many cases the custom to place an exit opening for it in the chimney over the room fireplace. The wall in the neighbourhood of this ventilator invariably becomes black; but as this wall is warm it is not probable that dust is deposited on it by the outgoing air, the explanation given by the housewife that the smoke from the chimney gets through the ventilator into the room is probably correct, though these ventilators are supplied with mica flaps which should swing open when air from the room strikes against them, and close when the air from the chimney does so.