In the manufactories of. chloride of lime (bleaching powder), and in places where it is used for bleaching wool and other materials, chlorine gas is given off into the air, and irritates the respiratory passages when breathed, from which we can see that these places should be very thoroughly supplied with fresh air.

A compound of chlorine called hydrochloric acid is given out in large quantities during the preparation of carbonate of soda. Soda is prepared in enormous quantities from sea salt, and the vapour evolved during its manufacture is an extremely irritating gas, which used to be allowed to escape into the air from tall chimneys, but is now, however, absorbed by passing the fumes through condensing towers in which there is coke washed with a stream of water.

During the preparation of sulphuric acid, sulphurous acid gas is set free, but the manufacturer finds it is a great disadvantage to him to allow this to escape, and so he takes care that as little as possible shall escape.

Now, wherever there is foul organic matter decomposing, a gas is emitted in greater or less quantities, called sulphuretted hydrogen, which has an exceedingly offensive smell like that of rotten eggs. This gas is given off in sewers and cesspools that are badly ventilated. When in sufficient quantity it causes suffocation, which is so sudden that it.goes in France by the name of le plornb, because the suffocated person falls down like a lump of lead. We can gather from this very clearly the great necessity of well ventilating places where this gas is at all likely to collect. In addition to this the fumes of ammonia are evolved. This is also an exceedingly irritating substance, and mostly affects the mucous membrane lining the eye-sockets, so that persons who work in an atmosphere impregnated with it suffer from inflammation of that mucous membrane. All this shows the great importance of not allowing accumulations of foul organic matters to be formed.

Let us pass on to consider the methods by which the air in our rooms can be frequently changed.

I have told you in a previous lecture that it is necessary that an individual should have 3000 cubic feet of air per hour to breathe. It has been shown by theoretical calculation from the changes produced by respiration in the air, that 2000 cubic feet per hour per head are required, but it is found on analysing the air in rooms in which there are large numbers of people, that this quantity is not sufficient to prevent the air from being stuffy, and 3000 are necessary. Sick rooms and hospital wards require more than ordinary places, and this quantity makes no allowance whatever for the impurity that gets into the air from lights. How, then, is this quantity of air to be obtained? It is perfectly clear that we cannot have apartments large enough to hold the required quantity, for suppose we remain seven hours in a bedroom with no supply of air from the outside, we should require a bedroom large enough to contain 21,000 cubic feet of air for each person in it It is clear, then, that the air of the room must be replaced a considerable number of times. How is this to be accomplished? Among the possible sources of the movement of air we have the property of the diffusion of gases. If there is a space containing gases of one kind in communication with a space containing gases of another kind, by the law of the diffusion of gases they will mix throughout; but foul organic matters do not obey this law.

Then we have the action of the winds, and they are an exceedingly powerful means of ventilation, but they are irregular. A wind that blows so slowly that it is imperceptible will change the air in a large room in a very short space of time. Then there are movements in the air that are caused by variations in its density or weight, and these variations in weight are generally produced by differences of temperature, j When the air is hot it is light, and when cold it is heavy, and the changes produced by this means are of great importance with regard to ventilation.

In this climate we cannot bear the air of a room to be changed more than three or four times in an hour. If changed more frequently there is a draught, and we can therefore see that since we must have 3000 cubic feet per hour, we should have from 750 to 1000 cubic feet of space each.

How, then, is the change to be accomplished and to be effected without draught ? In the first place how can the wind be utilised? This can be always done in a building which has windows on both sides all we have to do is to open the window nearest to the direction from which the wind comes, and also the one diagonally opposite, at the top, when sufficient currents of air will be produced to change the air in the room without any draught But, of course, in a quite still atmosphere this plan is of no avaiL Wind has been used for ventilating the holds of' ships, and in that case a cowl is so constructed that it always faces the wind, and so fresh air is conducted down into the hold by pipes, and the foul air is driven out through various passages. This method is known as Sylvester's mode. It was put into practice by a Dr. Neil Arnott to ventilate a large schooL He had a cowl which always faced the wind, and the air was carried by a pipe down into the basement, where it was warmed; then it rose upwards through apertures into the apartments of the school, and from thence was conducted by another pipe to the outside, which pipe was surmounted with another cowl, so constructed as to always face away from the direction of the wind, and in this way he utilised the wind whichever way it blew. Wind acts directly as a ventilating agent by displacing air that is before it, and indirectly by aspiration. When air is blown forcibly across a tube it causes a diminution of pressure in the air in the tube; and so air blowing across the top of a chimney causes an up-draught in the chimney.

These are the chief ways in which the wind has been utilised, but the objection is its irregularity. Sometimes it is too strong, at others too weak.