impurities that in several ways hindered the efficient action of the stove. The tortuous channels for admitting pure air were in part necessary for the advantageous objects just specified, and in part were also designed to delay the current of air by causing it to pass over a surface of greater length, and thereby to enable it to get better heated. Mr. Buchan, in contrasting the cost of coal with that of gaseous fuel, had not taken into account that the mere first money cost of fuel was not the sum and end of the considerations that would determine every person's choice. There was the ready convenience of the gas, the absence of dust and dirt attendant on the use of coal, the saving of labour in lighting, and in the necessary watchful trimming attendance upon a coal fire. There was the consideration that it is not necessary to keep a gas stove burning continuously, or at its full power. On the contrary, it can be used for short intervals of the day as required, and regulated as to the time it is burned, the intensity of the heating force, or the steady maintenance of the heat it imparts, as easily as an ordinary jet of gas is controlled for illuminating purposes. From such points of view it would oftentimes be found that in the use of gaseous fuel there may be the truest economy.

XIII. On the Heating Power of Coal Gas of Different Qualities. By Dr. W. WALLACE, Gas Examiner for the City of Glasgow.

[Read before the Society, December 3rd, 1879.]

THE extensive employment of coal gas for heating purposes, and especially for cooking, gives an interest to the question whether the heating power varies like the illuminating power, and if so, to what extent, in the gas used in different towns? As regards illuminating power, we know that it varies exceedingly. In Aberdeen and Edinburgh it is 30 candles for 5 cubic feet per hour; in Glasgow, Paisley, and Greenock, about 26 or 27 candles; in Liverpool, Manchester, and Carlisle, about 20; in London and Dublin, 16; in Birmingham and many other English towns, 14; and in some as low as 12 candles. But as regards heating power we have no definite information, although there is a general belief

that a rich gas has a greater calorific effect than one of poorer photogenic quality.

My attention having been directed to the subject by a correspondent, I caused a small apparatus to be made, in order, when opportunity offered, to test the matter practically. It is of very simple construction, consisting of a cylindrical tin-plate vessel 8 inches in diameter and 6 inches high, with a cylindrical opening in the centre 1 inches in diameter, through which the whole of the products of combustion pass. It contains exactly one gallon of water. Below is a brass tube bent into a circle 5 inches in diameter, and pierced with 34 small holes, from which the gas burns with flames about of an inch high. This tube is placed 1 inch from the bottom of the vessel, and is surrounded by an outer case having a sufficient number of small holes for the admission of air. On the top of the vessel there are two openings, in one of which a delicate thermometer is placed, while the other is provided with an open glass tube.

The opportunity of using the apparatus occurred recently, when I had occasion to test, on the same day, three samples of coal, which gave gas of 33, 26, and 15 candle power respectively. Each experiment occupied, as nearly as possible, 30 minutes, and consisted in raising one gallon of water from 60° to 160° Fahr., and measuring the quantity of gas consumed in the operation. The system is by no means a perfect one, but the results are strictly comparative. These are arranged in the following table:

The heat units represent the number of pounds of water heated one degree Fahr. by the combustion of one cubic foot of gas. I have taken Glasgow gas of 26 candles at 3s. 10d. per 1000 cubic feet as the standard of value, and it will be seen at a glance that, while the heating value rises and falls with the lighting value, the amount of difference is by no means so great in the former as in the latter.

VOL. XII.-No. 1.

Having made these simple but instructive experiments, I naturally wished to compare the results with the theoretical heating values obtained by calculation from the composition of the gases. Unfortunately, I had no apparatus at the gas-works where the experiments were made for analysing the gases, and the best I have been able to do is to make the calculations from two analyses by the late Dr. Letheby, one of 12 candle gas, and the other of London Cannel gas, the illuminating power of which is not stated, but which may be assumed to be somewhere between 22 and 23 candles. The analyses are given only, as is usual with gaseous mixtures, by volume, but I have, for convenience, calculated the composition by weight.

We see from this tabular statement that, while the heating power of the two gases is almost identical, weight for weight, the practical result when we take the same measure of gas is very different, the Cannel being much heavier than the common gas. I calculate that in my practical comparative test I have realised about 55 per cent. of the theoretical heating power, which is pretty nearly the proportion of effective heat obtained by the combustion of coal in a fairly well-constructed steam boiler.

The question naturally suggests itself, What is the comparative cost of heating by gas and coal? The calculation is a simple one.

The theoretical heating power of ordinary soft coal may be taken at 13, that is, the number of pounds of boiling water evaporated by 1 lb., equal to 29,120 lbs. for a ton of coal, value say 11s. 6d., being the price of 3000 cubic feet of Cannel gas, the heating power of which is, say 2500. The gas, therefore, costs 11 times as much as the equivalent quantity of coal, or, in round numbers, a pennyworth of coal gives as much heat as a shilling's worth of gas. When, however, we consider the handiness, the cleanliness and the convenience of gas, it is not surprising that it is extensively employed as a source of heat, as an illustration of which I may state that in my own laboratory from 250,000 to 300,000 cubic feet are consumed annually, almost the whole of which is burned for the production of heat.

XIV. The Ventilation of the Rooms of Houses, with Experiments. By HORATIO K. BROMHEAD, A.R.I.B.A.

[Read before the Society, March 31st, 1880.]

THE object of this paper is to consider the ventilation of an ordinary house room.

Ventilation is a difficult study to those who know all about it before they begin to give the subject any attention. The student who is not prepared to throw aside popular prejudice must meet with disappointment and be much disheartened.

It is possible to get good dictionary authority for the statement, that to "ventilate" is to fan with wind! or to open and expose to the free passage of air! If the question is studied in some new public halls and other buildings, the painful conclusion is forced upon one that to ventilate is to blow cold air on people's heads and feet-sometimes one, sometimes the other, and more often both!

That is not what is wanted. When a field or house or other place is drained, the word does not imply that clean water is poured into or upon it. The condition described is this, that certain pipes or apertures have been made with accuracy, experience, and skill, in accordance with the need for them, and so that the

impure or obstructing liquid can escape at once, and cannot remain and accumulate.

The word "vent" means to let out at a small aperture, to suffer to escape, or to pour forth. When a room is "ventilated," the word does not (or ought not to) mean that fresh air is poured into or upon it. A ventilated room should be so constructed that impure air can escape at once, and cannot remain and accumulate within it.

There are two arguments about this-the one theoretical, and the other mechanical. The theoretical argument is this:-A room that is in want of more ventilation is in want of more air, or in want of fresh air. Now that theory is quite wrong from a mechanical point of view. A room in want of ventilation is not in want of air, for it is quite full. It is not, properly speaking, in want of fresh air. To bring out the meaning of this, the following proposition is formulated:

When tainted air goes out of a room, it must be replaced by the pure air surrounding the room.

But that proposition cannot be reversed. When pure air goes into a room, its tainted air might go out; but that is not a necessity, and is not at all likely. In ordinary rooms, the probability is that the pure air will go out at some other opening as fast as it comes in, and still leave impure air in the room. Of course the relative lightness and ascending capacity of impure air is the cause of its accumulating at the ceiling, and remaining there, even though better air is going in and out of the room.

The mechanical argument amounts to this, that the ordinary construction of a room permits the free admission of fresh air, but does not offer facilities for the removal of tainted air. It therefore leads to the further statement, that ventilation ought not to be a system for giving more fresh air, but one for leaving as little as possible of tainted air in a room.

If a common tea-pot be placed so that the mouth of its spout is a little below the top of the tea-pot, and then filled with water, and the top of the water covered with anything like oil that will float on water, then a continuous stream of water may be poured in at the top, through the oil, and caused to flow out at the spout, but the oil will float and remain. On the contrary, if the mouth of the spout is raised so that it is higher than the top aperture of the tea-pot, then the water can be poured into the spout so as to cause the oil to overflow. The oil can be so removed