required to maintain a room at a temperature of say 50°. If the question is considered apart from the element of time 1 cubic foot of water raised 1° will raise 2,990 cubic feet of air to a like temperature; the element of time can only be determined by experiment. Mr. C. Hood1 found that an iron pipe 4 inches external diameter, loses 851 of a degree of heat per minute (i.e. 1° in 70 seconds) when the excess of its temperature is 125° above that of the surrounding air.

Therefore 1 foot in length of pipe 4 inches in diameter will heat 222 cubic feet of air, 1° per minute, when the difference of temperature of pipe and temperature of air is 125°. Before this calculation is applied to the practical heating of buildings, many corrections must be made, especially if there is much window surface, for the loss of heat from glass is considerable. Hood found that 1 square foot of glass in still air will cool 1.279 cubic foot of air as many degrees per minute as the internal temperature of the room exceeds the external temperature of the air. Thus if the difference between the internal and external temperature of a room be 30°, then 1.279 cubic foot of air will be cooled 30° by each square foot of glass. Wind of course increases the cooling power very much.

Neglecting losses from doors and crevices, and only considering the windows and the amount of air to be warmed. Hood reckons the quantity of air to be warmed as at least 3.5 to 5 cubic feet for each square foot of air per minute for each person the room contains, and 1.25 cubic foot for each square foot of glass.

The following table saves the necessity of calculation (see Table XVII.)

An example will show how it is used, supposing the temperature in the winter is likely to be 10° outside and the number of feet of pipe is required to be known to supply 40 persons with 12 cubic feet per minute of air at 50°.

125 p-2

1 Let p be the temperature of the pipe and t the temperature that the room is required to be kept at, then =x which will represent the number of feet of pipe that will warm 222 cubic feet of air 1° per minute, when p-2 is different from the proportions given above. If d represent the difference between the internal and external temperatures of the room and c the number of cubic feet of air which are d.c to be warmed per minute, then x p, that is, p will be the number of feet of 222 pipe 4 inches in diameter which will warm any quantity of air per minute.

=

The amount of air required at 50° is therefore 480 cubic feet. Now looking down the first column of the table, find the external temperature, which is 10°, next the temperature 50°, at which the air is required in the horizontal column, the figure under this i.e. at the intersection of the two lines is 150, which is the number of feet of pipe of 4 inches diameter, the temperature of the pipe being 200° which will heat 1,000 cubic feet per. minute, and as 480 cubic feet are alone required the simple rule of three 150 x 48072 calculation gives the length of pipe. 1,000

Or instead of so many feet per head, a rule may be drawn from experience that in rooms for every 1,000 cubic feet 12 feet of 4-inch pipe is required to maintain in cold weather the temperature to 50° a room measuring 20 x 40 x 15 requiring 144 feet.

TABLE XVII.

TABLE SHOWING THE QUANTITY OF PIPE, 4-INCH DIAMETER, WHICH WILL HEAT 1,000 CUBIC FEET OF AIR PER MINUTE ANY REQUIRED Number of Degrees. TEMPERATURE OF THE PIPE BEING 200° F.

Although as a rule it is much better to trust to radiators and hot water pipes only for corridors and waiting rooms, warming the room itself by open warm air grates, it cannot be denied that a general system which supplies warm air from a distance has certain advantages, the more particularly in those cases where expense is of secondary importance. For instance, in the ventilation of the House of Commons, the foul air is extracted from the ceilings by furnaces connected with powerful upcast shafts; the fresh air is cooled in summer by ice, and is heated in winter by a steam battery; it is brought through a cast iron perforated floor covered with a peculiar open coating of whipcord; this air is partly forced in as required by steam bellows, the air is also washed by passing it through canvas wetted with spray.

The warming and ventilation of a school at Bigelow, U.S., deserves mention, and is thus described by Dr. Hunter.1 "The ventilation is effected by a plan recently introduced. There is a common central shaft of wood lined with tin carrying the hot air pipes for supplying the rooms. The air enters near the ceiling at a temperature of 100° by an opening 20 × 35 inches, and is exhausted near the floor, passing to the shaft mentioned. The result is good." In this case no motive power is used, the lower cold air falls by its own weight through the lower openings while the warm air streams in through the upper apertures.

In all these general systems of warming, the success greatly depends on a sufficient quantity of warm air being delivered and also on the care taken in keeping all other channels closed save those which are intended to convey the air to the rooms or to extract it; air rushing in by other channels of course deranges the whole system.

1 Sixth Annual Report State Board of Health. Massachusetts.

CHAPTER VIII.

PRACTICAL EXAMINATION OF AIR.

(57) Examination by the Senses.

EXAMINATION by the sense of smell, is in a practical point of view of first importance, minute quantities of organic matter, traces of sulphuretted hydrogen, of coal gas, of ether, of carbon bisulphide, of scents of various kinds, empyreumatic matters and many other substances are detected by the sense of smell in such infinitesimal quantity as to rival and in some cases exceed the most delicate chemical and physical methods known to science; on the other hand the senses will not detect considerable quantities of carbon dioxide, carbon oxide, marsh gas, and several other vapours.

The nature of the organic more or less volatile matter which is given off by the skin and breath and makes the air feel so "stuffy" requires farther elucidation and to some extent varies in different individuals. It consists in part of volatile fatty acids and their ethers, and it may be hazarded as a conjecture that the faculty of dogs in scenting out their masters is dependent mainly on the kind. and combination of the fatty acid which the particular man evolves in his perspiration.

(58) Dust-Bacteria.

In ordinary living rooms the amount of dust will require no estimation, but in the case of factories and work rooms, the amount of dust per cubic foot may be desirable to be estimated. To make some sort of estimation is easy, but to get gcod accurate results is difficult for the reason that the dust is always imperfectly distributed. The best method is to fix up a large aspirator and pull two or three gallons through a tube at least one

inch in diameter which tube contains for a couple of inches of its length crystals of sugar or sodic sulphate packed sufficiently tight to allow the air to penetrate, the layer farthest from the aspirator being packed loosely and then increasing in density towards the aspirator; at the conclusion of the experiment the soluble filter is removed, dissolved in water, and the solution filtered through a small weighed filter, the filter washed, dried, weighed, and the weight returned as so much per cubic foot; of course the dust collected should also be submitted to microscopical examination.

It is scarcely necessary to say that the crystals must be pure and free from any kind of dirt; to collect the dust in this way is far better than by pulling it through liquids or by the use of small tubes. For quantitative estimations it is essential that there be no draught of any account through the tube, and the only way to avoid this is to use a tube of large diameter and to aspirate slowly.

For the mere qualitative examination of dust, air may be drawn through glycerin or through boiled distilled water. In some researches it will be advisable to surround a tube with ice and salt and draw the air through this tube, thus condensing moisture and any condensable volatile matters.

The bacteriological examination of air is also best performed by drawing the air through closely packed sugar or sodic sulphate or other soluble filter previously sterilized, and then dissolving the filter in water sterilized by boiling and cultivating in the manner detailed at page 160.

(59) Chemical Method of Estimating Organic Matter in the Air. The best chemical method of estimating organic matter in the air is its approximate estimation by means of permanganate of potash. A known bulk of air is drawn through a little distilled water and the amount of oxygen consumed determined by the Forchammer process.1

(60) Estimation of Carbonic Acid in the Air.

The simplest process of all, and one which might be used without the smallest knowledge of chemistry, is the phenol-pthalein method.

1 10 c.c. of the solution of permanganate [p. 164] and 10 c.c. of sulphuric acid 1:3 are added to a known bulk of water, say a litre; the whole is then heated for four hours to 26-6° C. (80° F.). At the end of that time the water is titrated with the hyposulphite solution [p. 165], using K1 and starch as an indicator. The value is obtained by running a control with distilled water.