have lost causticity represented by 10 c.c. of the oxalic acid solution. Therefore the 60 c.c. of lime water introduced have lost causticity represented by 20 c.c. of the oxalic acid solution, equivalent to 10 c.c. of CO2.

2000-60 (1940) c.c. of air in the jar are thus seen to contain 10 c.c. of CO2, and 1000 c.c. contain 5'15 c.c. of CO,; or the CO, is present to the extent of 5.15 parts per 1000, or 0.515 per cent.

If the air of the room is above 32° F., a correction must be made by adding o.2 per cent. to the result for every degree above freezing point. A correction for height above sea level may be made, if the place is in a mountainous district, by a simple rule of three from the observed height of the barometer.

Estimation of Organic Matter in Air.-The simplest method is to draw a measured volume of air to be examined, by means of an aspirator, through a washbottle, or a succession of wash-bottles, containing distilled water free from ammonia. The water absorbs the organic matters or a portion of them, and it can then be submitted to analysis for free and albuminoid ammonia according to Wanklyn's method. The results can be expressed as milligrammes of ammonia and albuminoid ammonia per cubic metre (= 1000 litres) of the air examined. They are necessarily indications only of the amount of nitrogenised organic matter in the air. The oxidisable matters in air, including putrescible organic matters, sulphuretted hydrogen, nitrous acid, and tarry matters, may be determined by submitting a part of the water used in the above experiment to the permanganate of potassium test. What has been said. in the chapter on examination of water (p. 82) applies equally to the test as now proposed for the estimation of oxidisable matters in air.

Microscopical Examination of Suspended Matters.-In Pouchet's aeroscope air is drawn through a glass funnel narrowed to a fine point, which just rests upon a drop of pure glycerine on a glass slide. The funnel and slide are enclosed in an air-tight bottle or box, which is connected by tubing with an aspirator, the open mouth of the funnel projecting above the top of the bottle or box. The suspended matters are caught in the glycerine and can be examined with an immersion lens.

Another method is to immerse a bent glass tube, previously heated to redness (to sterilise it), in a freezing mixture of salt and ice, and to slowly aspirate air through it. The moisture of the air is condensed into drops in the lowest part of the bend, entangling suspended matters with it, and a drop of this fluid can be examined microscopically.

Examination of Air for Bacteria, Fungi, and Moulds.— Hesse's apparatus is the most convenient. It consists of a hollow glass cylinder, one end of which can be closed with an india-rubber cap, whilst the other is connected with an aspirator. All parts of the apparatus having been cleaned with corrosive sublimate solution and then with alcohol, 50 c.c. of nutrient gelatine are introduced into the cylinder, and the whole sterilised by steaming for half-an-hour on 3 successive days. After the final sterilisation, the cylinder is rotated on its long axis, so that the gelatine solidifies in the form of a coating over the whole of the interior. The india-rubber cap is now removed from one end, while the other is connected with the aspirator, and the apparatus is ready for use. As the air is drawn slowly through, spores and germs fall on the gelatine, and colonies, visible to the naked eye, are formed in a few days, and may be counted. It is usually found that the colonies of moulds and fungi are

formed further from the mouth of the cylinder than the bacterial colonies-it thus appearing that the spores or germs of these organisms can be carried greater distances through the air than the bacteria. In pure air it would seem that the moulds and fungi are present relatively in larger numbers than the bacteria, whilst in air vitiated by respiration or organic effluvia of various kinds the reverse is the case.

CHAPTER IV.

WARMING AND LIGHTING.

WARMING.

In all cold climates means are provided for heating the interior of houses. It is a very difficult matter to supply a satisfactory degree of warmth for everybody, as individual susceptibilities to heat and cold are so various, depending as they do upon age, sex, robustness of constitution, and previous habitude. It may, however, be permissible to state that as a general rule, the temperature of a sitting-room or work-room should be about 60° F. to 65° F. The clothing of the individual should be so adjusted that a position of rest at this temperature in a well-ventilated room free from draughts shall cause neither a sensation of chill nor a feeling of undue heat.

Whilst this degree of warmth (60° to 65°) may be regarded as a suitable one for the average healthy individual, in the extremes of life-in infancy and old age-and in cases of sickness, a higher temperature reaching up to 70° is often desirable.

Radiation.

In this country houses are generally warmed by radiant heat from open fire-places. By radiation is meant the passage of heat from warm bodies to cold ones, the rays of heat passing through the air but without

warming it. This form of heat is no doubt the most healthy, for whilst objects within the range of the fire are heated, no impurities are added to the air of the room. It is, however, extremely wasteful, for the greater part of the heat at least) escapes up the chimney. The column of air in the chimney-flue is heated, and, becoming lighter than the external air, escapes at the roof of the house to be replaced by colder and denser air from below. An open fire, therefore, as we have seen in the chapter on ventilation, acts as a powerful ventilator.

The intensity of radiant heat is inversely as the square of the distance of the heated object from the source of heat. Thus if there are two objects, I foot and 3 feet distant respectively from an open fire-place, the more distant object only receives the amount of heat received by the nearer object. This fact shows the impossibility of warming equally all parts of a room, when the source of heat is an open fire-place.

Of late much has been done to improve open fireplaces by securing the greatest amount of heat production with the least consumption of fuel. Some of these improvements have been made at the suggestion of Mr. Pridgin Teale. They may be thus summarized:

The width of the grate at the back should be about one-third the width in front facing the room, the sides of the grate being sloped out at the necessary angle. The back and sides of the grate should be formed of fire-clay, and the back instead of rising perpendicularly should be "rifle-backed," i.e., curved forward so that the flames may play upon it (fig. 37). The curved portion becomes heated by some of the upward rays, which would otherwise be lost up the chimney, and radiates this heat into the room.

The floor of the grate should be formed of a solid slab