50 litres of air. But in dealing with air the results are expressed in terms of "milligrammes per cubic metre' (i.e., 1000 litres).

Therefore, if there is o'05 milligramme of free ammonia in 50 litres, there will be 1 milligramme of free ammonia in 1000 litres—or a cubic metre—of air.

Pure air contains albuminoid ammonia up to o'ı milligramme per cubic metre, and averages about o'08. In a hospital ward this ammonia has been estimated at 1.3. Generally speaking the external atmosphere (pure air) averages about 0.06 milligramme per cubic metre of free ammonia, which has been estimated as high as o-8 in hospital wards.

The process is made much more troublesome, and is but slightly increased in accuracy, by passing the air through a succession of small wash bottles containing ammonia free distilled water-instead of one large bottle.

In the same manner pure distilled water may be charged with the oxidisable organic matter; but before estimating this, any sulphuretted hydrogen, sulphurous and nitrous acids, and chlorine compounds, must be previously got rid of in the manner already seen, and likewise any tarry matter-which will generally give evidence of its presence by yielding a smell or turbidity to the water.

The water may also be tested for oxidised nitrogen (nitrates and nitrites), chlorine, &c. For processes vide "Water."

Another, rougher, plan is to surround a thoroughly clean U-shaped glass tube with a freezing mixture of salt and snow (or ice), and to aspirate the air through the tube, when the low temperature condenses the watery vapour in the air as it passes through, and

most of the organic matter is thereby collected. The U-shaped tube can then be washed with ammonia free distilled water, and the washings treated for "free" and "albuminoid" ammonia.

This plan may also be used for the collection and. examination of suspended matters.

CHAPTER V.

AMMONIA MARSH GAS-CARBONIC OXIDE-SULPHUR COMPOUNDS.

AMMONIA.

TRACES of this gas are present in every atmospherenormally, to the extent of about 0.06 milligramme per cubic metre-and such are derived from organic decomposition (sewage, &c.), from combustion (coal), and from manufacturing processes in which chemicals, &c., containing ammonia are employed. The ammonia exists generally in combination with an acid, so as to form a salt, such as the carbonate and chloride, and, less commonly, as the nitrate, or sulphate. Ammonia vapours do not appear to injure health, beyond affecting the conjunctiva when in excessive quantities, and it is doubtless one of the most wholesome forms in which nitrogen and hydrogen, as gases, pass into the air. It is in greatest quantity near the ground-over peaty land it is abundant during hot weather-and it is largely present at the back of houses where refuse matter is deposited. Its presence, however, is regarded with scant favour, in spite of its comparative innocuousness, for it is the result of decomposition, and it is a constant ingredient in the most impure airs; it has therefore very bad relations and keeps very bad company.

The gas may be detected by moistening strips of filtering paper with Nessler's reagent, and hanging

P

these up for some time in the air of the compartment; or if a sample of air is collected in a jar-by catching one of these prepared papers between the stopper and the neck, in such a way that it hangs down into the jar free of the sides for a few minutes. Or known quantities of air may be aspirated through doubly distilled water, and the ammonia estimated quantitatively by "Nesslerisation."

MARSH GAS (CH4).

This gas probably exists in traces in most atmospheres, although owing to the difficulties of its detection it is not easy to speak definitely upon this point. There are certainly traces in the atmosphere of towns, and over districts of abundant vegetation (especially when such districts are marshy) it exists in large quantities. As it is evolved from strata in which mining operations are progressing, it is known as "fire-damp;" and its character of exploding, when ignited in the presence of carbonic acid, is often disastrously exemplified.

There is no doubt that marsh gas, though apparently extremely innocuous, may create, after a while, symptoms of chronic cachexia (poisoning); and being inodorous and non-irritating, its presence would not be detected by the senses. Any escape of coal gas, containing as it does 35 per cent. of marsh gas, will charge the atmosphere with considerable and dangerous amounts of this substance, but fortunately in these cases the strongly smelling ingredients of the coal gas give timely warning.

* A. Muntz and E. Aubin found that a million volumes of such air, previously freed from all dust and CO2, yielded, when passed over red-hot cupric oxide, from 3 to 10 volumes of CO2.

CARBONIC OXIDE (CO).

Owing to the properties which this gas possesses of entering into combination with the hæmoglobin of the red corpuscles, displacing their oxygen, and thus paralysing their oxygen-carrying functions, it plays the part of a virulent narcotic poison; and since it gives no indication of its presence to the senses, its powers for evil are materially enhanced. It becomes, then, an urgent duty to examine the air for this gas in those. cases where there is any likelihood of its presence, and this will always be the case in the atmosphere of compartments where iron or copper stoves are employed, and especially so when the material is cast iron, and when the fuel is coke; where coal gas (which contains 6 per cent.) is incompletely burnt or escapes; or where there is a possibility of some of the products of combustion from furnaces, flues, &c., escaping into a compartment for the air in furnace-flues has been found to contain over 20 per cent. of carbonic oxide, and that of ordinary flues from domestic fire-places as much as 4 per cent. The carbonic oxide of the air of flues is always the product of incomplete combustion, that is to say, the carbon of the organic material burnt is either not fully oxidised to carbonic acid (CO,), owing to the supply of fresh air being insufficient, or else the carbonic acid, being formed low down in the furnace, gets reduced in passing through the rest of the furnace to carbonic oxide.

It will also be of service to remember, that of the gases generated from the explosion of gunpowder, carbonic oxide forms nearly 8 per cent. (7.5 per cent.); and that a serious drawback to the adoption of "water gas"