The effects of air containing the products of respiration in a concentrated condition, and of a deficient supply of air, have been shown only too well in the oft-quoted case of the Black Hole of Calcutta. In this case, 146 persons were confined in a space eighteen feet every way, with two small windows on one side. Next morning 123 were found dead, and the remaining 23 were very ill.
In the experience of this country, the highest death-rates are in the most densely populated districts. The death-rate from phthisis, childbirth, and typhus fever for instance, is far higher in cities than in country-places. The fact may be explained in various ways. Density of population commonly implies insufficient or unwholesome food, unhealthy work, and poverty; but especially impurity of the air, uncleanliness, and imperfect removal of excreta. Of these factors, the vitiated air is probably the most powerful for evil. Children suffer more than adults from close aggregation of population, largely owing to the greater ease with which infectious diseases spread in towns.
(2) Coal-gas is obtained by the destructive distillation of coal, free from access of air. The average composition of London coal-gas is hydrogen 50 to 53, saturated hydrocarbons 33 to 66, unsaturated hydrocarbons 3·5 to 3·6, carbonic oxide 5·7 to 7·1, carbonic acid 0 to 0·6, nitrogen 2·5 to 4·1, and oxygen 0·2 to 0·3 per cent. Of these the illuminants are olefiant gas (C₂H₄) and the higher hydrocarbons. Sulphuretted hydrogen and other sulphur compounds are present in small quantities, averaging 12 grains of sulphur per 100 cubic feet of London gas.
The inhalation of coal-gas, even in small quantities, is liable to produce headache, and may lead to chronic poisoning if allowed to continue. Where the escape of gas is more extensive, as when a tap is left turned on accidentally during the night, two dangers may arise. If a light is struck in the room an explosion occurs; or persons may be poisoned in their sleep by inhalation of the gas. The most poisonous gas in coal-gas is the carbonic oxide. The chief product of the combustion of coal-gas is carbonic acid. Some sulphurous acid is also produced, which is irritating to breathe, and injurious to bookbindings, picture-frames, etc. If the flame is imperfect, as when the pressure of gas is too great, some carbonic oxide may also escape.
In recent years Carburetted water-gas has been largely mixed with coal-gas in certain districts. This is made by passing steam over heated coke. Thus
| C | + | H₂O | = | CO | + | H₂ |
| (coke) | (steam) | (carbonic oxide) | (hydrogen) |
The product is water-gas which burns with a non-luminous flame and has no smell. For illuminating purposes it is enriched with vaporised paraffin oil, which gives it a high illuminating power, and a smell rather like that of coal-gas. In some towns as much as 60 per cent. of this carburetted water-gas is mixed with 40 per cent. of coal-gas. Now as the former contains about 30 per cent. of carbonic oxide, and the latter only 7 per cent., a mixture of equal parts of the two gases would contain 18·5 per cent. of carbonic oxide, and would therefore be much more dangerous than coal-gas. This has been found to be so in actual experience of escapes of gas.
In speaking of these products of different illuminants, it is necessary to adopt a standard of light. In this country the standard has hitherto been a light known as “one-candle power” which is given by a sperm candle burning 120 grains per hour, or in V. Harcourt’s standard flame by a mixture of air and pentane (C₅H12). A good fish-tail or bat’s wing burner for coal-gas gives an illuminating power equal to 16 candles, and burns from 4 to 5 cubic feet of gas per hour. Most flat flame burners known as 4 or 5, and supposed to burn that number of cubic feet of gas per hour, really consume nearly double this amount of gas. In the following table the amount of various products produced and of vitiation of air caused by various forms of illuminants is compared, when an illumination equal to 16 candles is produced in each instance:—
| AMOUNT BURNT. | CARBONIC ACID PRODUCED. | MOISTURE PRODUCED. | OXYGEN REMOVED. | AMOUNT OF VITIATION PRODUCED STATED IN TERMS OF THE NUMBER OF ADULTS WHO WOULD CAUSE AN EQUAL VITIATION. | |
|---|---|---|---|---|---|
| Sperm candles | 1740 grains | 6·6 c.ft. | 6·6 c.ft. | 9·6 c.ft. | 11 |
| Paraffin oil | 992 „ | 4·5 „ | 3·5 „ | 6·2 „ | 7 |
| Coal gas burned in Argand burner | 4·8 c.ft. | 2·5 „ | 6·4 „ | 5·8 „ | 4 |
| Flat-flame burner | 5·5 „ | 3·5 „ | 7·4 „ | 6·5 „ | 6 |
Thus as an adult expires 0·6 cubic feet of carbonic acid per hour, it follows that the amount of carbonic acid produced in one hour by the various illuminants named in the above table, burning so as to give a light equal to 16 standard candles, varies from 4 to 11 times the amount produced by the adult. Candle and oils possess the advantage over coal-gas that no sulphurous acid is produced in combustion. If the pressure in the mains is excessive, some gas may escape through the burner unburnt or carbonic oxide may escape.