Ammonia is derived from the putrefaction of animal and vegetable substances. It is from atmospheric ammoniacal compounds that plants obtain much of the nitrogen which is essential to the formation of many parts of their structure.
Nitric acid, like ammonia, is absorbed, and its nitrogen assimilated, by plants.
In addition to the gases and vapours already enumerated, as well as others which exist in minute quantity, or which are of only occasional occurrence, Pasteur and other investigators have discovered in the air living germs which are capable of exciting putrefaction and fermentation, and which are competent, in some instances, to engender disease when they are injected into the blood of animals. In fact, the spread of infectious diseases, e.g., smallpox, typhus fever, cattle plague, &c., is attributed to the presence in the atmosphere of the germs of such maladies. These germs are believed to be living beings, which develope and multiply at the expense of the tissues of the larger animals into whose systems they have found entrance.
Air, Vitiated. As has been stated in the previous article, the air consists chiefly of two gases, oxygen and nitrogen. In all open places it has a similar composition, as might be concluded from the constant mingling which takes place by the agency of currents continually in movement, although sometimes to an inconsiderable extent only. Dr Angus Smith regards air as very pure when it contains not less than 20·99 per cent. by volume of oxygen, and 0·030 of carbonic anhydride (acid). According as the proportion of the former gas diminishes and that of the latter increases beyond certain limits in the air by which we are surrounded, it becomes more or less deteriorated and unfit to be breathed, particularly as the increased amount of carbonic acid is, in crowded dwellings, assembly rooms, theatres, and confined inhabited spaces, associated with deleterious and putrescent exhalations from the person.
The following tables exhibit the amount of carbonic acid in close places in London.
I.
| Per-centage | |
| by volume. | |
| Chancery Court, closed doors, 7 feet from the ground, March 3 | ·193 |
| Same, 3 feet from ground | ·203 |
| Chancery Court, doors wide open, 4 feet from ground, 11·40, March 5 | ·0507 |
| Same, 12·40 p.m., 5 feet from ground | ·045 |
| Strand Theatre, gallery, 10 p.m. | ·101 |
| Surrey Theatre, boxes, March 7, 10·30 p.m. | ·218 |
| Olympic, 11·30 p.m. | ·0817 |
| Same, 11·55 p.m. | ·1014 |
| Victoria Theatre, boxes, March 24, 10 p.m. | ·126 |
| Haymarket Theatre, dress circle, March 18, 11·30 p.m. | ·0757 |
| Queen’s Ward, St. Thomas’s Hospital, 3·25 p.m. | ·052 |
| Edward’s Ward, St. Thomas’s Hospital, 3·30 p.m. | ·052 |
| Victoria Theatre, boxes, April 4. | ·076 |
| Effingham, 10·30 p.m., April 9, Whitechapel | ·126 |
| Pavilion, 10·11 p.m., April 9, Whitechapel | ·152 |
| City of London Theatre, pit, 11·15 p.m., April 16 | ·252 |
| Standard Theatre, pit, 11 p.m., April 16 | ·320 |
Dr Angus Smith states that out of 339 specimens of air obtained from various mines he found 35 normal or nearly so, 81 decidedly impure, and 212 exceedingly bad; he also adds that owing to the frequent firing of charges of gunpowder within the mines, and from other causes, the atmosphere is further contaminated with sulphuretted hydrogen, sulphate, carbonate, sulphide, sulphocyanide of potassium, and nitrate of potassium, carbon, sulphur, carbonate of ammonia, organic matter, sand, and sulphurous and arsenious acids.
The air of large cities, which are the seats of manufacturing industry, is always more or less charged with the exhalations given off by chemical and other works. The sulphuric-acid works contribute sulphuric, sulphurous, nitrous, and arsenious acids; copper works, in which pyrites is employed, give off large quantities
of sulphurous acid, mixed with arsenic and a little copper; manure works, in many cases, send out compounds of fluorine, besides sulphuric acid; glass works, sulphuric and hydrochloric acids; and alkali works, hydrochloric acid (although in small quantities), which very frequently contains arsenic. Of ammonia, Angus Smith remarks: “It is one measure of the ‘sewage’ of the air; it is the result of decomposition. It is not, in these small quantities, hurtful, so far as we know. The ammonia is in no case free, but combined probably with hydrosulphuric, hydrochloric, and sulphuric acid in towns. In country places it is, at all events partly, united to carbonic acid.