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. II. _London Air.——Carbonic Acid, Metropolitan Railway, November, 1869._ +--------+----------------------------+------------+---------+---------+ | | | |Carbonic | Oxygen, | | Date. | Place. |Time of Day.| Acid, |per cent.| | | | |per cent.| | +--------+----------------------------+------------+---------+---------+ | 1869. |Tunnel between Gower Street | | | | |Nov. 12.|and King’s Cross Stations; | 10 a.m. | ·150 | 20·60 | | |specimen taken at the open | | | | | |window, first-class | | | | | |carriage. | | | | | | | | | | | ” 12.|Tunnel between Gower Street | | | | | |and King’s Cross Stations; | 7·30 p.m. | ·078 | 20·79 | | |specimen taken at the open | | | | | |window, first-class | | | | | |carriage. | | | | | | | | | | | ” 12.|Tunnel Praed Street; | | | | | |specimen taken at the open | 10·30 a.m. | ... | 20·71 | | |window, first-class | | | | | |carriage. | | | | | | | | | | | ” 15.|Specimen taken during | | | | | |journey between Gower Street| 10·15 a.m. | ·338 | 20·66 | | |and King’s Cross, | | | |