=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._ Per-centage =I.= 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.