. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . isul-phide in traces ; carbonic oxide in traces ; ammoniumsulphide or carbonate in small quantity; sulphurettedhydrogen generally in faint traces ; watery vapour ex-cessive ; fine suspended particles of carbon and tarrymatters, together with the commoner forms of sus-pended matter in air [quod vide). Ground Air. Ground air may be sucked from considerable distancesinto a house, owing to the aspirating effect of thewarmed and expa
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . isul-phide in traces ; carbonic oxide in traces ; ammoniumsulphide or carbonate in small quantity; sulphurettedhydrogen generally in faint traces ; watery vapour ex-cessive ; fine suspended particles of carbon and tarrymatters, together with the commoner forms of sus-pended matter in air [quod vide). Ground Air. Ground air may be sucked from considerable distancesinto a house, owing to the aspirating effect of thewarmed and expanded air of the house itself; and thefoul air of cesspools has been shown to be sucked throughthe earth into a dwelling, for distances of over 20 feet. 234 LABORATORY WORK, When it is borne in mind that many houses containcellars built in and ventilated considerably below theground level, it is not difficult to see how groundair must enter materially into the constitution of theatmosphere of such cellars. A consideration of the character of ground air willshow its undesirability ! It contains an enormously highpercentage of carbonic acid, and the maximum amount. W ^-- FiG. 38.—Hesses apparatus for collecting ground air. of this impurity is always found between July andNovember—doubtless due to the amount of vegetablelife which dies off in the early autumn, and the circum-stance that at that season the temperature and moistureprevalent favour its rapid decomposition. The escape of large quantities of ground air into base-ments and cellars will, then, be best detected by ex-amining the atmosphere for carbonic acid, and any AIR OVER BURIAL GROUNDS. 235 considerable excess of this gas not otherwise accountedfor, points to such pollution ; other impurities likewisebeing present it becomes a question as to whether theseare derived from the same source, and the matter canbe settled by collecting a sample of the ground air inthe vicinity of the house, analysing it, and comparingit
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