. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . accurately fitted, with the trumpet-shaped end projecting externally (to collect the air),while the other extremity is made to dip well down intothe distilled water; the second perforation conductsanother bent glass tube, the end contained within the • The distilled ammonia free water should always be redistilledquite recently, as, even when comparatively fresh, it is often foundto contain minute traces of ammonia. 2o6 LABORATOR
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . accurately fitted, with the trumpet-shaped end projecting externally (to collect the air),while the other extremity is made to dip well down intothe distilled water; the second perforation conductsanother bent glass tube, the end contained within the • The distilled ammonia free water should always be redistilledquite recently, as, even when comparatively fresh, it is often foundto contain minute traces of ammonia. 2o6 LABORATORY WORK. flask being well above the surface of the water, andthe other connected directly by india-rubber tubing tothe aspirator. The capacity of the aspirator beingknown (and a convenient size is that of 25 litres),it is filled to the top with tap-water, the tap is thenturned so that the water passes slowly out—when air ofcourse enters the trumpet-shaped mouth of the bentglass tube to fill the vacuum created; such air iswashed in the distilled water (which has the propertyof taking up gases, &c., with e;xtreme readiness) beforeit reaches the Fig. 33.—An aspirator. Example.—It is desired to make an estimation of thenitrogenous organic matter;—then, if the aspirator betwice filled and allowed to empty, 50 litres of air willhave replaced the 50 litres of water, and these will havebeen drawn through the 500 of distilled water;therefore this 500 of water will contain the nitro-genous organic matter of 50 litres of air. Suppose the 500 of water are found to contain byWanklyns method 0*05 milligramme of free ammonia,then there will be 0-05 milligramme of free ammonia in ORGANIC MATTER IN THE AIR. 207 50 litres of air. But in dealing with air the results areexpressed in terms of milligrammes per cubic metre {, 1000 litres). Therefore, if there is 0*05 milligramme of free am-monia in 50 litres, there will be i milligramme of freeammonia in 1000 l
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