. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . on, thelid of the box is taken off for a second or longer, thenreplaced and put into the warm moist chamber toincubate. Very numerous forms of fungi, according tothe locality, rapidly develop. The moulds are earlyidentified; amongst the micro-organisms are observedsome which liquefy the gelatine, others of various shadesof red, yellow and other colours ; some assume definiteforms of growth. The number of separate colonies AIR. 4
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . on, thelid of the box is taken off for a second or longer, thenreplaced and put into the warm moist chamber toincubate. Very numerous forms of fungi, according tothe locality, rapidly develop. The moulds are earlyidentified; amongst the micro-organisms are observedsome which liquefy the gelatine, others of various shadesof red, yellow and other colours ; some assume definiteforms of growth. The number of separate colonies AIR. 459 should be counted, a process which may be faciUtatedby a hand lens, or by placing the box on the stage ofthe microscope and examining with a low power. Thenumber of distinct species may also be ascertained, andfrom these single cultures should be made; in thismanner a large number of interesting micro-organismscan be readily obtained from the air. Other methods of air analysis consist in drawingknown volumes of air by means of some form of as-pirator over solidified gelatine or through liquefied gelatine,cotton-wool, sand, or gvamilated or powdered sugar. In. Fig. 115.—Petris apparatus for examination of air. Hesses apparatus the air is drawn through a glass tubesome 18 inches or more in length, the floor or the wholeof which is covered with sterilised gelatine ; the micro-organisms are deposited on the gelatine surface andform colonies which are counted. In Petris apparatusthe air is drawn through a column of sand ; the sand isthen shaken up with liquefied gelatine and run on to aplate or spread out in an Esmarch tube (see later). InFranklands apparatus the air is sucked through a plugof glass-wool, which is afterwards thoroughly shakenup with gelatine. In Sedgwicks and in Tuckers 460 LABORATORY WORK. methods, the air is drawn through a column of granu-lated sugar, which when mixed with the liquid gelatinedissolves (an advantage over the sand employed byPetri), leaving th
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