. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . Fig. 113.—Filtering tube to keep back the bacteria. the ambient micro-organisms are both easy and instruc-tive. One of the simplest and self-evident, consists in 458 LABORATORY WORK. exposing a sterilised surface of gelatine to the air, for aknown period. Glass dishes (Petris dishes) with lids,fig. 112, of various sizes are made for the purpose; theyshould be of clear glass, and it is an advantage to haveeach surface polished. T
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . Fig. 113.—Filtering tube to keep back the bacteria. the ambient micro-organisms are both easy and instruc-tive. One of the simplest and self-evident, consists in 458 LABORATORY WORK. exposing a sterilised surface of gelatine to the air, for aknown period. Glass dishes (Petris dishes) with lids,fig. 112, of various sizes are made for the purpose; theyshould be of clear glass, and it is an advantage to haveeach surface polished. They are sterilised in the hotoven and then sufficient nutrient gelatine to cover thebottom is rapidly poured into them, in doing this the lidis only just raised to permit of the pouring in of thegelatine. The gelatine should be made to set aS soonas possible and the boxes should then be kept for two. Fig. 114.—Punch for making the gauze wads which fit in thefiltering tube. days in a moist atmosphere, to see whether they remainsterile. When it is desired to make an observation, 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 consis
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