. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . Fig. g8.—The hot air oven. Boiling.—Boiling, or subjecting to steam for fromten minutes to half an hour, sterilises many are very numerous forms of steam 99 represents the well known steam steriliserof Koch. * In sterilisation allowance must naturally be made for the thick-ness and conductivity of the object to be sterilised. The surgeonfor instance is satisfied w^ith boiling his instruments for t
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . Fig. g8.—The hot air oven. Boiling.—Boiling, or subjecting to steam for fromten minutes to half an hour, sterilises many are very numerous forms of steam 99 represents the well known steam steriliserof Koch. * In sterilisation allowance must naturally be made for the thick-ness and conductivity of the object to be sterilised. The surgeonfor instance is satisfied w^ith boiling his instruments for ten minutes,but insists on several hours for the ligatures. 428 LABORATORY WORK. The principle of this apparatus is that of the or-dinary kitchen steamer, and many of the American. Fig. gg.—Steam steriliser of Koch. vegetable steamers are more rapid and cheaper. It isfilled to the right level with water, and the apparatus STERILISATION. 429 to be sterilised is placed in the steamer ; it is furnishedwith a thermometer. Sterihsing in steam at 100° mustbe employed in the case of gelatine. Agar-agar willbear an exposure to 120° C. To make sure that thesterilisation is absolute in the case of the nutrientmedia (gelatine chiefly), successive sterilisation mustbe resorted to. Thus the gelatine should be kept at100° C. for a quarter of an hour on three consecutivedays. The reason for so doing is this :—Whilst thevegetative forms of the micro-organisms may be killedat one boiling, the spores need not; allowing an inter-val of a day, however, gives time for the spores todevelop into adult forms, and these are killed by thereboiling. It may be pointed out that the hay bacillusis isolated from the other numerous organisms in aninfusion of hay, by boiling the latter f
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