501
��tiB9UM ol such aniiDBls are tbe most perfect filters known, neliber permitting the entnujce, nor tolerat- ing llie eiistence, of siiy foreign material, unless the Usauea ftre diseased.
3. Sufflclently prolonged expoaare to a lenipera- ture of at least 110° C. This is the lowest iieeessary for the destruction of aporea, althongh 8U° C. is Bufn- cient to kill bacteria fn the growing condition. The length of the exposure must not be less than an hour: the longer the time beyond this, the greater the security.
4. Intermittent hefttlng, Invented b; Tyndall, and much used in Germany. This consists in making the spores germinate, la order to IsiU the full-grown bacteria at SU° C. For this purpose, the vessels eon- tainlDg the fluid to be
sterilized are kept at 20=-30° C. to favor the growth of the spores, and are every day raised to 80O C. for one hour, to destroy such bacteria as have become fully de- veloped. This method takes much time, and its results are always uncertain. [This is the French point of view, but must not be ac- cepted as that of the beat authorities. — Ed.]
Of all these methods, the third, that of de- stroying the germs once for all, is the one giving the greatest security and _
ease of manipulation. '" ' '
It htti but one fault, that
of ci>agulating all albuminous substances which can be solidified at the temperature of boiling water. [This fault is a very great one, and at once excludes the use of blood-serum as a culture-medium. — Ed.]
The latest and best method for employing this pro- cess is as foIlowB; The first thing Is to close the vessels meant to contain the sterilized liquids with stoppers permeable to air. The method of doing this will be described l.tter. The flasks are then kept at a tempemlure of 1((0= C. for at least three houra. tf the teraperature be higher, sterilization will occur sooner, but the cotton stoppers will be charred. The furnace In which this aterllizatlon is done should be double-walled, but its form la imimportant. The flasks should be allowed to cool slowly to prevent breakage; and, as the rareSed air contracts, the air which enters Is well Altered by the cotton plug.
The second step consists in preparing the sterilized liquid, and introducing It Into the flasks. The bouil- lon of Miguel la the beat we know of, and is this; Take of lean meat (beef) cue hllogratn, and boil it in four litres of water for five hours; skim It, and let It stand over night in a cool place; then take oS the fat, and neutrallie the fluid with caustic soda; filter, pot in water up to four litres, and boil for ten minutes.
��Prolong this second boiling to an hour, and do it in a Papin's pot, at 110° C. after putting in forty grama of common salt. Then the liquid, cooi«d. and passed through a doable filter. Is again placed in the Papin's pot for three hours, which completes the sterilisation. Instead of this natunl bouiUon , the following may be used: —
Peptone fehemicftlly putr) ............ B
���but is completely s<
��of the peptone. If a nutrient gdatine be de- sired, put from twenty- five to thirty grams of pore colorless gelatine into either of the above fluids before the last fil- tering. This last should then be done thmugh a hot filter. The other manipulations are the same, e.tcept that the sterilization must not be prolonged moi« than an hour; for, if it Is, the gelatine loses its power of solidifying. Agar- agar may be used in- stead of gelatine, and remains solid at 30° C. It is only partially dis- solved in boiling water, the end of an hour at 1 10° C.
��in Papin's pot, Filter hot, and again sterillw at 100°
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