268 MICEOBES, FERMENTS, AND MOULDS. 



Koch employs a more certain method. He makes 

 his sowings on glass plates, covered with sterilized 

 gelatine and kept at a temperature of 30, by means of 

 a slender platinum wire which has been made red hot, 

 then allowed to cool, and charged with a very minute 

 particle of matter, which is full of bacteria. The 

 colonies of different microbes isolate themselves, and 

 may be plainly seen on the glass plate with the aid of 

 a magnifier. Their variable size and characters often 

 enable experienced observers to distinguish them by 

 their aspect alone (Fig. 87, 1, 2). The test-tubes, 

 containing sterilized gelatine, are then inoculated 

 with the microbe which it is desired to study (Figs. 

 82, 105), after taking the usual precautions. 



The filters used to sterilize liquids are of Sevres 

 biscuit-ware heated to 120, or unglazed pottery. 

 Such is the Chamberland filter already described. 



Cultures for Experiments on Animals. The pro- 

 cesses we have just indicated are also necessary in 

 these experiments. Here likewise all the causes of 

 error which would arise from the want of cleanliness, or 

 from the impurity of the culture liquids, must be care- 

 fully avoided; and it must also be ascertained that 

 the effect produced on the animal is not due to any 

 other microbe than that of the experiment, nor to 

 any irritating and septic substance. The experiment 

 should be repeated several times by taking some of 

 the blood of the inoculated animal, and making a pure 

 culture, which may be used to reproduce the disease 

 in other animals. 



