CHAPTEE VII. 



Technique of making plates— Esmareh tubes, Petri plates, etc. 



Plates. — The plate method can be practised with 

 both agar-agar and gelatin. It cannot be practised 

 with blood-serum, because the serum, when once solidi- 

 fied, cannot be again liquefied. 



Plates are usually referred to as " a set." This term 

 implies three individual plates, each representing the 

 mixture of organisms in a higher stage of dilution. 

 The first plate is known usually as " the original," or 

 "plate 1," the first dilution from this as "plate 2," and 

 the second as " plate 3." 



In the preparation of a set of plates the following 

 are the steps to be observed : 



Three tubes, each containing from 7 to 9 c.c. of gela- 

 tin or agar-agar, are placed in the warm water bath 

 until the medium has become liquid. If agar-agar is 

 employed, this is accomplished at the boiling-point of 

 water ; if gelatin is used, a much lower temperature 

 suffices (35°-40° C). When liquefaction is complete 

 the temperature of the water, in the case of agar-agar, 

 must be reduced to 41°-42° C, at which temperature 

 the agar-agar remains liquid, and the organisms may 

 be introduced into it without fear of destroying their 

 vitality. The medium being now liquid and of the 

 proper temperature, a very small portion of the mixture 

 of organisms to be studied is taken up with a steril- 



