CHAPTER VII. 



Technique of making plates Petri plates. Esmarch tubes, etc. 



PLATES. The plate method can be employed with 

 both agar-agar and gelatin. It cannot be practised with 

 blood-serum, because the serum when once solidified 

 cannot be again liquefied. 



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

 term implies three individual plates, each representing 

 a mixture of organisms in a higher state of dilution. 

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

 " plate No. 1," the first dilution from this as " plate 

 No. 2," and the second as " plate No. 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 a 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 sterilized 

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