CHAPTEK VII. 



Technique of making plates— Esmarch 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 solidified, 

 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 state 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 sterilized, 



