TEXT-BOOK OF BACTERIOLOGY. 105 



and form on the cover-glass, but it is not tni the high power with 

 immersion is employed that the advantages of this treatment are 

 seen at their best. What previously seemed to be the uniform 

 mass of the colony resolves itself into a closely-crowded mass of 

 separate bacteria, which stand beside each other, shoulder to shoul- 

 der, or lie together without order, showing in the clearest manner 

 how such a colony is built up. 



The ease with which the process is conducted and its faultless 

 results have in a short time made it an indispensable part of our 

 method of investigation. When working with this form of cul- 

 tures, scarcely ever lay by a plate without taking one or more im- 

 pressions of it. 



Be on guard against a certain error in judging the plates. It 

 is scarcely possible to prevent germs from falling upon the gelatin 

 surface from the air during its preparation, and still less so during 

 the after-employment of it. Such germs naturally develop; and 

 although it is one of the advantages of a solid food medium that 

 they are bound down to the spot on whix;h they fall, and so cannot 

 spread beyond a narrow limit, yet their numbers are sometimes so 

 great — particularly' when the plates have been carelessly handled 

 or the moist chamber has been long left open — that they become 

 troublesome hindrances to the observer. 



It is chiefly mould fungi that get in in this manner, but they 

 are little likely to be confounded with bacteria. Thej' are recog- 

 nized by there being only one, or at most but a verj^ few, on the 

 ^i'hole plate, and also by their occurring exclusively on the surface. 

 If Koch's original method has not been used, but one or other of 

 its modifications, the manner of proceeding requires but little alter- 

 ation. With Petri's dishes the examination proceeds exactly in the 

 manner described; with Esmarch's test-tube preparations, place 

 the tube under the microscope and examine with a low power, to 

 become acquainted with the appearance of the different colonies. 

 Print preparations cannot, of course, be made with them. Al- 

 though this must be regarded as a defect, it is almost balanced hy 

 a very considerable advantage which is peculiar to the rolled test- 

 tubes. The plates cannot be kept for any length of time without 

 receiving pollutions from without. It seems that quite a number 

 of germs develop comparatively long — perhaps weeks — after the 

 inoculation, a circumstance which escaped notice formerly only 

 because the ordinary plate could not be kept in good condition be- 

 yond a certain limited period of time. 



The agar plates require no particular mention here. No micro- 

 organisms are known capable of peptonizing agar; it offers no 



